Dictonary of Pure and Applied Mathematics

a Volume in theComprehensive Dictionary

of PHYSICS

Edited by

Dipak Basu

PUreand

APPLIEDPHYSICS

DICTIONARY OF

Boca Raton London New York Washington, D.C.CRC Press

http://www.crcpress.com

Preface

TheDictionary of Pure and Applied Physics(DPAP) is one of three physics dictionaries beingpublished by CRC Press LLC, the other two being theDictionary of Material Science and High En-ergy PhysicsandDictionary of Geophysics, Astrophysics and Astronomy.Each of these dictionariesis entirely self-contained.

The aim of the DPAP is to provide students, researchers, academics and professionals in general,with definitions in a very clear and concise form. The presentation is such that readers will not haveany difficulty finding any term being looked for. Each definition is written in detail as informativeas possible supported by suitable diagrams, equations, and formulae whenever necessary.

With more than 3000 terms, the fields covered in the DPAP are acoustics, biophysics and med-ical physics, communication, electricity, electronics, geometrical optics, low temperature physics,magnetism, and physical optics.

Like most other branches of science, physics has grown rapidly over the last decade. As such,many of the terms used in older books have become obsolete and new terms have appeared inscientific and technical literature. Care has been taken to ensure that old terms are not included inthe DPAP and new terminologies are not missed. Some of the terms are related to other fields, e.g.,engineering fields (mostly electrical and mechanical), mathematics, chemistry, biology.

Authors are eminent scientists at research institutions and university professors from around theworld. Readership includes physicists and engineers in all fields, teachers and students in physicsand engineering at university, college and high school levels, technical writers and professionals ingeneral.

The uniqueness of the DPAP lies in the fact that it is an extremely useful source of informationin the form of meanings of scientific terms presented in a very clear language and concise formwritten by authoritative persons in the field. It would be a great aid to students in understandingtextbooks, help academics and researchers fully appreciate research papers in professional scientificjournals, provide authors in the field with assistance to clarify their writings and, in general, benefitthe enhancement of literacy in physics by presenting scientists and engineers with meaningful andworkable definitions.

Dipak Basu

©2001 CRC Press LLC

CONTRIBUTORS

Barry I. BarkerStanford University

Stanford, California

Dipak BasuCarleton University

Ottawa, Ontario, Canada

Christopher BoswellThe Johns Hopkins University

Baltimore, Maryland

H.R. ChandrasekharUniversity of Missouri

Columbia, Missouri

David CheekeConcordia University

Montreal, Quebec, Canada

Shenlin ChenBoise, Idaho

Lee ChowUniversity of Central Florida

Orlando, Florida

T.C. ChoyUniversity of Melbourne

Parkville, Victoria, Australia

J.M. CollinsMarquette University

Milwaukee, Wisconsin

Luis Cruz-CruzBoston University

Boston, Massachusetts

Robert T. DeckUniversity of Toledo

Toledo, Ohio

Vijai DixitSt. Louis University

St. Louis, Missouri

Douglas M. GingrichUniversity of Alberta

Edmonton, Alberta, Canada

Arthur A. GrossmanUniversity of California, San Diego

San Diego, California

Shirin Haque-CopilahUniversity of the West Indies

St. Augustine, Trinidad, West Indies

Takafumi HayashiThe University of Aizu

Fukushima, Japan

Cila HermanThe Johns Hopkins University

Baltimore, Maryland

Stanley JeffersYork University

Toronto, Ontario, Canada

Joe KhachanUniversity of Sydney

Sydney, NSW, Australia

Vasudevan LakshminarayananUniversity of Missouri, St. Louis

St. Louis, Missouri

Scott A. LeeUniversity of Toledo

Toledo, Ohio

Mirko MirkovCynosure, Inc.

Chelmsford, Massachusetts

Michael J. O’SheaKansas State University

Manhattan, Kansas

Vladimir OstashevNOAA

Boulder, Colorado


A.G. Unil PereraGeorgia State UniversityAtlanta, Georgia

Edward RothwellMichigan State UniversityEast Lansing, Michigan

Kenneth TranthamArkansas Technology UniversityRussellville, Arkansas

Kainam Thomas WongThe Chinese University of Hong KongShatin, NT, Hong Kong


Editorial Advisor

Stan Gibilisco

Acknowledgments

The following figures have been reproduced by kind permissions as mentioned.Kellner Schmidt optical system: Fundamentals of Optics (4th Edition), by F. Jenkins and H.

White, McGraw Hill, New York, 1976.Tangential and sagittal focal lines: Mirrors, Prisms and Lenses, by J. P. Southall, Dover Publishers,

New York, 1964.Lummer-Gehrke plate: Fundamentals of Optics (4th Edition), by F. Jenkins and H. White, Mc-

Graw Hill, New York, 1976.Moira fringes: McGraw Hill Dictionary of Scientific and Technical terms (5th Edition), by S. P.

Parker (Editor-in-Chief), McGraw Hill, New York, 1994.Heat capacity for 4He. The Lambda phenomenon: John Lipa and Joel Nisser of Stanford Uni-

versity.


AAbbe number Dispersion or separation ofneighboring wavelengths by transparent mate-rial can be characterized by the Abbe number.If for a given materialnD, nF , andnC refer tothe refractive indices for the FraunhöferD (589nm), F (489 nm), andC (656 nm), then thechromatism of the material is characterized byits v-value or constringence

v =nD − 1nF − nC

.

This is called the refractive efficiency,v-valueor Abbe number.The larger the constringence,the lesser the chromatism. The inverse of theAbbe number is the dispersive power.

Abbe sine condition (1) For an optical sys-tem, if one considers the object and image planesperpendicular to the optical axis, andθ to be theangle relative to the axis made by a ray from anaxial object point in object space of indexn andθ′ to be that in image space of indexn′, then thetransverse magnification,m:

m =n sin θn′ sin θ′

for all θ andθ′ .

For an object at infinity:

sin θ′ = − y

f ′

wherey is the height of a ray parallel to the axisandf ′ is the back (or secondary) focal distance.The condition is valid for a lens free of coma andthe relationships hold to a good approximationin most lenses, but not where discontinuities ap-pear in ray behavior (such as Fresnel lenses orzone plates). Deviations from this relationshipare called the “offense against the same condi-tion” and are associated with coma.

(2) The condition satisfied by rays refractedby spherical interfaces of an optical system (e.g.,microscope) to form an image free of coma; ifθ1 andθ2 are the angles made by non-paraxial

rays from the object and the image,h1 andh2

the object and image heights,n1 andn2 the re-fractive indices, then theAbbe’s sine conditionis

n1h1 sin θ1 + n2h2 sin θ2 = 0 .

ABCD law If qin andqout are the complexradii of curvature of the input and output beamsof an optical system, they are related by the ele-ments of the ABCD matrix according toABCDlaw: qout = A qin+B

C qin+D . SeeABCD matrix.

ABCD matrix The height (y) and angle (α)of a paraxial ray (measured with respect to theoptical axis) can be described by a simple2× 2composite system matrix as it passes throughan optical system with several elements (lenses,mirrors, etc.). The elements of the matrix (M )are referred to asA,B,C, andD. In the equa-tion[

yf

αf

]= M

[yi

αi

];M = MnMn−1 . . .

M1 =[A BC D

],

the subscriptsi andf refer to the initial (input)and final (output) rays and the system matrixMis a product of the ray matrices of each elementof the optical system. The optical element clos-est to the initial ray has the matrixM1 and theone closest to the final ray hasMn. The physicalsignificance of the elements of the system matrixis the following: The input (output) plane cor-responds to the first (second) focal plane whenD(A) is equal to zero. The output plane is theimage plane conjugate to the input plane whenB = 0 with A being the linear magnification.WhenC = 0, a parallel bundle of input raysemerges as a parallel bundle of output rays;Dcorresponds to the angular magnification.Seecardinal points.

aberration Optical systems form distortion-free images when the rays entering the systemare parallel to the axis and are close to it (parax-ial rays). However, this restriction leads to lowthroughput. Off-axis and non-paraxial rays leadto distortions which can be classified as (a) chro-matic and (b) monochromatic aberrations.See


aberration, chromatic and aberration, mono-chromatic.

aberration, chromatic The refractive indexof a material is dependent on wavelength. BySnell’s Law, light rays of different wavelengthswill be refracted at different angles, since indexis not a constant. Because the index of refractionis higher for shorter wavelengths, these are fo-cused closer to a lens when compared to longerwavelengths, when polychromatic or white lightis incident on it.Longitudinal chromatic aber-ration is defined to be the axial distance fromthe nearest to the farthest focal points.

aberration, lateral The inability of a systemto focus rays from a source on its axis to a pointon the axis, but instead focus at a point shiftedperpendicularly or laterally to the axis.

aberration, longitudinal When a system fo-cuses the obliquely incident rays on the opticalaxis but at a point that is not the focal point forthe paraxial rays.

aberration of optical systems Any error inimaging, for example due to dispersion (chro-matic), curvature of the surface (spherical),coma, astigmatism, distortion, etc., resulting ininability of an optical system to bring a broadbeam to focus at a unique point.

aberrations, monochromatic The aberra-tions in an optical system even for a monochro-matic light, thus arising totally from the geome-try of the system, in contrast to chromatic aber-ration (seechromatic aberration) due to disper-sion of the medium (different refractive indicesfor different wavelengths of the incident light).

aberration, spherical This distortion iscaused by marginal or non-paraxial rays (i.e.,rays that are parallel to the axis but are furtherfrom the axis). The image has a diffuse halo sur-rounding the sharp image formed by the paraxialrays. The marginal rays come to a focus at dif-ferent points on the axis compared to paraxialrays and diverge at steeper angles. (Seedia-gram.) This leads to longitudinal and transversespherical aberration. The position at which theimage blur is minimum is called thecircle of

least confusion.The spherical aberration is zerofor an optical system if the object and image arearranged to be at theconjugate pointsof one an-other. (Seeconjugate points.) For a thin lens, ifthe shape is such that the radius of curvature ofthe first convex surface is about six times that ofthe second concave surface, then thesphericalaberration is minimized. The meniscus shapeof eyeglasses is chosen for this reason. Opticalsystems such as cameras can reduce this defectby using smaller apertures (largef -numbers) toblock off the outer rays.

Aberration, spherical.

aberrations, Seidel Monochromatic aber-rations named after the German mathematicianLudwig von Seidel who studied aberration the-ory keeping higher order terms for parametersdescribing the departure of the incident beamfrom a paraxial beam. The five Seidel aberra-tions are astigmatism, coma, curvature of field,distortion, and spherical aberration.

abrupt junction A diode junction in whichthe dopant level or material type changesabruptly at the junction.

A diode is formed at the junction of two ma-terials with different Fermi levels (that is, eachwith a different chemical potential, the energylevel which is the highest filled at a temperatureof absolute zero). In semiconductors, one wayto alter the Fermi level of the material is to adddopants that are either electron donors or ac-ceptors. A diode constructed of the same basicsemiconductor material can be realized with dif-ferent dopants on each side of the junction. Thedopant level can be changed gradually acrossthe device, or change abruptly at the junction.In real devices, the dopant level changes in a re-gion small compared to the depletion layer, the


region of non-uniform carrier density about thejunction.

Such abrupt junction diodes have the benefitof simplified numerical analysis. For instance,the depletion region depth depends on the squareroot of the voltage difference (which includesthe intrinsic diode voltage and the applied re-verse bias voltage).

A junction that is not abrupt is sometimescalled a gradual or graded junction, and willhave different properties depending on the na-ture of the dopant profile.Seediode junction.

absolute zero The absolute minimum in tem-perature. Any system in thermal equilibrium atabsolute zerohas its minimum energy and mini-mum entropy. Absolute zero occurs at−273.15C (−459.67F). The absolute temper-ature scale has an arbitrariness in the size of thedegree, chosen such that the triple point of wa-ter is exactly273.16C. It is not possible for asystem to reach the absolute zero of temperature.This can be seen through the behavior of entropyas the temperature approaches zero. The thirdlaw of thermodynamics states that the entropyof a system atT = 0K is a constant. If thisis true, then the specific heat is zero at absolutezero:

CX

T=(∂S

∂T

)X

whereX describes the method of temperaturechanges (e.g.,V for constant volume). Since thethird law statesS approaches a constant at ab-solute zero, the slope of the entropy vs. temper-ature curve must become zero at absolute zero,resulting in a zero heat capacity. It can be shownthat this also results in the thermal expansion ap-proaching zero at zero temperature and that

V α

CX→

T→0constant

whereV is the volume,α is the expansion co-efficient, andCX is the heat capacity. For anadiabatic change in some variableX, we find

dT =V α

CXTdX

and this shows us that the change inX needed toproduce a temperature changedT grows withoutbound asT → 0. This completes the argument

that absolute zero is unattainable.See alsother-modynamics, third law of.

absolute zero, unattainablility of Seeabso-lute zero.

absorbance Seeabsorption, Lambert’s lawof.

absorptance Seeabsorbance.

absorption If a beam of light with the in-tensityI0 passes through a homogeneous sub-stance of thicknessx (unit: m) and emerges witha lower intensityI, according to the exponen-tial law of absorptionI = I0 exp(−αx). α isthe absorption (or extinction) coefficient (unit:m−1). α depends on the material and is a func-tion of the wavelength of light passing throughit. For example,α for glass can be as low as10−7 m−1 in the visible while it can be severalorders of magnitude higher for the infrared andultraviolet wavelengths of light. While derivingthis law, effects due to scattering are ignored.

absorption, acoustic Nonreversible trans-formation of acoustic energy into other forms ofenergy (for instance, heat). Two main kinds ofthis process are absorption of sound in its reflec-tion from a surface and absorption of sound inits propagation through gaseous, fluid, or solidmedia. The latter phenomenon results in expo-nential decrease of an amplitude and intensityof a sound wave with distance of propagation,e.g.,A = A0e

−αx. Here,A andA0 are the am-plitudes of sound pressure in a sound wave attwo fixed points,x is a length of a sound pathbetween these points, andα is theabsorptioncoefficient.

absorption, anomalous, acoustic Absorp-tion of sound byrelaxationprocesses. The term,anomalous absorption,had been used until themiddle of the twentieth century. However, it israrely used nowadays.

absorption bands/lines The absorptionspectra of atoms at small concentrations con-sist of discrete energies at which the absorptionis high. The electronic energy levels of eachatomic species are discrete and unique. An atom


in one state of energy can be excited to a higherstate by absorption of a photon whose energy isequal to the difference in energy between the twostates of the atom. A spectrograph which dis-plays the intensity of white light passing throughthe atoms of interest vs. wavelength will showdark lines due to the missing photons of specificenergies absorbed by the atoms. Molecules, onthe other hand, have vibrational and rotationalenergy levels superimposed on the electronicenergy levels giving rise to a large number ofclosely spaced lines which smear and appear asbands. Bands also appear at higher concentra-tion of atoms due to broadening of spectral lines.Energy levels of solids (crystalline and amor-phous) also exhibit bands.

absorption, Beer’s law of For solutions withconcentrationC (unit: gm per liter) in a cell ofpath lengthb (unit: cm), the logarithmic form ofBeer’s law for the absorbance,A, can be writtenasA = log( I0

I ) = abC = εbC. Herea is calledtheabsorptivity(unit: liters per gram-cm). IfCis in units of moles per liter,ε is called themolarabsorptivity(unit: liter per mole-cm).

absorption coefficient Seeabsorption.

absorption coefficient, acoustic (1) A ratioof absorbed by a surface energy of a sound waveincident on this surface to the energy of the wavebefore interacting with the surface.

(2) An inverse distance at which the ampli-tude of pressure in a sound wave propagatingin a medium decreases by a factore ≈ 2.7183.The absorption coefficient,α, can also be de-termined byα = x−1 ln(A0/A), wherex is alength of a sound path between two fixed points,andA andA0 are the amplitudes of sound pres-sure at these points.

absorption edge Refers to the threshold en-ergy below which photons are transparent in asubstance.

1.Absorption edgein x-rays: The probabilityof absorption of photons in elements increaseswith wavelength until an absorption edge isreached when a sharp drop occurs. The sharpdrops correspond to the binding energies of elec-trons in inner shells.

2. Absorption edgein semiconductors: Theelectronic energy states consist of completelyfilled bands of energies (Valence Band) followedby unoccupied bands (Conduction Band) sepa-rated by aband gap.The absorption of photonsshows a sharp increase at and above the band gapenergy due to the promotion of electrons fromthe filled Valence Band to the empty Conduc-tion Band. The direct band gap semiconductors(e.g., gallium arsenide) have a sharp edge unlikethe indirect gap (e.g., silicon) semiconductors inwhich the absorption is assisted by lattice exci-tations (phonons). The edge is also very sensi-tive to temperature. At low temperatures (whenthermal energy is much less than the band gapenergy) the edge is much sharper. At highertemperatures, the phonons and impurity statessmear the edge.

absorption, Lambert’s law of The exponen-tial law of absorption converted to logarithms tobase 10 can be written asA = log( I0

I ) = αx2.303 .

HereA is called theabsorbanceof the sub-stance,I0 andI are the incident and transmittedintensities through a substance of thicknessx(unit: m) and absorption coefficientα (units:m−1). Seeabsorption.

absorptivity Seeabsorption, Beer’s law of.

absorptivity, molar Seeabsorption, Beer’slaw of.

acceptor An impurity introduced into a semi-conductor crystal lattice to accept an electron,allowing a bond missing an electron (a positivehole) to migrate and thus carry charge. For ex-ample, in a silicon lattice, an atom of boron canbe substituted for one of the silicon atoms. Sili-con atoms have four valence electrons, whereasboron atoms have only three. However, theboron atom completes the covalent bonds withthe neighboring silicon atoms by accepting anelectron (and becoming negatively charged).The crystal remains electrically neutral, so ahole (seen as a positively charged lack of an elec-tron) can be a charge carrier through the lattice.

Acceptor impurities (for silicon or germa-nium) include boron, aluminum, gallium and in-dium. Semiconductors with more acceptor thandonor impurities have conductivity dominated


by diffusion of holes (positive charges) and arelabelledp-type. See alsodonor.

access coupler An access couplertransfersoptical signal from one conductor to another inan optical transmission system. The couplingcoefficient, measuring the efficiency of a cou-pler, is defined as the ratio of optical power onthe incident side to the optical power on the otherside of the interface.

access line An access lineembodies the cir-cuitry linking a user and a switching center(where message packets are routed onwards totheir eventual destination for addresses servedby other centers or are delivered to local ad-dresses served by this center). An access linemay be specifically adjusted to provide ampli-tude and phase equalization to the signal passingthrough it; this is labeled a special-grade accessline. An access line connecting an individualuser or a group of users to a communication net-work is called a network access line.

access time Access timein a communicationsystem refers to the time lag from the beginningof an access attempt to an access success. Ac-cess time is defined only for successful accessattempts. Access time in a computer systemrefers to the time lapse from when data are de-livered to when data storage starts.

accidents, laser Lasers are dangerous due tothe high intensity of laser light. The laser beamcauses localized burning resulting in skin le-sions, retinal damage, damage to mucous mem-branes, etc. The extent of damage is a functionof laser wavelength and intensity and may bealleviated by wearing safety glasses and opaqueclothing. Care must yet be taken to prevent dam-age to the safety glasses and starting fire to theopaque clothing.

accommodation (1) To observe objectsclosely and far away, the lens in the eye ac-commodates, i.e., it adjusts its dioptric powerto attain maximal sharpness of retinal imagery.When accommodating to a near object, the cil-iary body contracts and hence the curvature ofthe lens decreases, increasing the power of thelens. The front surface of the lens moves for-

ward slightly. For a distant object, the ciliarybody relaxes and the lens assumes a flatter shape,thus increasing its radius of curvature and con-sequently increasing its focal length.

(2) Optical: the ciliary muscles tense or re-lax causing the lens to either (a) bulge resultingin a small radius of curvature and a concomi-tant shorter focal length allowing the viewer tofocus on close objects, or (b) relax resulting ina larger radius of curvature and a concomitantlonger focal length allowing the viewer to focuson more distant objects.

(3) Sensory: with constant, or nearly con-stant stimulation, sensory receptors may adaptto the stimulus and fail to respond. That is,a slowly rising depolarization at the molecularlevel may not initiate an action potential (Seeaction potential); hence no stimulation.

(4) Evolutionary: a change in an organismthat allows it to better interact with its environ-ment, sometimes referred to asselective evolu-tion.

accumulation time The amount of time asignal (radiation, x-ray, light, etc.) is actuallybeing recorded. Every recording instrument hasdead time; the fraction of running time requiredfor a signal to be processed before the next signalcan be processed. Hence, the accumulation timeis the running (or clock) time less the dead time.

accumulator An electrochemical cell thatcan be electrically recharged repeatedly is calledanaccumulator.It is also called secondary cell.Seesecondary cell.

achromatic colors Colors in which the at-tribute hue is lacking. Colors ranging from black(absence of light) through various shades of grayto dazzling white are called achromatic. The at-tribute that distinguishes them is brightness.Seecolor.

achromatic doublet A common way to elim-inate chromatic aberration is to use anachro-matic doubletthat consists of a convex and aconcave lens of different glasses cemented to-gether. The focal lengths and refractive pow-ers of the lenses differ (by shaping their surfacecurvatures) producing a net power, and the dis-persion powers of the components are chosen


such that they are in inverse proportion to thesepowers. The result is a compound lens that hasa net focal length but reduced dispersion overa major portion of the visible spectrum. If thedoublet, made up of 2 lenses of individual pow-ersF1 +F2, has a net power ofF diopters, thenthe conditions defining the doublet are:

F = F1 + F2 ,

andF1

v1= −F2

v2.

The powers (F, F1 andF2) are measured for thesodiumD-line (589 nm) andv1 andv2 are theAbbe numbers of the materials of the two lenses.Achromatic prisms are formed by combining 2prisms with equal and opposite dispersions.

achromatism An optical element or systemis said to exhibitachromatismif chromatic aber-ration has been eliminated in the element, e.g.,by use of an achromatic doublet.

achromatopsia Total color blindness. Theoptic system responds to all frequencies of visi-ble light; however, the person “sees” only shadesof gray.

acoustic bridge Device that is used to mea-sure acoustic impedance of a substance.Acous-tic bridgeis closely analogous to electric bridge.For example, in one of possible arrangements ofan acoustic bridge, two tubes (one filled with asubstance whose impedance is known and theother with a substance of unknown impedance)form the ratio arms.

acoustic capacitance The ratio of the vol-ume displacement in an acoustic system to thepressure applied to the system.

acoustic density Deviation of the density ina medium from itsambientvalue caused by pas-sage of a sound wave.See alsoacoustic pres-sure; displacement, acoustic.

acoustic efficiency The ratio of the acous-tic energy radiated by a transducer to the energy(for example, electrical energy) supplied to thistransducer.Acoustic efficiencyis a dimension-less quantity, and, according to the principle of

energy conservation, it is always less than one.It is customary to express acoustic efficiency inpercent.

acoustic grating A periodic structure or sur-face that affects an incident sound wave in sucha way that, after transmission or reflection, anumber of diffraction maxima and minima oc-cur. Acoustic gratingis similar to optic grating.An example ofacoustic gratingis a number ofrods with widthh placed in a row at a distancelbetween each other. If a plane sound wave is in-cident normally on this acoustic grating, the farfield of the transmitted wave has maxima that aredetermined by the formulasinα = nλ/(h+ l).Hereα is the angle between the normal to thegrating and the direction of the diffracted wave,n = 0,±1,±2, . . . , andλ is the sound wave-length.

acoustic lens Material of special shape andkind that is used to focus sound waves. Thisfocusing is based on the phenomenon of soundrefraction. Acoustical lenses are similar to op-tical lenses. Acoustic lenses can be made fromgaseous, fluid or solid substance. In the first twocases the substance is encased.

acoustic loss A decrease in amplitude andintensity of a sound wave in its reflection froma surface or in its propagation through gaseous,fluid and solid media due to attenuation, geomet-rical spreading and other mechanisms. For ex-ample: excitation of a surface wave and acous-tics barriers.

acoustic pressure Deviation of a pressure ina medium from itsambientvalue, caused bypassage of a sound wave. At a given point,acoustic pressurep oscillates in timet with thefrequency of the sound wave. The root-mean-squared acoustic pressurep = ( 1

T

∫ T

0p2(t)

dt)1/2 does not depend ont and is often usedin practice. Here,T is a time interval of averag-ing, which should be properly chosen. Anotherquantitative measure of acoustic pressure thatis often used is the sound pressure levelL =20 lg p

p0, wherep0 represents a reference pres-

sure. In atmospheric acoustics,p0 = 2·10−5Pa,while in underwater acousticsp0 = 10−6Pa.


acoustics in moving media The branch ofacoustics that studies the effects of medium mo-tion on sound propagation, and the effects ofsource and receiver motion on the emitted andreceived sound. Medium motion significantlyaffects sound propagation in the atmospherewhere wind velocity and its fluctuations are rela-tively large: in the ocean if a sound wave crossesa region with strong currents, in ducts with meangas flow, etc. Mean flow in a medium and its reg-ular and random inhomogeneities cause bend-ing in the path of a sound wave, phase change,diffraction and scattering of this wave. Sourcemotion always results inDoppler effectand alsogenerates a sonic boom if the source speed is su-personic.

acoustics of buildings A part of architec-tural acoustics that deals with noise propagationthrough a building or its parts. Building acous-tics treats outdoor noise transmission throughwalls of a building and the level of this noise inrooms, and considers indoor noise transmissionfrom one room to another. It also designs build-ings to reduce transmission of outdoor noisethrough walls, for example, by locating win-dows of a building away from a highway.

acoustics of rooms A part of architecturalacoustics that deals with sound propagation in-side a room (auditorium, concert hall, studio,etc.). In a room, a listener hears direct soundfrom a source plus a series of itsechoesdueto reflection and scattering by walls and objects.The latter sound is called reverberant sound. Re-verberant sound exponentially decreases in thecourse of time because of absorption by walls,objects and air. A time interval for which thelevel of the reverberant sound is decreased by 60dB is known as reverberation time. Reverbera-tion time is the main characteristic of acousticquality of a room. If reverberation time is small,sound is toneless. If reverberation time is large,parts of speech or music overlap and are difficultto be heard.

acousto electronics The discipline that dealswith conversion of radiosignals by means of ul-trasound devices. Acousto electronic devicesallow one to amplify an amplitude of a radiosig-nal and to modulate this amplitude, change a

phase of a radiosignal and its spectrum, delaya radio impulse and to change its lengths, inte-grate, decode and encode radiosignals, and todo other conversions. The frequencies of ultra-sound waves used in acousto electronics are 10MHz and higher.

acousto-optic deflector A beam of light canbe deflected by an angleθ by Bragg diffrac-tion from a refractive index grating producedby acoustic waves launched in a crystal.Seeacousto-optic effect.

acousto-optic effect Interaction of opticaland acoustic waves in a crystal. By applyinga periodic mechanical stress in a crystal (by at-taching a piezo-electric transducer, for example)longitudinal acoustic waves can be launched init. This leads to a moving refractive index grat-ing in the crystal with the spacing,d = v

f , wherev is the speed of the sound wave in the crystalandf is the frequency of the transducer. Thisgrating can scatter light according toBragg’slaw leading to diffracted beams.

acousto-optic modulator A device that canmodulate a beam of light by acousto-optic ef-fect. TheRF signal used to launch acousticwaves in the crystal can be (a) amplitude modu-lated which results in the modulation of the in-tensity of the undiffracted beam, (b) frequencymodulated resulting in the modulation of the fre-quency shift of the first order beam or (c) mod-ulated by changing the direction of the soundwave in the crystal.Seeacousto-optic effect.

acousto optics The discipline that deals withthe effects of sound waves in solids and fluidson light or electromagnetic wave propagatingthrough these media. A progressive or standingsound wave in a medium periodically changesits dielectric permittivity in space and/or time.These changes cause diffraction and scatteringof a light beam or an electromagnetic wave prop-agating through such a medium. This phenome-non, also known as light diffraction by ultra-sound, is the basis of many acousto optical de-vices that are widely used to control the direc-tion of light propagation, and its polarization,amplitude and spectrum. Among such devicesare acousto optical filters, scanners and deflec-


tors. Acousto optical devices allow one to con-trol a light beam by changing amplitude and/orfrequency of a sound wave.

actin A proteinaceous filament constituentof muscle tissue and noncontractile tissue caus-ing motility, usually in association with myosin.Actin is a globular protein of molecular weight42,000 Daltons.Seemolecular weight.

actinic radiation The portion of wavelengthsof the total radiation at which a specific sen-sor is excited within the pass band of an opticalsystem. This is a term used in the context ofmetrology.

action potential A membrane electric poten-tial exists between the exterior and the interior ofa living cell; usually of value 50 mV to 100 mV,with the zero potential reference in the outsidemedium and the inside of the cell negative po-larity. This resting potential is due to an unevendistribution of ion species (co- and counter ions)inside and outside the cell. Whenever the mem-brane potential rapidly or suddenly changes, asfor example during muscle movement, it is re-ferred to as an action potential.

action potential, sodium conductanceSodium conductance is a measure of the abil-ity of sodium ions,Na+, to diffuse across acell membrane. It is observed that depolariza-tion of a cell membrane leads to an increase insodium conductance across the membrane. Theaction potential (seeaction potential) causes atransient depolarization of the cell membrane;hence, action potentials create a transient in-crease in sodium conductance.

activation analysis An atom or a nucleus thatabsorbs energy may be transformed into eitheran excited state atom or nucleus, or, if the en-ergy absorption is sufficient, the nucleus may betransformed in a nucleus of higher atomic mass.Many of these excitations are metastable and thedecay may be observed and recorded yielding asignature of the original atom or nucleus present.

activation analysis, neutron In neutron acti-vation analysis, a beam of neutrons (sometimesslow (thermal), sometimes fast) is incident on a

sample of material for which the constituents aredesired to be identified. Some of the nuclei inthe sample will capture a neutron, thus increas-ing the atomic mass by one. Often the new,or daughter, nucleus will decay via beta emis-sion: the form and energy of emission identify-ing the isotope originally present in the sample.Different nuclei have disparate capture efficien-cies (cross sections) for neutrons of various en-ergy (speed), hence the proper neutron energiesmust be selected for the anticipated sample nu-clei composition.

activation analysis, photon When high en-ergy photons are incident on atoms within a sub-stance, some of the atoms will absorb the pho-ton if either the photon energy matches one ofthe allowed molecular, atomic, or nuclear en-ergy level differences, or if the photon energyis at or above the ionization energy of the atom.The atom is then placed in a metastable, excitedstate that decays to its ground state. This yieldsa characteristic frequency signature without al-tering the original atom present, unlike neutronactivation, which permanently alters the originalatom present.

activation energy (1) Chemical: the mini-mum amount of energy that must be providedto start a self-sustaining chemical reaction, thatis, on the molecular level, the amount of energyper mole to break the requisite chemical bondsso the atoms are free to recombine.

(2) General: the amount of energy (mechan-ical, heat, light, etc.) that must be supplied to asystem for the system to begin functioning in adesired way.

active device A device that introduces energyinto the primary circuit, instead of being a purelystorage or dissipative device. Some examplesof active devices are transistors, amplifiers andmixers.See alsopassive device.

active voltage In a circuit element operatedwith alternating current, the voltage waveformcan be viewed as the sum of two waveforms,one in phase with the current, one out of phase.The in-phase component is sometimes termedthe active voltage. Energy dissipation in the de-


vice is proportional to the active voltage timesthe current.Seephasor.

acuity, stereoscopic This is the smallest dif-ference in distance of an object perceivable bystereoscopic cues and is usually specified by an-gle of stereopsis. This is a measure of depthperception. The angle of stereopsis is the differ-ence between the angle subtended at the centersof the entrance pupils of the two eyes by the twopoints at different distances from the eyes.

acuity, visual Visual acuity depends on theeyes’ ability to differentiate details at differentdistances from the eye. It is a measure of theresolving power of the eye. There are 3 types ofresolution possible:

1. Minimum separable resolution:the abil-ity to discriminate between two closely spacedpoints.

2. Minimum visible resolution:the ability tosee the smallest resolvable angle subtended bya black bar on a white background

3. Minimum legible resolution:the abilityto read the smallest angle subtended by blockletters on a test chart such as a Snellen chartwhich is commonly used.

Acuity is denoted in terms of6/x (knownas a Snellen fraction) or in decimal form. In theSnellen chart, a standard test distance of 6 meters(20 feet) is chosen. At this distance, in the rowof letters labeled6/x, each letter will subtend 5minutes of arc and each individual feature in theletter (i.e., the gap in the letterC) will subtend1 arc minute. In general the numerator of theSnellen fraction indicates the fixed distance ortest distance and the denominator denotes thedistance at which each letter in a given Snellenrow subtends 5 arc minutes.

Visual acuity varies with region of retinastimulated, state of light adaptation of the eye,general illumination, background contrast, sizeand color of objects, refractive state of the eye,character of retinal image and time of exposure(viewing time).

adaptation of the eye When the eye (or thevisual system) is presented with a stimulus fora period of time, the system will “adapt” to thestimulus and will be less responsive or sensitive.This can be formally defined as the change in

sensitivity due to continuous or repeated sensorystimulation. Examples of adaptation include:

1. dark adaptation, wherein the eye adjustsand becomes more sensitive to light as illumi-nation is reduced (such as being in a dark room),and its converse, light adaptation (walking froma dark room to a well lit room); and

2. chromatic adaptation which is an alteredsensitivity to color that results in apparentchanges in hue and saturation due to prolongedviewing of a specific color.

ADC (Analog to digital converter) A circuitor device that quantizes an incoming analog sig-nal and puts out a digital representation of theinput. A necessary precurser for digital signalprocessing, ADCs must be designed with the ap-propriate dynamic range, linearity and stabilityfor the signal expected.

adder, analog An amplifier circuit that pro-vides the output proportional to the sum of theinputs. Also called asummer.Requires at leasttwo inputs and one output.

adder, cascade A device constructed of cas-caded half adders and the appropriate logic cir-cuits to allow the true addition of binary num-bers. A simple cascade adder is also known asa full adder. Seehalf–adder.

adiabatic nuclear demagnetization A cool-ing method that uses the properties of nuclearparamagnets in high magnetic fields to reachtemperatures below 1 mK. If a system of non-interacting nuclear spins, magnetic momentµ,is in thermal equilibrium at temperatureT in amagnetic fieldB, its entropy is a function ofµB/kB T . If the system is then thermally iso-lated, and the magnetic field is decreased isen-tropically, the system temperature will decreaselinearly with the magnetic field.Tf = Bf

BiTi.

The most common simple nuclear paramagnetused in nuclear demagnetization cryostats ishigh purity copper. It is possible to cool a Cudemagnetization stage to a fewµK by startingat a magnetic field of roughly 8T and a tempera-ture of 5 to 10 mK . When higher initial temper-atures and/or lower initial magnetic fields areto be used, PrNi5 is a good choice of nuclearrefrigerant. The 4f electrons in Pr form a non-


magnetic singlet ground state in zero magneticfield. In a large magnetic field, some of theseelectrons will become aligned with the magneticfield, enhancing the local magnetic field seen bythe nuclei. This hyperfine enhancement is verylarge for PrNi5 – the field produced by the elec-trons at the nucleus is roughly ten times the ex-ternal field. The concomitant entropy decreaseallows use of PrNi5 at higher temperatures andlower fields than those used with Cu demagne-tization cryostats. As a result of the large inter-nal field, the minimum temperature attainablewith PrNi5 is approximately 0.4 mK as the in-teractions lead to magnetic ordering.See alsocooling, magnetic.

adiabatic process in sound propagationProcess with no heat flow in a medium due tosound propagation. Usually, adiabatic processis mathematically formulated as the followingcondition:

(∂/∂t+ v ·∇)S = 0 .

Here, S(R, t) is the entropy per unit mass,R = (x, y, z) are the Cartesian coordinates,tis time, v is the medium velocity vector, and∇ = (∂/∂x, ∂/∂y, ∂/∂z). Propagation ofsound waves in air and water can be consid-ered to be accurate as an adiabatic process ifthe sound frequencies are much less than109

Hz and1012 Hz, respectively.

adjacent channel An adjacent channel refersto a channel adjoining another channel in thefrequency domain, in the time domain, or in thespatial domain. For instance, in a frequency-division multiplexing communication system,the channel with carrier frequency just aboveor just below the carrier frequency of channelA embodies an adjacent channel to channel A.In the case of time-division multiplexing, thechannel at the time slot right before or right afterthe time slot of channel A is an adjacent chan-nel of channel A. Adjacent channel interferencemay occur if signal power from adjacent chan-nels spills into the desired channel, say, due tofrequency drift in a frequency-multiplexed sys-tem or due to mis-synchronization in a time-division-multiplexed system. The capability ofa receiver to differentiate signals in the desired

channel from its adjacent channels is termedad-jacent channel selectivity.

admittance The inverse of impedance. If thevoltage in a circuit element isV (the phasor) andthe current isI (also a phasor), the admittanceY is the ratio of current “admitted” to appliedvoltage:Y = I

V . This is in general a complexvalue, and may depend upon the frequencyω.Some examples of admittance are given here fora pure resistor (YR = 1

R , a real number), a purecapacitor (YC =

√−1ωC) and a pure inductor

(YL =√−1

ωL ), both imaginary. The admittanceof elements in parallel is the sum of the indi-vidual component admittances. Sometimes theadmittance is broken into the length of the pha-sor and the phase shift angle. For AC devices,the admittance is the change in current over thechange in voltageY = ∂I

∂V .

admittance, acoustic Reciprocal of theacoustic impedance.

admittance, input The admittance calcu-lated by dividing the current driven into a de-vice by the applied voltage. For antenna designor other small signal inputs, it is crucial to tunethe input impedance to match that of the trans-mission line, or a large portion of the signal willbe reflected back instead of being processed bythe device.

admittance, output The admittance calcu-lated by dividing the current output of a deviceby the voltage signal output. Inverse of the out-put impedance.

adsorption pump A pump which uses cry-opumping to achieve low pressures.See alsocryopumping.

aerodynamic sound (noise) Sound (noise)generated (1.) by interaction of a flow with asolid surface, (2.) by low speed flow. Examplesof aerodynamic sound of the first kind are eoliansounds produced by twigs and wires in the wind.Sources of this aerodynamic sound are associ-ated with aerodynamic forces acting on a solidsurface; they have a dipole character. Sourcesof the aerodynamic sound of the second kindare velocity fluctuations in a flow, which have


a quadrupole character. The latter aerodynamicsound can be mathematically treated by usingLighthill’s acoustic analogy: sound generationin a non-moving homogeneous medium by theturbulent velocities in a flow.See alsoair jetnoise.

afocal system A telescope or a beam ex-pander is often referred to as anafocal systemi.e., one without a focal length. Both the objectand the image are located at infinity. The angularmagnification is the ratio of the apparent angu-lar size of the image to that of the object. Whenthe object is centered on the axis the magnifica-tion, M , is M = tan α2

tan α1whereα1 andα2 are

half-field angles in the object and image space,respectively.

air jet noise Noise (sound) generated bysubsonic and supersonic air jets. Noise pro-duced by a subsonic turbulent jet can be ap-proximately treated by using Lighthill’s acousticanalogy (seeaerodynamic sound) which yieldsthat the noise power is proportional to the eighthpower of the jet speed. Noise generation by asupersonic jet is completely different from thatby a subsonic jet or a low speed flow, for exam-ple; shock waves may play an important role innoise generation. As a result, the noise power ofa supersonic jet is proportional to the jet speedto the power of no greater than three.

air liquefier, Claude-Heylandt The Claude-Heylandt liquefier produces liquid air by a com-bination of isentropic expansion of a gas andJoule-Thompson cooling. Incoming gas is pres-surized to the order of 200 bars, then allowedinto an expansion engine, where the pressure de-creases to 1 bar. This gas exchanges heat with anincoming stream of air and then returns to thecompressor. The incoming stream of gas, af-ter being cooled in a series of heat exchangers,passes through a Joule-Thompson valve wheresome fraction of the air is liquefied. The re-maining gas returns to the compressor coolingthe incoming gas on the way.See alsoheat ex-changers.

air, liquid Air, composed of 78% nitrogen,18% oxygen, and 1% trace gases, becomes aliquid at 78.8 K. Liquid air is a colorless, odor-

less liquid. The liquid oxygen is actually a paleblue, but is normally not visible in liquid air.Due to the liquid oxygen present, liquid air isa flammable liquid, and, as with all cryogenicliquids, is a severe frostbite hazard.

Airy disk The pattern of diffraction for cir-cular aperture has rotational symmetry about theaxis. The central maximum is a circle of lightthat corresponds to the zeroth order of diffrac-tion and is known as theAiry disk.The near fieldangular radius of the Airy disk is given by1.22λ

DwhereD is the diameter of the aperture andλis the wavelength of light used to illuminate thedisk.

Airy function In the context of the transmit-tance of a Fabry-Perot interferometer, a func-tion of the form 1

1+F sin2( δ2 )

is called theAiry

function. F is the coefficient of finesse and isa measure of the sharpness of fringes andδ isthe phase difference between successive inter-fering rays. The figure below displays the Airyfunction for different values ofF .

Airy function.

Airy’s disc The Fraunhofer diffraction pat-tern due to a circular aperture contains a centralbright spot with concentric rings. The centralspot is called the Airy’s disc. The angular ra-dius of the Airy’s disk is1.22λ

D whereD is thediameter of the aperture andλ is the wavelengthof light.

Alfven waves Transverse hydromagneticwaves propagating along magnetic field linesin plasma or in a conducting fluid. Alfvenwaves were theoretically predicted in 1942 byH. Alfvén, Swedish physicist. The speed of theAlfven waves does not depend on frequency and


is given byva = H/√

4π%. Here,H is themagnetic field strength, and% is the density of amedium. The Alfven waves play an importantrole in astrophysics.

algorithm A sequence of precisely definedprocedures, often mathematical or logical op-erations, to perform a particular task possiblyon a given set of data. This sequence of stepshas a singular starting point and terminates ei-ther with the task accomplished or an indicationthat the specified task is impossible. The ac-tual path taken through the defined procedureswould depend on the input data set and on theprior process step. A practical algorithm accom-plishes the given task within the time constraintsof the problem and the memory limitations ofthe computer with sufficient robustness to con-tamination in the data set. An algorithm mayalso be embodied in the hardware architectureof the computer processor as a special-purposededicated machine.

aliasing If signals are sampled at certainrates during measurements, a well-known phe-nomenon in information theory calledaliasingplaces a limit on the highest frequency that isunambiguously processed. If∆t is the timeinterval between successive measurements, theNyquist frequency,fN , defined asfN = 1

2∆tdetermines that the signal and noise present ata frequencyf will be folded back at frequen-cies 2nfN − f and 2mfN + f . Heren is apositive integer andm a negative integer cho-sen such that the aliasing is in the range of 0 tofN . If the signals are band limited up to a max-imum frequency,fmax, the sampling should besuch thatfN ≤ fmax. Stated differently, fora proper measurement of a spectrum the high-est frequency component should be sampled atleast twice. This is also referred as thesam-pling theorem. The diagram below illustratesthis principle. A function due to a superpositionof three harmonic waves of frequencies 0.3, 0.8and 1.1 Hz is shown in the top part of figure. Ifthis data is sampled at intervals of∆t = 1 sec,the Nyquist frequency,fN , is 0.5 Hz. The signalcomponents at 0.8 and 1.1 Hz do not satisfy thesampling theorem while the 0.3 Hz does. The0.8 and 1.1 Hz will be aliased to 0.2 Hz (corre-sponding ton = 1) and 0.1 Hz (form = −1),

respectively, as shown in (b) of the lower figure.The panel (a) shows the true spectrum wherethe sampling theorem is satisfied for the highestfrequency component present in the data.

Aliasing. t (sec), f (Hz).

These ideas have applications in digital signalprocessing and Fourier transform spectroscopy,to name a few. Any function with two variablesrelated by Fourier transforms (e.g., time and fre-quency, distance and spatial frequency) can beprocessed by the above criteria.

alnico Manmade magnetic material exhibit-ing high coercivity (seecoercivity), comprisingapproximately 8% Al, 13% Ni, 24% Co and 3%Cu.

alternating current Alternating current(AC) is current that is alternating in the direc-tion of the current flow. The typical alternatingcurrent is sinusoidal in shape. Alternating cur-rent has an advantage over direct current (DC) inthat its voltage magnitude can be changed easilythrough a transformer.

alternator An electrical device for generat-ing alternating current.

ambient Pertaining to a medium where asound wave can propagate. If there is no soundwave propagating in a medium, the pressure,density and fluid velocity in the medium arecalled the ambient pressure, density and fluidvelocity. If there is a sound wave in the medium


and the approximation of linear acoustics holds,the pressure, density and fluid velocity in themedium are sums of the ambient and acousticpressure, densities and fluid velocities.

ammeter An instrument used for the mea-surement of direct or alternating electrical cur-rent. It consists of resistors in series with a gal-vanometer.

amount of charge that flows in an electrolyticcell The rules are:

1. The mass of an element eroded at an elec-trode is directly proportional to the amount ofelectric chargeQ passed through the electrode.

2. If the same chargeQ is passed through sev-eral electrodes, the mass lost at each electrode isdirectly proportional to the atomic mass of theelement, and to the number of moles of elec-trons required to erode one mole of electrodeelemental material.

ampere Unit of electric current in the Sys-teme International (S.I.). One ampere of currentis defined as the current when flow in each of twoinfinitely long parallel wires of negligible radiusseparated by a distance of one meter in a vac-uum causes a transverse force per unit length of2x10−7 newton/meter to act between the wires.Equal to one coulomb of charge passing througha circuit element per second.

ampere balance An apparatus for measur-ing current, balancing the torque from forcesbetween coils on the balance beam and statorcoils (in the same circuit) with the torque fromforces on the balance beam due to weights. Ifproperly calibrated, the current through the coilscan be read off from the position of the weights.

ampere hour The integrated amount ofcharge passing through a circuit element inan hour. Therefore 1 ampere-hour = 1coulomb/second times 3600 seconds/hour, or3600 coulombs.

Ampere’s law This theorem states that theline integral of a magnetic field around a closedpath is equal to the current enclosed by the path.

ampere turn In the case of a coil ofN turnscarrying currentI the line integral of the fieldaround a closed loop yieldsNI, the magneto-motive force. The units of magnetomotive forceare ampere turns.

Amperian path An arbitrary closed pathused in the definition of Ampere’s law. In Am-pere’s law, a closed line integral of the magneticfield is equal to the total current enclosed in thepath times the permitivity of the free space,µo.This arbitrary closed path in Ampere’s law issometimes called Amperian path.

amplifier A circuit or device designed to pro-duce an output proportional to the input signal,or possibly some other function of the input. Of-ten this proportionality is a single multiplicationby a gain factor (g) so that the output voltageV out is related to the input voltageVin by the re-lationshipVout = gVin. However, sometimes theoutput current (or voltage) is a function of theinput voltage (or current). In general the gainmay be complex.

amplifier, AC coupled An amplifier circuitconstructed with a simple resistor-capacitor net-work to isolate a DC input voltage (or interme-diate voltage in a multi-staged device) from theinput directly to the amplifier (or following stageamplifier). Alternating current passes throughthe capacitor, while any DC offset voltage is notamplified. This allows operation of any com-ponents previous to the amplifier at operationalvoltages, or can be used from controlling tem-perature effects. Also called an RC-coupled am-plifier (for the resistor-capacitor isolation circuitused).

amplifier, antilog An amplifier with the out-put level an exponential function of the input;thus the input is a logarithmic function of theoutput.

amplifier, audio-frequency An amplifier de-signed to operate at frequencies similar to thefrequencies of audible sound, 100 Hz to 3 kHz.More generally, the termaudio amplifieris usedto imply a DC signal will not be amplified. Inthis usage, the design frequency may be muchhigher than sound.


amplifier, bipolar An amplifier that is basedon a bipolar transistor. A bipolar transistor isa device containing two diode junctions. Sinceeach junction has a polarity, the name bipolar isan apt descriptor of the device. The transistorhas three active leads, labelled emitter, base andcollector.Seetransistor, junction; bipolar code.

amplifier, broad band An amplifier circuitcapable of operating over a wide frequencyrange with a nearly constant gain.

amplifier, cascaded An amplifier where twoor more amplifier stages are connected in series(cascaded). This is done to provide increasedgain. Mixing between stages may allow shiftingof the frequency to allow filtering or optimalamplification in the intermediate stages.

Many radio receiver designs are staged insuch a way, with filtering done in the intermedi-ate frequency stages.

amplifier, cascode A low noise amplifierconstructed of two amplifiers in a special seriesarrangement. If bipolar amplifiers are used, thefirst stage is a common emitter circuit, with inputat the base and output to the collector. This goesinto the second stage, a common base amplifier.The signal is input to the second stage emitter,with output to the second stage collector.

A cascode circuit need not be of bipolar de-vices. Any amplifiers, if arranged in an analo-gous fashion (stage 1: common source or com-mon cathode, stage 2: common gate or commongrid) can make up a cascode amplifier.

The high output impedance of the cascodecircuit allows its use to drive circuits while keep-ing the effects of the amplified output away fromthe input. The rest of the parameters of the cir-cuit behave much like the first stage would alone.

amplifier, classes A, B, AB, C Amplifierclasses are defined by what fraction of the inputwaveform cycle results in amplified output. Thisis changed by changing the relationship of theamplifier response to the input signal.

Class A amplifiers are those for which theoutput signal follows the input at all times. Con-sider the amplifier response curve that shows therelationship between input voltageVi and outputvoltageVo (or current or other signal). Since the

response of the amplifier is well behaved for allVi , the output signal includes the entire wave-form and makes this a class A amplifier.

Class A amplifier operation.

Class B amplifiers are those that provide anamplified output for half of the input waveformcycle. This is due to the response cutoff valuefalling right at the input cycle average voltage.

Class B amplifier operation.

Class AB amplifiers are those that provideoutput signal for less than the full input cycle,but more than half the cycle.

Class C amplifiers provide an output signalfor less than half the cycle.


Class AB amplifier operation.

Class C amplifier operation.

amplifier, common base A bipolar amplifierwith the base itself connected to the commonvoltage. Thus, the input signal is the emitter tocommon, with output collector to common. Thebase to common connection generally includesan impedance circuit chosen for stability.

This type of amplifier circuit has a low in-put impedance and high output impedance. Theoutput will be nearly in phase with the input.

amplifier, common collector A bipolar am-plifier with the collector connected to the com-mon voltage. The input signal is connectedto the base, while the output circuit connectsthrough the emitter. The circuit has a low outputimpedance, with higher input impedance. Theoutput is nearly in phase with the input. Of-

ten used as an impedance matching stage in amultiple-stage amplifier circuit.Seeamplifier,cascode.

amplifier, common drain An amplifier cir-cuit based on a field effect transistor, in whichthe drain is connected to the common. The inputis to the gate, while the output is to the source.Analogous to the common collector amplifier.

amplifier, common emitter A bipolar ampli-fier designed to amplify the input base to emittersignal to an output collector to emitter circuit.Since the emitter is connected to a common “lo-cal ground” voltage, the amplifier is termedcom-mon emitter.In most uses, the connection fromemitter to common includes a resistor for stabil-ity. So the signal is base to common, while theoutput is collector to common. The circuit hasa high output impedance, but a lower input im-pedance. The output will be out of phase withthe input signal by 180.

The common emitter amplifier circuit is themost common bipolar amplifier for single stageamplification. Most useful circuits, however,have multiple stages.

amplifier, DC An amplifier constructed suchthat it is capable of the amplification of a directcurrent signal (DC offset). Since this means thatthe frequency of zero is within the bandwidthof the amplifier, this is the antonym of audioamplifier.

amplifier, difference A circuit that producesan amplified output proportional to the differ-ence of two input signals. This is especially use-ful with some signal transmission lines that maypick up induced voltages due to the electromag-netic environment. If these induced voltageswill be nearly identical (common mode noise),the output will not be affected.

amplifier, feedback An operational ampli-fier circuit in which there is a return path fromthe output to one of the inputs. For resistive re-turn paths this feedback may be negative (tend-ing to mitigate the input) or positive (tending toreinforce the input).

However, in general the return path may con-tain any impedance, introducing a magnitude at-


tenuation and phase lag. For instance, an inte-grator circuit uses a capacitor in the feedbackcircuit.

amplifier, high gain An amplifier circuit de-signed with high gain, producing a substantialoutput level with a very low signal input level.

amplifier, ideal A mathematical constructfor the representation of an amplifier circuit incalculations. The input impedance is modeledas a high resistance between inputs, and the out-put voltage (with respect to ground) is a perfectlinear function of the input.

amplifier instability Variations in the re-sponse of an amplifier circuit. Instabilities arisefrom many causes including thermal effects,coupling to other parts of the circuit (crosstalk),and environmental effects. Stable amplifier cir-cuits provide reliable gain for a broad range ofoperating parameters, but must be optimized forthe intended use. For example, the space envi-ronment poses special problems for the elimina-tion of circuit instabilities.

The most pernicious types of instability arebased on the device itself, as when a capaci-tive path back through a device can allow thecircuit to meet the requirements for oscillation(seeBarkhausen criterion). This may occur ei-ther by a feedback path or by causing parametricamplification. Often remedied by changing theimpedance of the bias or input circuit paths.

amplifier, linear An amplifier that producesan output that is linearly dependent on the in-put. The region over which this linearity holds isthe usual operating range. However, amplifiersare sometimes deliberately operated outside thisrange, as in an oscillating circuit.

amplifier, logarithmic An amplifier produc-ing an output that is a logarithmic function of theinput.

amplifier, narrow band An amplifier circuitin which the gain drops appreciably for frequen-cies off the operating frequency. The width ofthe region with higher gain is narrow enough toallow the use in oscillators or filtering applica-tions. Also termed a tuned amplifier.

amplifier, negative resistance An amplifierin which the real part of the input impedanceis negative. What this means is that when anincrease in voltage is applied to the input, thecircuit will allow less positive current (or morenegative current) to flow. The simplest way toconstruct this is in a positive feedback circuit.Useful in latching and memory applications.

amplifier, nonlinear An amplifier circuit thatprovides an output that is not a linear function ofthe input. For example, a logarithmic amplifieris a type of nonlinear amplifier.

amplifier, overdriven An amplifier circuitoperated outside of the normal design range ofthe amplifier. For a linear amplifier, if the inputsignal level is above some threshold, the outputresponse will no longer be a linear function ofthe input due to the saturation of the amplifier.This operation of the circuit outside the amplifi-cation region is sometimes useful, for examplein oscillator circuits. Most often, however, anoverdriven amplifier is the cause of unintendedperformance problems.

amplifier, parametric An amplifier that re-lies on a nonlinearity and a varying parameterof a device to perform amplification. For in-stance, a variable capacitor may be the param-eter varied at some frequency (called the pumpfrequency). Suppose an input signal at some dif-ferent (possibly lower) frequency traversed thecircuit with the variable capacitor. A mixing oc-curs between the frequencies, introducing sumand difference frequencies into the circuit. Non-symmetric treatment of the sum and difference(for instance filtering the difference) may allowthe sum to be output, containing signal informa-tion and with a potentially high gain relative tothe original signal input level.

amplifier, power An amplifier device or cir-cuit designed to handle higher power levels, i.e.,higher currents, than most devices. Attention ispaid in the design to thermal management aswell as electronic characteristics.

amplifier, push-pull An amplifier circuitconstructed of two (often class B) amplifiers,one for amplifying each polarity of the input


signal. Thus the output of the push-pull circuitreproduces the entire waveform, while the op-erating voltages may be lower than needed fora single (possibly class A) circuit, with betterefficiency.

A simple push-pull amplifier.

amplifier, radio frequency An amplifier cir-cuit optimized for use in the radio frequency(RF) bands. Usually, RF amplifiers are mul-tistage devices with frequency mixing used toplace the desired signal in a shifted frequencyregion in different stages of the device. Such anintermediate frequency (IF) allows the construc-tion of more complex filtering and amplificationstages that are tuned for use at the IF. Some ra-dio amplifiers (especially those used above 30MHz) have multiple IFs.

In addition to designating the operating fre-quency range, the term RF amplifier allows dis-tinction from the rest of the receiver circuit,which may include amplifiers that handle the au-dio portion of the signal. Filtering circuits mayexist here as well, but do not in general replacethe filtering in the IF stages of the circuit.

amplifier, video A high-bandwidth ampli-fier circuit (∼ 100 MHz) designed to amplifywithout signal distortion, even in the higher fre-quencies of use. Named for its utility in videoand CRT applications.

amplitudes of waves, acoustic Maximumabsolute values in oscillations of pressure, fluidvelocity, and density in a medium, caused bypassage of a sound wave. For a plane monochro-

matic wave, these oscillations are given byξ =ξ0 exp(ik ·R−iωt). Here,ξ stands for acousticpressure, fluid velocity, or density,ξ0 is the am-plitude of the sound wave,k is the wavevector,R = (x, y, z) are the Cartesian coordinates,ωis the angular frequency, andt is time.

analog operations Procedures acting on ana-log signals that provide analog results. For in-stance, it is easy to construct a circuit to providethe analog sum of two inputs. This sum has beensynthesized from the continuous response of thecircuit, so any small change in the inputs mayprovide the correct smoothly varying output.

Other analog operations besides the analogsum include the difference, integration, differ-entiation, multiplication, etc. Analog circuitsmay be used in the construction of analog com-puters, for which digital calculations may be illsuited. Therefore the termanalog operationsisused as an antonym of digital operations.

analyzer The state of polarization of lightthat has been passed through a dichoric polar-izer can be tested by a second dichoric polarizer,which can then function as an analyzer. Whenthe transmission axis of the analyzer is orientedat 90 relative to the transmission axis of thepolarizer, the light is effectively extinguished.As the analyzer is rotated the light transmittedby the pair increases reaching a maximum whentheir transmission axes are aligned. The trans-mitted intensity is given by Malus’ Law whichstates that the irradiance for any relative angleθbetween the transmission axes is given by:

I = Io cos2 θ ,

whereIo is maximum transmitted intensity.

anamorphic system An optical system withdifferent powers (or magnifications) in differ-ent meridians. Such systems are used to correctastigmatism of the eye which arises from theuneven curvature of the cornea.

anastigmatism In an anastigmatic lens, bothastigmatism and curvature of field are corrected.Such lenses must contain negative lenses. Typ-ical anastigmatic lenses include:

1. the Celor (or Gauss) type,2. the Cooke triplet and


3. the Dager type.The Cooke triplet in particular, consists of a

negative lens at the aperture stop with two pos-itive lenses: one in front, the other in back. Ifthe last positive lens is a cemented doublet, it iscalled a Pepan lens and a Heliar if both positivelenses are cemented. The Celor type is made upof two airspaced achromatic doublets, one oneach side of the stop of the system. The Dagertype consists of two lens systems (each havingthree or more lenses) placed symmetrically withrespect to the stop.

AND A logic operation that is true if and onlyif all the inputs are true. For a binary (2 input)digital logic circuit where true is 1 and false is0, all cases are as follows:

input A 0 0 1 1input B 0 1 0 1output 0 0 0 1

Anderson bridge A bridge used to mea-sure inductance values over a wide range whileonly requiring a fixed capacitance of a moderatevalue. A schematic diagram of Anderson bridgeis shown below. The resistors are chosen suchthatR1R3 = R2Rx. Adjustments ofr andRx

are used to balance the circuit. At balance, theinductance is given by

L = C [R1R3 + (R3 +Rx) r] .

Some circuit element values that give optimumsensitivity are:

R1 = R2, R3 = 2R1, Rx∼= 2R1

∼=√

2LC

.

Andronikashvili’s experiment An experi-ment, first performed by Andronikashvili, whichmeasures the density of the normal fluid in a su-perfluid. Thin disks, closely spaced, are placedin a superfluid helium bath at the end of a tor-sional fiber. This set of disks is set into oscilla-tory motion and the period measured. Any nor-mal fluid is locked to the disks due to the viscos-ity of the normal fluid and thereby contributesto the system’s moment of inertia. As normalfluid is converted into superfluid, the moment

Anderson bridge.

of inertia decreases and the period decreases.Thus, the measurement of period provides a di-rect measurement of the density of normal fluid,ρn. See alsohelium-4, superfluid; superconduc-tivity, two-fluid model.

angle of incidence When a ray of light isincident on a surface, the angle between the in-cident ray and the surface normal is called theangle of incidence.

angle of minimum deviation If a light ray isincident on a prism’s front surface, the emergentray will be deviated. The amount of deviationwill vary with angle of incidence. For a specificvalue of the angle of incidence, the deviationwill be a minimum. When minimum deviationoccurs, the ray of light will pass symmetricallythrough the prism. Measurements of minimumdeviation angle are used to calculate the refrac-tive index,n of the prism:

n =sin [(A+ δ) /2]

sin (A/2),

whereA is the prism angle, andδ is the mini-mum deviation angle. For small prisms:

δ = A(n− 1) .

angle of polarization For a light wave goingfrom an optically rarer dielectric medium to anoptically denser medium, that value of the angleof incidence for which the angle of transmission


= 90− angle of incidence. According to theFresnel’s laws, at this angle of incidence, thecoefficient of reflection of the electromagneticwave with the electric field vector lying in theplane of incidence (containing the incident rayand the normal to the surface) vanishes. Accord-ing to Brewster’s law, the tangent of the angle ofpolarization equals the relative refractive indexn of the two media.

Angles of reflection, refraction.

angle of reflection When a ray of light is inci-dent at an interface dividing two uniform media,part of the incident ray will be reflected back.The angle of reflection is the angle between thesurface normal and the reflected ray. The angleof reflection equals the angle of incidence andthis is known as the Law of Reflection.

angle of refraction When a ray of light isincident at the surface between two uniform me-dia, the transmitted ray (also known as the re-fracted ray) remains within the plane of inci-dence and the angle between the refracted rayand the surface normal is called the angle of re-fraction.

angstrom unit Unit of length equal to 10−8

cm or 10−10 m; symbol: Å.

angular dispersion The angular separationof the wavelengths of a diffracted beam. For agrating with the spacinga and order of diffrac-tionm, the angular dispersion is given bydθ

dλ =m

a cos θm.

angular frequency For a harmonic wavewith a frequency of oscillationf (unit: Hz) andperiodT (unit: s), the angular frequencyω isgiven byω = 2πf = 2π

T . (unit: radian persecond).

angular magnification of eyepiece Ratio ofthe angle subtended by the eye (with the aid ofthe eyepiece) with the virtual image of the objectto the angle subtended by the object without theeyepiece. For an eyepiece with a focal lengthf , the angular magnification isNf for the eye

relaxed andNf + 1 for the image viewed at the

near point (nearest position of accommodation)N .

angular magnification of microscope Iffo and fe are the focal lengths of the objec-tive and the eyepiece separated by a distanced of a microscope, the angular magnification is(

Nfe

)(fe+fo−d

fo

)whereN is the near point dis-

tance (approximately 25 cm for young adults).

angular magnification of telescope If fo andfe are the focal lengths of the objective and theeyepiece of a telescope, the angular magnifica-

tion is(− fo

fe

).

aniosotropic, materials These materials ex-hibit different physical properties in different di-rections within the material.

aniosotropy (energy) Energy stored in a fer-romagnetic crystal by virtue of the work donein rotating the magnetization of a domain awayfrom the direction of easy magnetization.

anisotropic media Media in which certainproperties are different along different directions(as opposed to isotropic media in which all direc-tions are equivalent). For example, certain crys-tals have different values of elastic constants orrefractive indices along different orthogonal di-rections leading to differences in propagation ofsound or light velocities; the crystal structure orthe periodic arrangement of atoms determinesthis property. Cubic crystals (e.g., diamond)are optically isotropic, uniaxial crystals (e.g.,quartz) have two different refractive indices andbi-axial crystals (e.g., mica, topaz) have three.


anode The electrode from which positive cur-rent enters a device. This may also be seen as thepoint where electrons leave a medium or device.

anomaloscope An optical instrument that al-lows an investigator to display one solid color ina half field of view and a mixture of two comple-mentary colors in the second half field of view.The patient’s perception or lack of the similari-ties of the two fields of view allows a measureof the degree of color blindness of the patient.

anomalous dispersion The refractive index,n, of a dielectric in the wavelength regions oftransparency decreases slowly with the wave-length of light. This leads to “normal disper-sion” with the red being refracted less than theblue. However, in the vicinity of absorptionbands,n increases rapidly with increasing wave-length leading to the so called “anomalous dis-persion” (see figure). Longer wavelengths willbe refracted more than the shorter ones. Thiseffect can be demonstrated in the visible wave-lengths by observing refraction through somedyes placed in an empty glass prism. If the ab-sorption is not too high to black out the light, areversed spectrum of the visible light becomesobservable.

Anomalous dispersion.

antenna A device constructed to radiate orintercept electromagnetic energy. Sometimescalled anaerial.

antenna, aperiodic An antenna having aroughly uniform input impedance and antennapattern over a wide band of frequencies. Exam-ples include traveling-wave antennas such as theRhombic antenna. Also called a “non-resonant”antenna.Seeantenna, rhombic.

antenna array A group of individual anten-nas called elements acting in unison to provide adesired antenna pattern through constructive anddestructive interference. Arrays can be used toprovide a much higher gain than available froma single element, to shape an antenna main beamor sidelobes, and to provide a means of steeringthe antenna pattern without physically movingthe antenna.

Antenna arrays are classified by a variety ofcharacteristics. They can be classified accord-ing to configuration as linear arrays (all elementsaligned along a single line), planar arrays, circu-lar arrays, or three-dimensional arrays. Lineararrays are often classified as end-fire or broad-side, depending on whether the main beam iscentered along, or perpendicular to, the axis ofthe array. Parasitic arrays (such as the Yagi–Uda) make use of the interaction among ele-ments to provide individual element excitation,rather than using individual feeding structures.Phased arrays use precise control over the ampli-tude and phase of each element to provide elec-tronic steering of the main beam and placementof pattern nulls. Adaptive (or “smart”) arraysuse feedback information to automatically steertheir main beam toward a desired signal, whilenulling out undesired signals.

If all of the array elements are identical, andif the elements are assumed to have no inter-action between them, the antenna pattern of anarray is the product of the antenna pattern of asingle element multiplied by an “array factor”that takes into account the geometry of the arrayconfiguration. Pattern synthesis is the processof selecting the element placement to provide aprechosen pattern.

Many different types of antenna elementsmay be used to form arrays. Yagi–Uda, log-periodic, and curtain arrays are usually com-posed of dipoles. Satellite antennas often usearrays of helices, and phased array radars com-monly use slots or patches. Even large reflect-ing antennas may be used in arrays, such as thevery-large-array interferometer radio telescopethat employs 27 25-m dishes arranged in a Y-configuration of 20-km long legs.Seeantenna,Yagi; antenna, steerable.

antenna backlobe The radiation side lobe ofan antenna pattern located spatially opposite the


antenna main beam.Seeantenna beam; antennasidelobe.

antenna beam Also called aradiation lobe,aportion of the antenna pattern containing a local-ized maximum, bounded on all sides by signifi-cantly lower power density. The “main beam” isthe beam with the largest local maximum, whileall other beams are called patternsidelobes.Thesidelobe level is the ratio of the maximum side-lobe power density to the main beam maximum,and is usually expressed in dB.

Beams are often classified by their shape andtheir spatial extent (seeantenna beamwidth) andinclude “pencil beams” that are highly concen-trated along both angular directions and “fanbeams” that are narrow along one angular di-rection and wide along the other.

antenna beamwidth A measure of the angu-lar extent of an antenna beam in a chosen cross-sectional plane, usually given in degrees. Sev-eral methods exist for describing beamwidth.Most often it is taken as the angle between theadjacent 3-dB (half-power) points on the powerpattern, but it is also described in terms of the10-dB or 20-dB points. For patterns with manysidelobes, the beamwidth is sometimes taken asthe angular width between nulls (zeroes) adja-cent to the beam.

In general, the narrower the antenna mainbeam, the higher the gain and the directivity.Seeantenna pattern; antenna beam; antenna gain;antenna directivity.

antenna, dipole An antenna consisting oftwo segments called “legs”, generally madefrom straight wires and of equal lengthl, andfed from the center by a two-wire transmissionline. A dipole may be used either alone or inarrays such as log-periodic and Yagi-Uda, andas such is the most commonly used antenna el-ement.

The pattern of a dipole depends on the lengthof the dipole legs as a fraction of the operatingwavelength,l/λ, and the input impedance de-pends on bothl/λ and the radius of the wires,a/λ. Dipoles are usually operated in the reso-nant mode, withl/λ slightly less than 1/4 fora/λ << 1. In this case the input impedance isreal and approximately72 Ω and the pattern is

very nearly a sine of the angle measured fromthe antenna axis.Seeantenna, half-wave.

Coaxial cable can also be used to feed adipole antenna if an appropriate “balun” is used.Seeantenna feed system.

A monopole antenna operating above aground plane (or approximately so, in the caseof a monopole above the earth) acts as a dipolethat radiates one half the power and has one halfthe input impedance of a dipole in empty space.

antenna directivity Symbol:D. A dimen-sionless parameter, greater than or equal to unity,describing the effectiveness of a transmittingantenna in concentrating its radiation intensityalong a certain direction. Expressed as

D(θ, φ) =U(θ, φ)

W4π

whereU(θ, φ) is the radiation intensity (powerradiated per unit solid angle) andW is the totalradiated power. Thus directivity relates the ra-diation intensity along a particular direction tothe radiated power averaged over the total solidangle4π (the radiation intensity of an isotropicradiator). Often the word directivity is used todescribe the maximum value ofD(θ, φ) over allangles.

Directivity is related to antenna gain and an-tenna effective area.Seeantenna effective area.

antenna effective area Also calledeffectiveaperture,a parameter with dimensions of m2

describing the ability of a receiving antenna tocapture the power carried by the wave impingingon it. Specifically

Ae =PL

Wi

wherePL is the power inW delivered to theload attached to the antenna,Wi is the powerdensity of the impinging wave inW /m2 andAe

is the effective area. Thus, a largerAe impliesa greater ability to intercept power.

Effective area depends on several variables,including the orientation and efficiency of theantenna, the polarization of the impinging wave,and the value of the load. The effective area ismaximized when the orientation of the antennais polarization matched to the impinging wave,


the load is conjugate matched to the antenna in-put impedance, and the antenna is lossless. Themaximum effective area of an antenna acting as areceiver,Aem, is related to the maximum direc-tivity of the same antenna acting as a transmitter,Dm, by

Aem =λ2

4πDm .

Although the effective area of aperture-typeantennas (such as horns and reflector antennas)is often of the same order as the physical antennaaperture area, there is little correlation betweenthe effective area of wire-type antennas (suchas dipoles and loops) and the physical area theypresent to the impinging wave.

antenna efficiency A dimensionless param-eter, less than or equal to unity, describing theability of a transmitting antenna to convert inputpower into radiated power. Defined through

e =Pr

Pin

wherePr is the radiated power andPin is theinput power to the antenna. The radiated powerwill be less than the input power for antennasexhibiting conductor or dielectric loss, so thatPin = Pr + Pl wherePl is the power loss.

Often the impedance mismatch between thefeeding line and the antenna is included as aloss mechanism and the mismatch efficiency isexpressed as

em = 1− |Γ|2

so that the total antenna efficiency is given byet = e · em.

antenna feed system The physical connec-tion between an antenna and the transmissionline or waveguide supplying or drawing power.Often the feed is assumed to include all or partof the transmission line.

Antenna feed structures are as varied as theantennas they are designed to feed. A feed struc-ture is usually constructed to match the inputimpedance of the antenna with the characteris-tic impedance of the feeding transmission line.It may also be constructed to provide a transitionbetween a balanced antenna (such as a dipole)

and an unbalanced transmission line (such ascoaxial cable). In this case it is called abalun.

For a reflector antenna, the antenna feedrefers to the primary radiator and its own as-sociated feeding system.

antenna gain Symbol: G. Antenna direc-tivity corrected for efficiency. A dimensionlessquantity, usually expressed in dB, correspond-ing to the ratio of the radiation intensity in a cer-tain direction to the input power averaged overthe total solid angle4π. Expressed as

G(θ, φ) = etD(θ, φ) .

The word gain is sometimes used to describe themaximum value ofG(θ, φ) over all angles.

antenna, half-wave A dipole antenna of totallength2l equal to one half wavelength. A verythin half-wave dipole has a sinusoidal currentdistribution, a maximum directivity of 1.64, andan input impedance of73+j42.5 Ω. A very thindipole antenna can be made resonant by reduc-ing its length slightly, tol/λ = 0.24, producingan input impedance of approximately70+j0 Ω.For a dipole of larger diameter, the length mustbe reduced more, and the resulting resonant in-put impedance is also reduced.Seeantenna,dipole.

antenna, horn A class of high-gain apertureantennas used extensively in the microwave andmillimeter-wave bands. The simplest horn an-tennas are constructed by flaring out the mouthof a circular or rectangular waveguide. Theflare geometry controls the pattern shape andthus the antenna gain. Rectangular horns areclassified as sectoral E-plane, sectoral H-plane,and pyramidal, depending on whether the flare isalong the wide side, narrow side, or both sides ofthe guide, respectively. A typical standard-gainpyramidal horn has a gain of about 20 dB.

Several modifications can be made to the sim-ple horn geometry to improve gain, sidelobelevel, bandwidth, and polarization characteris-tics, resulting in such types as the ridged horn,the corrugated horn, the aperture-matched horn,and the TEM horn.

Horn antennas are commonly used as singleelements or in arrays, and as feeds for reflec-tor antennas such as microwave satellite dishes.


Other important applications include use as astandard for calibrating other high-gain anten-nas and as a radio-astronomy telescope.

antenna, isotropic A hypothetical antennathat radiates uniformly in all directions. Anisotropic antenna cannot exist physically, but isuseful for defining various antenna propertiessuch as directivity and gain. The directivity ofan isotropic radiator is unity and its gain is 0 dB.Seeantenna directivity; antenna gain.

antenna, lens An antenna combining a pri-mary radiating element or array with a converg-ing lens to produce increased gain. The lensacts much the same as the reflector of a dish-type antenna, collimating the radiation from theprimary radiator and narrowing its main beam.

The primary element of a lens antenna isoften a low-gain antenna such as a dipole,slot, or small horn. The lens may be con-structed from dielectric material, as with anoptical lens, or from artificial dielectrics com-posed of small conducting objects imbedded infoam. For lower frequencies the weight of thelens becomes a significant factor and alternativelens designs using stacked metal plates or wiremeshes may be more appropriate.

Lens antennas are classified both by the typeof material used to construct the lens, and by thegeometry of the lens. Important lenses catego-rized by shape include the Luneberg lens, whichis spherical with a radial grading of the dielectricconstant, and the Schmidt lens, which is used tocorrect aberrations in spherical reflector anten-nas.

antenna, loop An antenna consisting of oneor more turns of wire, often contained in a plane,formed into typically a circular, square, or rect-angular shape. Small loop antennas are thosewhose perimeters (number of turns times cir-cumference) are generally less than a tenth of awavelength. These have sinusoidal antenna pat-terns regardless of the shape of the loop, witha sharp null on the loop axis useful for direc-tion finding, station nulling, and radiowave nav-igation. Small loop antennas have low antennaefficiency due to large resistive losses, strongmismatches due to a highly inductive input im-pedance, and low input resistance. The input

resistance can be enhanced by increasing thenumber of turns or by winding the antenna on aferrite core, forming the “loop-stick” antennascommonly found in AM radio receivers. Smallloops are also used for probing near-zone fieldsand currents.

Loops can also be made of resonant size, withperimeters approximately one wavelength in ex-tent. A resonant square loop has an input im-pedance of about100 Ω, and is often used byradio amateurs in Yagi–Uda arrays calledcubi-cal quads.

antenna, paraboloid A reflector antennain which the main reflector is paraboloidal(parabolic surface of revolution). These anten-nas are often used for radio astronomy becauseof their narrow main beams, and are used as the“dish” antennas in many home satellite televi-sion systems.Seeantenna, reflector.

antenna pattern The angular variation ofthe radiation-zone electromagnetic field of anantenna, generally expressed in terms of thespherical-coordinate variablesθ andφ. Usu-ally plotted in polar coordinates and normalizedto the maximum value using either natural unitsor dB. Either the field magnitude (field pattern)or the power density (power pattern) may beplotted, with power density proportional to thesquare of the field magnitude.

Occasionally, the spatial variation of thenear-zone field is considered. Describing thisusing a near-zone antenna pattern is complicatedby the need to include distance as an additionalparameter.

antenna, reflector Antenna combining a pri-mary radiating element or array with a large con-ducting surface to produce increased gain. Thereflector acts much as the mirror in an opticaltelescope, collimating the radiation from the pri-mary source and decreasing the width of its mainbeam. It may be constructed from solid metalplating, or from perforated plating or wire meshto reduce weight. The primary source is usu-ally a low-gain antenna such as a dipole, slot,or small horn placed at the focal point of thereflector. More complicated primaries such ascircular corrugated horns and dielectric conical


antennas provide more efficient illumination ofthe reflector.

Reflector antennas are often categorized ac-cording to the reflector shape, and include planarreflectors, corner reflectors, spherical reflectors,parabolic cylinder reflectors, and paraboloidalreflectors.

Shaping of the reflecting surface can improvethe performance of a reflecting antenna by in-creasing its gain or decreasing its sidelobes. Adual-reflector Cassegrain antenna allows for fur-ther improvements by providing shaping on twosurfaces, but only at the cost of increased “aper-ture blockage” — the tendency of the feed toblock a portion of the impinging wave.

antenna, rhombic A traveling-wave wire an-tenna used mostly at medium and short-wavefrequencies, consisting of four straight wire legsplaced parallel to the ground in the form of arhombus. One narrow apex of the rhombus isfed by a two-wire transmission line while theopposite is terminated by a resistance chosen invalue (typically600 − 800 Ω) to eliminate re-flections of the traveling waves generated at thefeed. When the legs are several wavelengthslong a highly directional pattern is produced,with the main beam aligned along the rhombus,and elevated at an angle to the ground due to theground effect. The elevation angle allows the re-ception of sky waves reflected by the ionosphere.Rhombic antennas typically have two large side-lobes adjacent to the main beam, and may have abacklobe produced by traveling waves reflectedfrom an imperfect apex termination.

antenna sidelobe Any antenna beam that isnot the main beam of the antenna pattern.Seeantenna beam.

antenna, steerable A highly directive an-tenna with a main beam direction that canbe changed either mechanically or electroni-cally. A reflector antenna is often mounted ona gimble, allowing it to be mechanically ro-tated through azimuth and elevation. A reflec-tor antenna may also be steered by moving itsprimary feed. The 305-m reflector antenna inArecibo, Puerto Rico is mounted in a naturalvalley, and steered by moving a feed suspendedfrom three towers. In contrast, a phased array

antenna is steered by precisely controlling thephase of the signal supplied to a fixed-positionarray elements such as patches. This type ofarray is commonly used in radar applications,where rapid scanning of the main beam is re-quired.

antenna subreflector The smaller reflectingsurface in a dual reflector antenna system, suchas the Cassegrain.Seeantenna, reflector.

antenna, Yagi An antenna array consistingof one driven element and several parasitic ele-ments, arranged in a linear configuration to pro-duce a main beam aligned with the antenna axis.Also called aYagi–Udaarray after the two in-ventors, Hidetsugu Yagi and Shintaro Uda, whodeveloped the array in the 1920s. Usually con-sisting of dipole elements, Yagi–Uda arrays mayalso be constructed from loops or other simpleelements. These easily constructed arrays areused heavily in the short-wave bands up throughthe low microwave bands.

Dipole Yagi–Uda arrays consist of severalparallel elements, including a single resonantdipole (called the “driver”) fed by a two-wiretransmission line. A single parasitic (short-circuited, undriven) element, called a “reflec-tor”, is located behind the driver and has a lengthslightly greater than the driver. Several parasiticelements called “directors” are located in frontof the driven element.

The Yagi–Uda array works on the principle ofcurrent induction. Through very careful choiceof length and placement of the parasitic ele-ments, the driver induces the proper current tocreate constructive interference of the individ-ual element patterns along the antenna axis. Thegain of the array increases with the number ofdirectors used, but little additional gain is real-ized beyond the 11 dB of a five-director Yagi.Beamwidth and sidelobe levels are also impor-tant design considerations for Yagi–Uda arrays.

Seeantenna array; antenna, dipole.

antiferromagnetism This is a weak mag-netism similar to paramagnetism insofar as it ischaracterized by a small positive susceptibility.

anti-jamming Anti-jamming refers either tothe capacity of a device (typically a radar, navi-


gation guidance system, or communication sys-tem) to resist jamming without critical deteri-oration in its effectiveness, or to measures un-dertaken by the device to mitigate the effectsof such jamming. One common anti-jammingmeasure is the broadening of the information-bearing signal’s spectrum using various spreadspectrum modulation techniques.

antinode That portion of a standing wavewhere its amplitude is maximal. Antinode isalso called a loop.See alsonode.

anti-reflecting films Thin layers of dielec-tric films deposited on a material so that light ofdesired wavelengths is not reflected due to de-structive interference. One application is to min-imize reflected glare from window panes andglass covering paintings and photographs. Aperfectly anti-reflecting film for light of vac-uum wavelengthλ should have a thickness of(λ/4n1) and satisfyn1 =

√n0 ns wheren1, n0

andns are the refractive indices of the film, theincident medium (air, in most cases) and the sub-strate material, respectively. Multiple layers ofmaterials of high and low refractive indices in aquarter-wavelength-thick stack can also achieveanti-reflecting properties over a broad range ofwavelengths. For a three-layer stack with ma-terials of refractive indicesn1, n2 andn3 on asubstratens, the condition for anti-reflectanceis n1 n3

n2=√n0 ns.

antiresonance A regime of an acoustic sys-tem consisting of two or more parts with inter-action between each other when the effectiveimpedanceof the system is very high (in limit,infinite). An example of such a system is a mem-brane vibrating in the water.

anti-Stokes lines The inelastic scatteringof light by matter leads to frequencies thatare higher than those of the incident photonscalled anti-Stokes lines (in contrast to the lower-frequency Stokes lines and elastically scatteredRaleigh lines at the same frequency). IffM areinternal excitation frequencies of a material andfI the incident photon frequency, then the anti-Stokes frequencies arefanti-Stokes = fI + fM .If fM correspond to sound waves or acousticfrequencies, the scattering is namedBrillouin

scattering;for vibrational, rotational and elec-tronic excitations in molecules or crystals thephenomenon is calledRaman scattering. Seescattering, Brillouin; scattering, Raman.

aperiodic vibrations Vibrations withoutrepetitive pattern. This term is used as oppositeto that of periodic vibrations. An example ofaperiodic vibrations is random vibrations, whenamplitude, phase and/or frequency of vibrationsare changed randomly.

aperture The diameter (usually measured ininches) of the opening or objective of the tele-scope, camera, etc. that determines the amountof light ultimately reaching the image. Some-times, the aperture is quantified as the angle be-tween the lines from the opposing ends of a di-ameter of the objective to the principal focus.

aperture acoustic (1) Surface that effectivelyradiates sound. An example of acoustic aper-ture is the mouth of a horn. An area occupiedby anarray of acoustic sourcesis often calledan acoustic aperture too.

(2) An opening in a screen through whichsound waves can propagate from a source lo-cated behind the screen.

aperture ratio Determines the light passingpower of a lens for a non-parallel beam from anearby object. Its value is given by2n sin(θ)whereθ is the angle between the lens axis andthat emergent ray in the image space, whichstarts from an axial object point and passesthrough the rim of the lens.

apertures, complementary Seecomplemen-tary apertures.

aplanat A lens or an optical system that isfree of both spherical aberration and coma. Sucha system satisfies theAbbe’s sine condition.Alens made of a dielectric of refractive indexnwith the radii of curvaturer1 andr2 can havean image free of spherical aberration and comaif the object is located at a distancer1. Theimage that is virtual will be at a distancenr2should the radii of curvature satisfy the conditionr1 =

(n+1

n

)r2. Such a lens is called aplanatic

and the object and image points are calledapla-


natic points. Aplanatic lens systems made ofimmersion oil and index-matched thick plano-convex lens are used next to the objective lensof microscopes.

aplanatic achromatic doublets Two lensescemented together to correct for spherical aber-ration, coma and chromatic aberration.Seeaplanat; aberration, chromatic.

aplanatic points The two conjugate pointsof anaplanat. Seeaplanat.

aplanatic refraction Refraction at a spheri-cal interface in whichthe Abbe’s sine conditionis satisfied for incident and refracted rays.

apochromatic correction Seeaberration,chromatic.

apodization Literally, to remove the feet.Any process in which the aperture function isaltered to produce a redistribution of the en-ergy in the diffraction pattern. It is usually em-ployed to reduce the secondary diffraction max-ima. This procedure is accomplished by alteringthe aperture with suitable masks so that the re-sultant diffraction pattern has reduced secondarymaxima resulting in cleaner images as shown inthe figure below. The diffraction pattern due toa grating without a mask (a) and with a stan-dard mask (b) is shown in (c) in solid and dot-ted curves, respectively. The intensity scale isdrawn in the log scale to visualize the secondarymaxima that are a few percent of the principalmaximum without the mask. It is clear thatthey are substantially reduced in the apodizedspectrum. The principal maximum suffers somebroadening.

Apodization can also be done after the obser-vation by a mathematical operation. The mea-sured diffraction pattern of the aperture is con-voluted with the Fourier transform of a suit-able mask. InFourier transform spectroscopy,apodizationis performed by multiplying thein-terferogramwith the mask function and obtain-ing the spectra by a Fourier transform.

apparent size The size of the retinal imageof an object. It is proportional to the angle that

(a) unmasked grating, (b) grating with a mask, and

(c) the secondary maxima without mask (solid curve)

and with mask (dotted curve).

the object subtends at the first principal point ofthe optical system.

applique An auxiliary circuit equipment ap-pended onto a standing communication systemto offer substitute or supplementary utility.

arithmetic operations Operations that treatthe inputs or operands as numbers. In digitalcircuits, binary numbers can be added and sub-tracted. It is also possible to multiply the binary-represented numbers, in addition to division, etc.These operations are arithmetic.See alsologicoperations.

array of acoustic sources A number ofacoustic sources coupled together. Radiationpattern of an array of acoustic sources can bequite different from that of an individual source,e.g., the former can be much narrower than thelatter. This property of an array of acousticsources is often used in forming a narrow ra-diation beam.

Arrhenius equation k = Ae−Eact/RT wherek is the rate constant,A is the Arrhenius factor (aconstant for a given system),e is the exponentialfunction,R is the universal gas constant (8.315J/mole-K),T is the absolute temperature of thesystem, andEact is the activation energy of thereaction.Seeactivation energy.

Arrhenius plot A generalized plot of anyphysical parameter that is a function of tempera-ture. Following the form of the Arrhenius equa-tion (seeArrhenius equation), one makes a plotof the physical parameter, say for example, cor-


relation timeτc, vs. 1/T (the reciprocal of theabsolute temperature). One often observes thedata follow a straight line on such a plot, theslope of which is the activation energy for thatprocess. Another utility of such a plot is thatwhere phase transitions occur in the system, thephase transition is observed as an abrupt changein slope at the temperature of the phase transi-tion.

Arrhenius plot.

arteriography The use in the bloodstream ofa dye opaque to x-ray radiation (for example, io-dine) which allows a screen display (fluorescentor computer) of artery systems exposed to x-rayradiation. Sometimes called x-ray imaging orangiography.

artificial inductor A circuit designed to syn-thesize the effects of an inductor without havinga physical inductor coil. This may be an activedevice or a passive network device. If the effec-tive admittance approximatesY '

√−1

ωLeffectivefor

any simulated inductanceLeffective near the an-gular frequency of operationω, the device canbe used in place of an inductor. Often usefulin integrated circuits, in which capacitor and re-sistor elements are easy to construct in smallareas using planar processing techniques, whileinductors are not.Seeadmittance.

artificial reactor A device constructed tomimic the complex admittance desired in a cir-cuit. For instance, examine the possibility ofan artificial capacitor without having a physical

capacitor in the circuit. The admittanceY willbe nearly completely imaginary, and scale lin-early with operating frequency. The effectivecapacitanceCeffective can be found from the re-lationshipY =

√−1ωCeffective. The artificial

capacitor may be constructed of an active cir-cuit, or may be a passive network design, andcan be used in many circuits in place of a capac-itor. Of course, capacitors are among the easiestof elements to construct in the normal ways, butthe example shows the possibility of tuning theamplitude and phase response as a function offrequency for any desired impedance.See alsoartificial inductor; admittance.

aspect ratio In images, the aspect ratio is theratio of one dimension of the total image (say,horizontal) with the other (in this case, the ver-tical). For video image reconstruction or anyCRT process, the aspect ratio should be tuned tothe design value so the images appear realistic.It is well known that current television designhas a different aspect ratio for the viewed imagethan that used in motion picture applications.Now the aspect ratio for high-definition televi-sion (HDTV) is different than that for currentvideo as well.

aspherical mirrors Paraboloids, ellipsoidsand hyperboloids which produce perfect imagesbetween a pair of conjugate points correspond-ing to their two foci.

association constant When macromoleculessuch as proteins self-assemble by thermody-namic means, i.e., due to a minimum conditionin free energy provided by hydrogen bonds be-tween the macromolecular assemblies, then theself-assembly reaction constant is referred to astheassociation constant.

association kinetics When macromoleculesself-assemble in response to the kinetic pathwayof the reactants, the final association is not de-pendent solely on the minimum in free energydue to nearest neighbor interactions; rather thefinal association is dependent on the reaction ki-netics of the individual processes making up theself-assembly. The study of these reaction pro-cesses is referred to asassociation kinetics.


τ c P

1/T

astigmatic surface The effect of astigmatismis that the rays of a narrow oblique bundle, in-stead of being brought to a focus at a single point,pass through two small focal lines at right anglesto the path of the chief ray in the image space. Ifthe chief rays proceeding from the various objectpoints lying in the meridional plane of a sym-metrical optical instrument are constructed, andif along each of these rays, the positions of theimage points of the pencils of the meridian andthe sagittal rays are determined, the loci of thesepoints will be two curved lines, both symmetricwith respect to the axis, which touch each otherat a common vertex on the axis. These curvedlines are the traces in the meridional plane of thetwo astigmatic image surfaces generated by re-volving the traces around the axis of symmetry.The focal lines of a narrow pencil of meridionalrays lie on one surface, and the sagittal rays arefocused on the other surface.

astigmatism When an object point is awayfrom the optical axis of a lens or a mirror by aconsiderable distance, the cone of incident raysare asymmetric with respect to the optical sys-tem leading to the aberration calledastigmatism.The image of a point object results in two mutu-ally perpendicular line images displaced fromone another. Rays in the vertical (or merid-ional) plane and in the horizontal (or sagittal)plane lead to these two images. The image isdisc shaped at some intermediate point calledthecircle of least confusion.Projection systemsand photographic enlargers suffer from this de-fect due to the closeness of the lens from objectsover a large area. Two or three lens systems thatcorrect for this defect also flatten the curvatureof the field.Seecurvature of field.

astigmatism of the eye In the case of vision,astigmatism occurs in the eye because the corneais not a perfect sphere, and hence there is a con-tribution due to additional cylindrical curvature.This could occur due to oblique incidence oflight on the cornea or lens. If there is an astig-matic refractive error, rays of light from a singlepoint object are focused as 2 line images at dif-ferent distances from the system, at right anglesto each other. This is due to different refractionof the incident light by the dioptric system in

different meridians. Astigmatism of the eye isin general classified as:

1. against-the-rule: Astigmatism in whichthe meridian of greatest refractive power of theeye is in or within 30 of the horizontal.

2. with-the-rule: Astigmatism in which themeridian of greatest refractive power is in orwithin 30 of the vertical.

3. irregular: Astigmatism in which the twoprincipal meridians of the eye are not at rightangles to each other.

astigmatism, radial This is a monochro-matic aberration of a spherical lens. For an op-tical system imaging an off-axis point, the chiefray (or principal ray) will go from object pointthrough the center of aperture of the system. Theplane perpendicular to the plane containing thechief ray (the tangential plane) is called the sagit-tal or radius plane. When evaluating the imageat the tangential conjugate, there will be a line inthe sagittal direction. A line in the tangential di-rection will be formed at the sagittal conjugate.In between these conjugates, the image will beeither elliptical or circular. The separation ofthese conjugates is called radial astigmatism.

atmospheric acoustics The discipline thatdeals with sound radiation, propagation, andscattering in the atmosphere, and use of soundwaves for the remote sensing of the atmosphere.There are several factors that can simultaneouslyaffect a sound wave in the atmosphere: absorp-tion of sound in air, interaction of a sound wavewith the ground, temperature and wind velocitystratification resulting in refraction of a soundwave, scattering of sound by atmospheric turbu-lence, terrain and different obstacles such as bar-riers, houses, etc. Among modern concerns ofatmospheric acoustics are studies of noise prop-agation from highways, factories, airports, andsupersonic aircrafts; source detection, rangingand recognition by means of acoustical systems;acoustic remote sensing of the atmosphere, etc.

atmospheric duct A layer of the earth’s at-mosphere that traps and guides electromagneticwaves by reflection and refraction. Electro-magnetic waves propagating in the earth’s tro-posphere normally bend concave down towardthe earth due to the negative gradient in the re-


fractive index of the atmosphere with increasingheight. When the refractive index of the atmo-sphere changes rapidly, or in a discontinuousfashion, the waves are in essence reflected fromthat region of the atmosphere. A “groundbase”duct occurs when the reflecting layer is close tothe earth so that the reflected waves bounce re-peatedly between the layer and the earth. An“elevated” duct occurs when the reflecting layeris high above the earth, and the reflected wavesare bent back upward before reaching the earth’ssurface. In a fashion similar to metallic wave-guides, cut-off frequencies may be computed forvarious ducted modes, giving the lowest possi-ble frequency of a ducted wave. For a typicalduct a few hundred meters thick, waves withfrequencies in the VHF range and higher are al-lowed, while waves with lower frequencies arecut off.

Atmospheric ducting is often associated withtemperature inversions. Bounding layers mayextend for over 1800 km along stationaryweather fronts. Ducting over water has providedVHF communication distances of over 4500 km.

attenuation, acoustic A decrease in ampli-tude and intensity of a sound wave in its reflec-tion from a surface or in its propagation throughgaseous, fluid or solid media. This decreaseis caused by bothabsorptionof a sound waveand its scattering due to inhomogeneities on thesurface or in a medium. For propagation of asound wave in a medium, its attenuation is de-scribed by the formulaA = A0e

−δx, which issimilar but not identical to that for absorption ofsound. Here,A andA0 are the amplitudes ofsound pressure at two fixed points,x is a lengthof a sound path between these points, andδ isthe extinction coefficient which is a sum of theabsorption coefficient and the scattering coeffi-cient.

attenuation coefficient When a plane wavetravels through a medium, the intensity of thewave will drop exponentially as a function ofdistance traveled,x, e−αx, due to the absorptionof the medium. The coefficientα is called theattenuation coefficient.

attenuation constant A constant that de-scribes the exponential decrease of the intensity

of anEM wave traveling through a medium dueto absorption.

attenuator A device designed to attenuatethe input signal without distorting the waveform.It can be an electric circuit to attenuate the elec-tric signal, or it can be an absorbing material toattenuate the optical input.

audibility, limits of Frequency and intensityranges in which a sound can be heard by ear. Ahuman ear is able to detect sound in the range15 Hz to 20,000 Hz. At a given frequency inthis range, sound can be heard if its intensity isabove the threshold of audibility and below thethreshold of feeling. Both thresholds depend onfrequency and other factors, for example, an ageof a person. A sound with intensities above thethreshold of feeling cause pain and may causetrauma.

audio frequency Sound frequency that canbe heard by ear. Audio frequency is in the range15 Hz to20,000 Hz.

audiogram A plot of hearing loss as a func-tion of frequency for the audible range. Audio-grams are widely used inaudiometry.

audiometry A procedure of investigating im-paired hearing. Both pure tones and speech areused in audiometry to measure the threshold ofaudibility of a person. As a result, the hearingloss is determined as a ratio (in decibels) of themeasured threshold to that of the normal ear. Ifpure tones are used in audiometry, it is custom-ary to plot the hearing loss vs. frequency for theaudible range. Such a plot is calledaudiogram.

audiometry, bone conduction Bone con-duction is the process of conducting acousticsignals to the inner ear through the cranial bonesrather than through the ear canal. Audiometryprobed with an oscillator placed on the foreheador head to produce a sound response in the au-ditory nerve is referred to as bone conductionaudiometry. This technique is considered to beproblematical for lateralization effect (does notisolate one ear) and masking. Oscillator place-ment strongly affects the results.


audiometry, brain-stem electric responseAudiometry detected not by the patient relayingwhether or not they heard the response; rather,by detection of the response via electrodes im-planted in the auditory nerve or in the brainstem.

audition limits The same asaudibility, limitsof.

auditorium acoustics See acoustics ofrooms.

aurora borealis Also known asnorthernlights; This phenomenon of shimmering lightsin the northern skies is due to the charged parti-cles of the solar wind that stream through spaceand ionize air molecules in the upper atmospherewhich in turn emit light. The magnetic fieldlines of the earth trap the charged particles. Theelectrons and protons swirl around the field linesand proceed towards the poles due to the Lorentzforce, which is normal to both their velocity andthe magnetic field direction. The colors in theauroras are due to the emission of excited oxy-gen atoms in the red (630 nm) and green (558nm) spectrums. Excited nitrogen atoms emit anumber of lines between 391 and 470 nm, and650 and 680 nm. A number of atmospheric fac-tors on earth and the solar wind lead to a varietyof color displays.

autocollimator Any optical system with theproperty that the incident parallel light emergesas parallel light but is travelling in the oppositedirection. This is accomplished, for example, inthe Kellner-Schmidt optical system by placinga small-aperture convex mirror at the focus ofa large concave mirror, both mirrors having thesame center of curvature. Parallel light entersthe system through a correcting lens (to correctspherical aberrations) placed at the center of cur-vature and, after double reflection, emerges inthe opposite direction. For an autocollimatingeyepiece,seeeyepiece, Gaussian.

automatic bias The bias voltage of an ampli-fier element produced by voltage differences dueto the device current. This may be accomplishedby placing resistors in the circuit, so when thecurrent flows, the voltage drop across the resis-tor is sufficient to bias the device elements.

Kellner-Schmidt optical system.

For example, in a common emitter amplifier,instead of providing two voltages for the correctbias of both junctions, a resistor from the base tocommon will carry most of the collector current,providing the voltage difference (the automaticbias) needed for the base-emitter junction. Thiscircuit can be made to work with only one ap-plied voltage.

automatic frequency control The techniqueof automatically adjusting the local oscillator orintermediate frequency to compensate for fre-quency shifts in the input signal. Often used inradio receivers. A by-product (in FM reception)is that the control voltage for the adjustment canbe the amplitude of the demodulated signal.

automatic gain control (AGC) The tech-nique of automatically adjusting the amplifiercircuit gain to give a constant output level. Thuswhen signal strength diminishes (as may be thecase in radio reception), the amplitude of the out-put (or the volume) will not change dramatically.In actual AGC circuits, the output is allowed tochange slightly with signal variation to facilitatetuning.

automatic volume control (AVC) Automaticadjusting of audio amplifier gain to give a con-stant volume level.Seeautomatic gain control.

autoradiography Visualization of the spa-tial distribution and concentration of tissue ra-dioactivity. Usually detected by placing the tis-sue (human body) in close proximity to photo-


graphic film and upon developing the film thedistribution of tissue radioactivity is recorded.

auxiliary channel An auxiliary channelrefers to a channel adjunct to the main transmis-sion channel but with a transmission directionindependent of that of the main channel.

Avrami equation Used as a model for crystalgrowth rate as a function of the density of thecrystalline and the melt phase(m) with respectto time. Generally expressed in two equivalentforms:m = moe

−ktn

or,y = 1−m/mo = 1− e−ktn

.Wherem is melt mass of the crystal,y is

the fraction of crystallized crystal,k is the rateconstant,t is the time, andn is an integer.

axial modes, of laser cavity Also knownas longitudinal modes.These are the resonant

modes at which a laser can oscillate. IfL isthe length of the laser cavity resonator, the ax-ial mode frequencies arefm = m

(c

2L

)where

m is an integer (usually very large) andc is thespeed of light. The separation between succes-sive modes is the free spectral range of the res-onator and a typical laser transition line is broadenough to accommodate several modes. A sin-gle axial mode can be sustained at the expenseof others by inserting an etalon of appropriatelength in the cavity.

axicon A refracting element that can imagea point as an axial line. A common form ofan axicon is a refractor in the shape of a plano-convex shallow cone. Axicons are used in autocollimators and alignment telescopes to detectmisalignment of illuminated point objects.


BBabinet’s compensator Also known asBabinet-Soleil compensator. A device that canproduce a continuous change in phase retarda-tion between two orthogonally polarized beamsof light. In this device, two quartz wedges —the slow direction in one being perpendicularto the slow direction in other — can be gradu-ally slid against one another. The light passingthrough such a device suffers a retardation thatis proportional to the distances in each wedge.

Babinet’s principle A principle that relatesthe fields diffracted by a screen with anaper-ture and by the complementary screen. Babi-net’s principle states that the sum of these fieldsis equal to a field that would be in the absenceof the screen. The complementary screen isformed from the original one by replacement ofall its transparent parts by opaque parts, and byreplacement of all opaque parts by transparentones.

back emf The electromotive force (emf) gen-erated in an AC electric motor that is out ofphase with the initial applied voltage of the mo-tor. Back emf is due to Lens law; namely, achange in the magnetic flux inside a closed loopwill induce an emf to oppose the change in mag-netic flux.

back focal length This is the distance fromthe back (or secondary) vertex to the secondary(or rear) focal point.

backlash The hysteresis inherent in a device(such as a tuning element) will cause slightlydifferent dial readings for the same operationdepending on the direction of travel. The mag-nitude of the difference is termed thebacklash.

backscatter Electromagnetic waves propa-gating in a direction directly opposite their di-rection of origination. In radar, the backscatter

signal is the reflected wave returned to the send-ing antenna.

backward channel A backward channeltransmits supervisory control signals, error-control signals or acknowledgment signals in adirection reverse of the instantaneous directionof the information signal in the forward channel.

backwave Wave propagating in the backwarddirection.Seebackscatter.

baffle Surface that is an extension of the di-aphragm of a loudspeaker or anapertureof asource. Baffles are used to increase the poweroutput of an acoustic source and to change itsradiation pattern, especially at low frequencies.For example, a vibrating circular disk radiatesprimarily as a dipole if its radius is less than thewavelength of the emitted sound. The same diskbaffled (surrounded) by a large surface radiatesprimarily as a monopole and with much greateroutput.

balance, amplitude/phase In splitter or cou-pler circuits (especially in microwave devices),the balance is a measure of the symmetry of thesignal or power division process. The ampli-tude balance (usually specified in decibels) ap-proaches 0 dB if the signal strength is identicalin the two output paths. The phase balance (of-ten stated in radians or degrees) is zero if thereis no phase lag between the two outputs.

balance, bridge A bridge is a circuit de-signed to make some measurement by setting upa series/parallel connection. Imagine a squarecircuit, with each side of the square containingcomponents, one of unknown admittance, theothers known, some of which can be tuned. Adriving circuit is connected across one of thediagonals, and the current (or voltage) is mea-sured across the remaining diagonal. The sidesare tuned until there is no current flow (or volt-age difference) measured, allowing calculationof the unknown admittance.

The process of nulling the measured quantityacross the bridge is called balancing.

Balmer lines/series; band These are spec-tral lines observed in the emission or absorp-


tion spectra of hydrogen in the visible and nearultraviolet. They correspond to transitions be-tweenn = 2 andn = 3, 4, 5 . . .∞ energy lev-els. The first three lines of the Balmer seriesareHα,Hβ , andHγ lines at wavelengths of6562.79 Å, 4861.33 Å and 4340.47 Å, respec-tively. Transitions to higher levels get closerwith increasingn and the continuum (forn =∞) appears as a band.See alsohydrogen spec-tra.

band analyzer A device that allows mea-surement of amplitudes or intensities of a com-plex sound in many contiguous frequency bands.Many acoustic signals that are dealt with in prac-tice and technique have complicated spectra.Band analyzers are widely used to measure thesespectra.

band gap In semiconductors, the electronicenergy states consist of completely filled bandsof energies (valence band) followed by unoccu-pied bands (conduction band). The separationin energy is called theband gap. Seeabsorptionedge.

band pass filter A device that can transmita narrow band of frequencies of light while re-flecting or absorbing the rest. Usually it is con-structed by depositing thin layers of dielectricson a transparent substrate. The desired wave-length of transmission by constructive interfer-ence is achieved by a proper choice of film thick-nesses. Two dielectric mirrors (each consistingof a stack of high and low refractive index mate-rials) separated by a spacer of another dielectricfilm can produce a very narrow band pass filter.Seeinterference.

bandwidth The frequency band useful for theproper operation of a circuit will have a width,measured as the difference between the maxi-mum usable frequency to the minimum usablefrequency. High bandwidth devices are eithertunable over a large range of frequencies, or canpass an entire range at once. Low bandwidthdevices find use in filters and clock or oscillatorcircuits.

A more general use of the term bandwidthsignifies the rate of information transfer.

band width The range of frequencies con-tained in a wave packet. The band width,∆v,is inversely proportional to the average durationof the pulses.Seecoherence time.

bandwidth constrained channel Abandwidth-constrained channel (or narrowbandchannel) passes only selected spectral com-ponents of the transmitted signal within thechannel’s frequency passband. Other spectralcomponents of the signal lying outside thechannel passband will be significantly atten-uated. The bandwidth-constrained channeleffectively functions as a bandpass filter. Ifthe transmitted signal possesses significantspectral power outside the channel’s passband,then the transmitted signal will undergo seriousspectral distortion, resulting in inter-symbolinterference (ISI) in the time domain.

bandwidth unconstrained channel Abandwidth-constrained channel (or wideband orbroadband channel) passes all spectral compo-nents of the transmitted signal with little or nospectral distortion. No actual channel is com-pletely unconstrained in bandwidth, but it maybe effectively considered as so for particularclasses of transmitted signals.

Barkhausen criterion The condition for os-cillation in a feedback amplifier circuit.

Consider an amplifier with gaing1. A voltagesignal input (Vin of magnitudeV0) is amplifiedgiving the voltage signal outputVout with mag-nitudeg1V0. Next, a feedback path through asecond amplifier (with gaing2) is added. Theproductg1g2 is called the loop gain, since it is thetotal gain due to going once around the feedbackcircuit loop. The effective gain of the circuit is

g′ =g1

1− g1g2,

whereVout = g′Vin, which is to say that for agiven desired output voltage the input voltagerequired is

Vin =(1− g1g2)

g1Vout .

Note that when the loop gain is identicallyone (g1g2 = 1, the Barkhausen criterion), the


output voltage is achieved with no input voltagelevel.

This is exactly what happens when an au-dio system with the microphone too close to thespeaker screeches with no other input to the mi-crophone. To stop the oscillation, the gain ofthe circuit must be decreased, either by movingthe microphone away from the speaker or byreducing the volume.

In the design of real oscillator circuits, theloop gain is greater than one, so that the oscil-lation amplitude increases until the amplifiersno longer respond linearly. At saturation of thiscircuit, the loop gain can only reach unity; thusa nearly constant output level is maintained. Ingeneral, real gains are complex, so the two con-ditions for oscillation must include phase infor-mation:

Barkhausen criteria:1) The feedback loopgain must be greater than one at the frequenciesof interest. 2) The sum of phase shifts aroundthe feedback loop must be an integer multiple of360.

A sine wave oscillator can be constructedwith the appropriate filters at the output of eachamplifier. However, smooth oscillations are notthe only waveforms that can be generated in thismanner. For instance, a stable multivibrator is atwo stage amplifier feedback circuit that oscil-lates between quasi-stable states if the Barkhausencriteria are met, yielding nearly square-wave pulsetrains.

Oscillator design is not the only field wherethese conditions become important. In manyapplications it is important to ensure there is notoscillation. The criteria were named after physi-cist G. Heinrich von Barkhausen (1881–1956).

Barkhausen effect This effect occurs whenthe grid of an amplifier tube is sufficiently ca-pacitatively coupled to the plate that oscillationsat very-high or ultra-high frequencies may takeplace.

bar magnet A rectangular block of magneticmaterial producing a static magnetic field.

barrel distortion An image distortion re-sulting in decreasing magnification for the raysaway from the axis. It results from the limita-tions of some ray bundles by aperture and stops.

barrier capacitance The capacitance of thedepletion region in a diode junction. Under re-verse bias, the depletion region (also called thebarrier region or space-charge region) does notallow current to flow (except for the dark currentof the device). The depletion depth depends onthe magnitude of the reverse bias voltage and thedopant profile of the junction. The capacitanceis inversely proportional to the depletion depth.

barrier, insulating Insulating materialplaced between signal lines or electrodes of adevice, electrically isolating them by increasingthe interelectrode impedance.

barrier potential The intrinsic voltage pro-duced at the junction of two materials with dif-ferent energy bands (say,p andn-type silicon,or a metal-semiconductor junction). Before be-ing joined the materials would have a differentchemical potential (or Fermi level). Upon reach-ing thermal equilibrium, however, there will beonly one chemical potential. Diffusion of chargecarriers in the semiconductor will allow equilib-rium to be reached, but in the process will pro-duce a space-charge region near the junction,depleted of carriers (the depletion region or bar-rier region). There will be a non-zero electricfield in this region, thus a difference in elec-trostatic potential. The magnitude of the bar-rier potential depends upon the resistivities (ordopant profiles) in the materials, but for manyphotodetectors is measured in tenths of volts.

bars, vibration in Studies of different kindsof vibrations that can occur in bars. A bar is asolid elastic object the length of which is muchgreater that its characteristic transverse size. Af-ter excitation, longitudinal and transverse vibra-tions can occur in a free bar. Transverse vibra-tions can further be subdivided into those due totwisting and bending of a bar. Any vibration ina bar can be represented as a sum of longitudi-nal and transverse vibrations. Studies of vibrat-ing bars are important in practice and technique.For example, vibrations in construction beamsare modeled as vibrations in bars. Furthermore,vibrating bars are used as parts of many musicalinstruments.


base In a bipolar transistor, the base is thecenter electrode and/or bulk, which is separatedby junctions from the emitter on one side, andthe collector on the other. In annpn transistor,the base is thep-type semiconductor material,while for pnp transistors, the base is then-type.Often, the lead connecting to the base bulk isalso termed the base.Seetransistor, bipolar.

base-emitter breakdown In a bipolar tran-sistor, when the base-emitter junction is reversebiased past the peak voltage, avalanche conduc-tion occurs. This is calledbreakdown,and inmost transistors will destroy the device.

basilar membrane A soft partition that di-vides the cochlea located in the inner ear length-wise. The cochlea is a cavity in a form of asnail shell, which is connected to the middle earby two membranes called the oval window andthe round window. The cochlea is filled with afluid (the cochlea fluid). The basilar membranehas about thirty thousand nerve endings. Thecochlea fluid and the basilar membrane are setinto vibrations by movements of the oval win-dow caused by sound traveling from the outerear. The nerve endings “feel” these vibrationsand pass on information about them to the brain.Vibrations of the basilar membrane have maxi-mal amplitude at a certain point along the mem-brane, the position of which depends on a fre-quency of sound. This is a mechanism that al-lows distinction between different frequencies.

bats, sound from Ultrasound emitted by batsto orientate and to find prey. Bats not only emitultrasound but also hear echoes from objects andflying insects that enables them to fly aroundthese objects and find prey in darkness. In otherwords, bats use a principle of echolocation. Batsradiate ultrasound through the mouth or the nos-trils. The radiated ultrasound is usually in theband20 − 100 kHz. The level of the radiatedultrasound can reach the value of120 dB at adistance of10 cm from a bat.

battery Two or more cells connected togetherto form one unit that can convert chemical en-ergy directly into electric energy. There aremany different types of battery, two major typesof which are (1) dry battery and (2) wet battery.

baud The baud represents the minimum timeinterval between successive signaling symbols.It derives from the name of Emile Baudot, aFrenchman considered by many as the fatherof automatic telegraphy. The baud embodiesthe shortest unit of modulation rate in a particu-lar signaling scheme. The baud rate equals thenumber of discrete signaling events in unit timeand, as such, determines the signal bandwidth.The baud rate always exceeds or equals the bitrate in bi-level signaling schemes.

beacon A coded signal transmitted for usein identification or for navigational use in thedetermination of position, direction, or distance.The signal may be optical or in the form of radioor radar waves.

Optical beacons, in the form of lighthousesand channel buoys, are used for guiding ships.Aircraft guidance is aided by rotating opticalairport and airway beacons. Airport beaconsare color coded to identify the airport as civilianor military and land or sea.

Radio beacons are passive stations radiatinga coded signal for use in bearing determinationor the analysis of radiowave propagation condi-tions. Seebeacon, radio.

Radar beacons may be either active or pas-sive. A passive radar beacon consists of reflec-tors installed at a lighthouse or buoy to enhancethe reflection of radar signals transmitted by aship. The known locations of the beacons areused to find the ship’s bearing and position.

Active radar beacons are used for identify-ing and locating aircraft. The ATCRBS (airtraffic control radar beacon system), also calledSSR (secondary surveillance radar), consists ofa ground-based radar system and an airbornetransponder unit. The ground-based system isused to query the transponder, which then re-sponds by transmitting a selected code, and pos-sibly other data such as the aircraft altitude. Be-cause the transponder is actively transmitting,the signal received by the ground station is gen-erally larger and more reliable than that returnedby a radar echo.

beacon, marker A radio beacon used foridentifying a specific region or position. Air-craft marker beacons are used to designate criti-cal positions on precision instrument approaches


to airport runways. These beacons radiate a 75MHz low-power signal of 2 W in an upward-pointing fan-shaped pattern. The signal is mod-ulated to allow audio identification.

beacon, radio A beacon consisting of a sig-nal transmitted at radio frequencies. These bea-cons are generally used in air or sea navigation toidentify position or bearing. A system of coastalbeacons operating in the 285–325 kHz band ex-ists for nautical direction finding. These stationsradiate signals of 100 W to 10 kW, providingground-wave coverage up to 1800 km over thesea.

Aviation radio beacons transmit azimuthallyuniform signals in the band 200–1600 kHz us-ing from 10 W to 2 kW of power. Aircraft auto-matic direction finding systems (ADFs) can usethe signals from these non-directional beacons(NDBs) to determine bearing at distances up to320 km.

Radio beacons are also used to analyze prop-agation conditions. By monitoring beacons in arange of frequencies and from a variety of loca-tions, an optimum radiowave propagation chan-nel can be determined.

beam The totality of all ray pencils emanat-ing from a source or aimed toward an image.In the case of a point source, there is only onepencil and the beam is made up of a single pen-cil. With extended sources, the beam consistsof all pencils emanating from every point on thesource.

beam divergence The radius of the spot sizew at a distancez from the beam waist in aGaus-sian beamis λ z

πw0whereλ is the wavelength of

light andw0 is thebeam waist.

beam expander This is an optical systemwith two lenses of focal lengthsf1 andf2 ar-ranged so that the separation is equal tof1 +f2.Both lenses could be positive or one of themcould be negative (see figure). The magnifica-tion is given by the ratio of focal lengths. Thedevice can be used as a beam reducer if the beamtraverses in the opposite direction.

beam, Gaussian The output of a laser, forexample, is the TEM00 or fundamental mode,

has a perfect plane wavefront and a Gaussiantransverse irradiance profile. When consider-ing a Gaussian beam propagating along theZ-direction, the intensity distribution at theZ = 0(flat wavefront) plane is given by:

I(x, y, o) = Io exp

[−2(x2 + y2

)ω2

]= Ioe

−2r2/w20 ,

whereIo is the intensity at the beam center andωo is the measure of beam width, known as spotsize of the beam; it represents the distance atwhich intensity falls off toIo/e2 (13.5%).

As the beam propagates along thez axis,diffraction occurs and the transverse intensitydistribution after a propagation distancez is

I(x, y, z) = Ioω2

o

ω2(z)exp

[−z(x2 + y2

)ω2(z)

]

= Ioω2

o

ω2(z)e−2r2/ω2(z)

whereω(z) is thez-dependent spot size of thebeam given by:

ω(z) = ωo

(1 +

λ2z2

π2ω4o

)1/2

.

beam, radio Beam of an antenna radiatingradiofrequency electromagnetic waves.Seean-tenna beam.

beam splitter Any device (the simplest beinga partially silvered mirror) providing transmittedand reflected beams of desired relative intensity.One may use frustrated total reflection by fix-ing the distance between two accurately parallel


hypotenuses of two simple prisms (for example,by coating the hypotenuse of one prism with asolid film of low refractive index material of de-sired thickness and placing the other prism incontact with it). A typical application is for mi-croscopes where one could adjust the fractionsof light from the source going to the eyepiece,photographic film and to the light meter.

Beam splitter.

beam, waist The smallest radius (i.e., at thefocal plane) of a Gaussian beam.Seebeam,Gaussian.

beam waist The minimum transverse sizeof the beam after reflections from the concavemirrors inside a laser cavity.

bean critical state model In type II super-conductors, magnetic flux quanta, vortices formwhen the magnetic field is above a critical field,Hc1. These vortices move from the edges ofthe superconductor into the interior until theyare pinned by defects or impurities. The beancritical state model assumes that the pinning isas strong as possible, such that the vortices areunable to move from the pinning sites. As a re-sult of this assumption, the flux density alwaysproduces the critical current density for the su-perconductor. That is,

|∇×B(r)| = µ0Jc

whereB is the flux density averaged over manyvortices,µ0 is the permeability of free space,andJc is the critical current density. Physically,this means that as the magnetic field is increased,more vortices form, “pushing” the other vorticesaround until they produce a maximal screeningcurrent over a minimal area of the superconduc-tor.

beat frequency Superposition of two wavesof closely spaced frequencies results in a wavewhose amplitude is modulated by the beat fre-quency which is the frequency difference of thetwo waves. One can determine an unknown fre-quency by beating it with a reference frequencyand detecting the beat frequency. One popularapplication of this idea is the tuning of acousticalinstruments. Advent of lasers has made it pos-sible to detect a beat frequency of a few Hertzout of 1014 Hz. The ring laser gyroscope takesadvantage of such precision.

beat reception When two periodic signalsare close in frequency, the sum of the signalsexhibits an interference that is sometimes con-structive, and sometimes destructive. This re-sults in an oscillation of the amplitude envelopeof the sum that has a frequency of the differenceof the two initial signals. This low frequencyoscillation is termed the beat frequency. Thereis also another higher beat frequency that has afrequency of the sum of the two initial frequen-cies.

Beat frequency envelope.

One method of receiving an amplitude mod-ulated signal is to utilize the beat frequencyto convert the signal back to audio frequen-cies. This can be done without a transistor-basedmixer, by adding an appropriate, locally gener-ated frequency. However, a mixer is still oftenused in beat reception, so that the local oscillator


is at the intermediate frequency of the receiver.In either case, this reception method allows de-modulation of a single sideband by using thelocal oscillator to provide the carrier synthesis(or carrier insertion).

Circuit diagram for a simple beat frequency receiver.

beats Periodic variations in the amplitude ofa sum of two harmonic oscillations whose fre-quencies are close to each other. In the simplestcase, these harmonic oscillations have the sameamplitudesB and different angular frequenciesω1 andω2 so that they are given by:B cos(ω1t)andB cos(ω2t), wheret is time. The sum of theoscillations is given by:

B cos(ω1t) +B cos(ω2t) = A(t) cos(ωt).

Here, ω = (ω1 + ω2)/2 is the angular fre-quency of the resulting oscillation, andA(t) =2B cos((ω1−ω2)t/2) is its amplitude. If|ω1−ω2| ω1+ω2, the amplitudeA(t) periodicallyand slowly varies in time in comparison with fastvariations with the frequencyω. This slow pe-riodic dependence ofA ont is called a beat, andthe frequency of this dependence(ω1−ω2)/2 iscalled the beat frequency. Beats are an exampleof amplitude-modulated oscillations.

Beer-Lambert relation A combination oftwo separate laws relating the amount of lightpassing through an absorbing medium to theproperties of the medium. Lambert’s law statesthat equal paths in the same absorbing mediumabsorb equal fractions of the light passing alongthose paths. Beer’s law states that the absorp-tion coefficient of a medium is directly propor-tional to the concentration of the absorber. Puttogether the Beer-Lambert Relation is

I = Ioe−Kx

whereIo is the light intensity incident on themedium, I is the light intensity exiting themedium,K is the absorption coefficient (relatedto the extinction coefficient), andx is the pathlength in the medium.

Beer’s law Seeabsorption, Beer’s law of.

bel A logarithmic unit of the ratio of twoquantities having dimensions of energy, inten-sity, power, etc. IfI2 and I1 are such quan-tities, then their ratio in bels is given byN =lg(I2/I1). Bel is named after A. Bell, Americanscientist and inventor, and is abbreviated by B.

bell A widening object of tapered shape,closed at the narrow end and open at the wideone. Bells are made of metals (copper, tin, etc.)and are usually of nonuniform thickness that in-creases to their open ends. Bells are used assound sources and musical instruments. Theyare set into vibrations by hitting them close tothe open end. A vibrating bell radiates as aquadrupole.

Bernstein model A mathematical model ofthe heredity of human blood factors based on thetriple allelic theory.

betatron A device for accelerating electronsto speeds approaching the speed of light. Elec-trons are injected into strong magnetic fieldsmaintained in a toroidal-evacuated chamber.The device is used to produce X-rays by hav-ing the electron beam impact on a metal target.

Bethe-Slater curve This is the relationshipbetween the exchange energy for the transitionelements vs. the ratio of the interatomic distanceto the radius of the3d shell.

B-field In magnetic induction, the number oflines of magnetic flux per unit area of a surfaceperpendicular to the field.

bias circuit The circuit that provides theneeded DC bias for device operation, often sep-arate from the input and output active circuitpaths.


bias current The current through a devicethat is due solely to the bias voltage. Since thebias voltage is necessary for operation but doesnot provide information, the bias current is of-ten a main source of inefficiency in a device.In photosensitive applications, the bias currentis sometimes referred to as the “dark current”,since it flows with no light signal present.

bias, forward A real diode, any diode junc-tion, or any rectifying portion of a device (suchas the emitter to base path in a transistor) has anasymmetry in the current response dependingupon the polarity of the applied voltage. Whenoperated far from breakdown, one polarity of ap-plied voltage will allow more current flow (af-fording a lower effective resistance) while theother polarity will yield a lower current (a highereffective resistance). An applied voltage withpolarity yielding the lower resistance (more cur-rent flowing) is termed aforward bias voltage,or justforward bias. See alsobias, reverse.

bias, reverse The voltage applied to a circuitelement or portion of a device that is of a polarityyielding higher effective resistance is termed thereverse bias voltage,or merelyreverse bias. Seebias, forward.

bias voltage The voltage needed to operatea device, provided by the bias circuit. For manyapplications, the polarity of this voltage is im-portant.Seebias, reverse; bias, forward.

binary circuits Logic circuits with only twologic states, which may be labelled 0 and 1.Seecircuit, logic.

binary coded decimal (BCD) A way of rep-resenting decimal numbers in binary format. In-stead of being a true binary (base 2) number,each decimal (base 10) digit is separately repre-sented in a binary format.

binary symmetric channel A binary chan-nel represents a communication channel overwhich signals are transmitted only as a sequenceof binary-valued symbols. A binary symmetricchannel is a memoryless channel (i.e., each unitof channel output depends only on the corre-sponding unit of channel input but not on other

channel input units) with equal conditional errorprobabilities (i.e., the probability that a transmit-ted ‘1’ symbol becomes received as a ‘0’ equalsthe probability of a ‘0’ received as a ‘1’). Thechannel may thus be fully characterized by asingle error probability parameter, typically de-noted asp. Such a simple channel model oftensuffices for many practical applications.

binaural Pertaining to sound, process or sys-tem that deals with listening with two ears byhumans and animals. Binaural listening allowsthe listener to determine the direction of a soundsource up to3 in the horizontal plane. This phe-nomenon is based on the binaural effects, i.e.,the ability of humans and animals to distinguishthe intensity and time arrivals of sound from asource at both ears. The ear that is closer toa source “hears” more intense sound and earlierthan the other. Binaural effects are a backgroundfor performance of stereophonic audio systems.

binoculars Instruments (e.g., binoculartelescopes) offering comfortable telescopic-enhanced viewing of distant objects while al-lowing both eyes to remain active. The finalimage is made erect with the help of Porro orother types of prisms. Thus, the distance be-tween the objective lenses can be made largerthan the interpupillary distance. The designa-tion, e.g.,6×30, means that the angular magni-fication is6× and the diameter of the objectivelens is 30 mm.

bioelectricity Electrical energy (cur-rent/voltage) produced within a biologicalorganism, as in muscle tissue (see alsoactionpotential), nerve synapses, photosynthetic path-way, etc. Bioelectronics focuses on externalelectronic control of physiological response inplants and animals.

biofeedback A learned response whereby aphysiological output such as heart rate, bloodpressure, metabolism, anxiety is controlled byconscious monitoring of the output (feedback)leading to control of the physiological process.

biological control theory The theory of or-ganism population control through the use of


naturally occurring enemies, pests, pesticides,predators, etc.

biological effects, electric fields (1) Static:Static electric fields inside an organism maycause muscle reaction, nerve stimulation, death,or accommodation. External electric fields be-low the dielectric breakdown value (about 3 mil-lion volts/meter in air) appear to little damagelarger organisms but do cause a stimulus to ap-pear in the sensory nerves.

(2) Time dependent:Time dependent elec-tric fields can have varying degrees of effects onbiological form and function, leading to defor-mity and death. These effects are not strong ifthe electric field strengths are weak or the fre-quency of oscillation is low.

biological effects, electromagnetic fieldsLow frequency electromagnetic fields (less than1000 MHz) appear to have no distinguishable ef-fect on biological form and function. At higherand higher frequencies, the biological effectsgo from burning (infrared), sensory perception(visible), DNA and cellular damage (UV, X-ray), to death (gamma ray).

biological effects, gravity For all forms oflife on earth, the gravitational field is alwayspresent and the organisms adapt to the presenceof the gravitational force (mg). This force is ap-parently responsible for plants knowing whichway to grow “up”, for the bone size and distri-bution of walking mammals, for the limit to thesize of animals, a limit to the maximum heightof animals, and a limit to the longevity of ani-mals by the work needed from the heart muscleto pump blood through the organism against thegravitational force.

biological effects, ionizing radiation Highfrequency electromagnetic waves in the UV, X-ray, and gamma ray end of the spectrum possessenough energy to break atomic bonds and to lib-erate electrons from atomic orbitals. This elec-tron liberation is called ionization. Ionizationleads to damage to DNA, RNA, protein structureand function and cell death by breaking theseatomic and molecular bonds allowing the con-stituent atoms to recombine in non-functioningconfigurations. Some of this damage may be re-

paired via RNA function; however, if the dam-age is extensive or the DNA/RNA templates aredestroyed, cell death will be the end product ofionizing radiation. If the repair is not correct,cell deformity, including cancer, may result.

biological effects, magnetic fields Staticmagnetic fields, more so than electric fields,appear to strongly affect some biological or-ganisms. It is believed that some birds andmammals know North and South by sensing theearth’s magnetic field. Stronger magnetic fieldsappear not to be dangerous unless the organismis in possession of a ferromagnetic component,in which case the organism will be acceleratedtoward and held at one of the poles. The move-ment of an organism in a magnetic field willcause induced electric currents (Faraday Effect).If the magnetic field strength is small and the or-ganism’s speed is low, then the induced currentsare very small and cause no damage or disrup-tion. However, care must be exercised aroundlarge magnetic fields, such as with magnetic res-onance magnets, for even moderate speeds nearthese magnets will induce appreciable currents.

biological effects, microgravity Recent ex-periments in the space shuttle program and theMir program have provided a wealth of data onthe effect of small gravitational fields (micro-gravity) on the form, development, and func-tion of biological organisms. These space shipshave very small gravity because they are essen-tially in free-fall orbit about the center of theearth. There is a residual gravitational field dueto the moon, sun, and the nonspherical shapeand the nonuniform density of the earth. Sig-nificant effects of microgravity on humans, forexample, include muscle atrophy, loss of bonemass, disorientation on returning to the earth’sgravitational field, etc.

biological effects, noise (1) Signal process-ing noise is a normal component of signal trans-mission within an organism. Optic nerve trans-mission is an example. If the noise level be-comes too large, then the receptor is confusedabout the signal and loses its proper response.This can lead to mental confusion for processedsignals; or to disablement and death if the nerve


signals to the heart, for example, have too muchnoise.

(2) Sometimes referred to as unwanted ge-netic mutations (not recommended).

(3) Noise has psychological and physiolog-ical effects depending on amplitude, threshold,or timbre. Physiological response may includesleeplessness, anxiety, elevated blood pressureor heart rate. Psychological responses may in-clude stress, anger, depression. Severe acousti-cal noise may lead to deafness, dysfunction, ordeath.

biological effects, non-ionizing radiation(1) Photosynthesis.Visible light supplies theenergy for photosynthesis in green plants.

(2) Infrared. Infrared radiation leads to localheating and may assist or hinder plant/animalgrowth and sustenance.

(3) Radiofrequencies.Appears to have littleto no effect on biological systems. Studies arestill in progress.

biological effects, statics Statics, the sci-ence of equilibrium structures relating to theirforces and moments of interaction, attempts tounderstand the structure of biological organismswith respect to their structure. For example:the variance of blood pressure with height, themaximum size of a mammal for bone structureintegrity, or bone joint differences resulting indifferent walk patterns for various species.

biological effects, ultrasound Ultrasoundis acoustic frequencies in the frequency rangeabove human hearing, approximately 20 kHz.Ultrasound is used for two-dimensional imag-ing of internal body structure because of the ap-parent nonharmful character of low intensity ul-trasound. Sometimes referred to as echographyor sonography.

biological effects, ultraviolet radiation Ul-traviolet radiation is electromagnetic radiationwith frequencies beyond the visible spectrum.At these frequencies radiation is usually identi-fied by wavelength — from approximately 185nm to 390 nm. Ultraviolet radiation is usuallyharmful since it can energetically break DNAbonds leading to cell death or harmful muta-

tions. Some ultraviolet radiation is necessaryfor normal growth and calcium metabolism.

biological effects, X-rays Similar to theharmful effects of ultraviolet radiation; however,there are no beneficial effects of X-ray radiationsince X-rays are so much more energetic than ul-traviolet radiation that chemical bond breakageleading to cellular death and harmful mutationis guaranteed.

biological half-life The time required for onehalf of an injected radioactive substance to beexcreted by a biological organism.

biological kinetics (1) A study of the pro-cesses and rates of change of biological organ-isms.

(2) A study of the motion of biological sys-tems.

biological rhythm A regularly occurringprocess in the maintenance or growth of a bi-ological organism. Heartbeat for example is acardiac rhythm.

biomaterials Materials derived from, or atleast compatible with, biological organisms. Aspecific class of biomaterials are those devel-oped for synthetic prostheses.

biomechanics Sometimes referred to as bio-physics. However, more properly biomechanicsis the science devoted to elucidating the under-lying forces of interaction responsible for thegrowth, maintenance, function, and form of bi-ological organisms.

biorthogonal code A set of2K biorthogo-nal codewords may be formed from a set ofKorthogonal codewords by including the negativeof theK orthogonal codewords to theK orig-inal codewords. The correlation coefficient be-tween any two members in a biorthogonal codeset equals either0 or−1.

biosphere (1) That part of the earth compat-ible with living organisms.

(2) A closed thermodynamic model of a func-tioning ecological system.


biosphere, energy stored in The amount offree energy available for support of life func-tions. Without any free energy, life would notexist. The energy is composed of mechanical,chemical, thermal, and nuclear parts.

biostimulation Any process, heat, light,touch, chemical contact, etc. that induces a re-action in a biological organism.

biotelemetry Remote monitoring of the con-ditions within a biological organism without anydirect connection to the organism.

Biot-Savart’s law This law gives the dif-ferential contribution to the magnetic field dBproduced at a distancer from a differential lineelementdl that carries a currentI. In SI units

dB = µoIdl × r14π r3

andr1 is a unit vector in the direction ofr.

Biot’s law The rotation of polarizationof plane-polarized light in an optically activemedium is nearly proportional to the inversesquare of its wavelength. The specific rota-tion ρ (degrees/mm) is given by Biot’s law asρ = A+ B

λ2 whereA andB are constants spe-cific to the material.

bipolar code A bipolar code, also called analternating binary codeor analternate mark in-version(AMI) code, represents a tertiary codewherein a “low” bit is signified by a 0 and a“high” bit is signified by a 1 or−1, such that suc-cessive “high” bits would have opposite signs.Bipolar coding possesses limited innate errorself-detection capability because error musthave occurred if the aforementioned alternatingsign rule is violated. Bipolar coding is charac-terized by a spectral null at DC.

biprism Biprisms consist of 2 prisms placedbase to base.

biprism, Fresnel Fresnel used a biprism toshow interference phenomenon. It consists oftwo acute angled prisms placed side by side, andis constructed as a single prism of obtuse angle,with the acute angles on both sides about 30.

bird acoustics The discipline that stud-ies songs and other sounds produced by birds,mechanisms of their production and their func-tions. Acoustic source of bird’s songs andsounds is the syrinx located near the trachea andbronchi. Songs and sounds produced by birdsare very complicated acoustic signals with rapidmodulation in both amplitude and frequency.Frequency range of songs and sounds is from100 Hz to 10,000 Hz. Functions of songs arebelieved to be territorial maintaining, individualrecognition, mate attraction, and stimulating re-production.

birefringence When a beam of light passesthrough a uniaxial or biaxial crystal, it under-goesdouble refractionwith an ordinary and ex-traordinary ray polarized in orthogonal direc-tions. Birefringence is a measure of the differ-ence between the refractive indices of the opti-cal indicatrix of a crystal. If the refractive indexparallel to the optic axis is larger than that atright angles to it, the crystal is said to beposi-tive uniaxial birefringent(e.g., ice, quartz). Theopposite is said to benegative uniaxial bire-fringent (e.g., calcite). Optically active sub-stances, such as quartz, possess different refrac-tive indices for left and right circularly polar-ized light. This phenomenon is calledcircularbirefringencewhich leads to a rotation of po-larization of linearly polarized light as it passesthrough the material.

Bitter patterns These reveal the domainstructure of ferromagnetic materials. In thetechnique used by Bitter, a drop of colloidal sus-pension containing fine ferromagnetic colloidalparticles is applied to the ferromagnetic crystalsurface. The pattern of magnetic structures re-vealed by the colloidal particles are calledBitteror powder patterns.

blackbody radiation A blackbody is a per-fect absorber or emitter of radiation of all fre-quencies incident or emitted at all angles. Thespectral intensity distributionM per unit area ina unit wavelength interval (units:W /m2 - µm)of a blackbody as a function of wavelengthλ(unit: µm), at a temperatureT (unit: degree


Kelvin), is given by the Planck’s formula

M(λ, T ) =(3.7415)108

λ5

(1

e14388

λT − 1

)(W/m2 − µm

).

The wavelength at which the intensity peaks isinversely dependent on temperature and is givenby the Wien’s law

λmaxT = (2.8978)103 (µm−K) .

The total radiation (unit:W /m2) emitted by ablackbody at the temperatureT is given by theStefan-Boltzmann law

∞∫0

M(λ)dλ = (5.6697)10−8 T 4 .

The blackbody radiation spectrum at 300, 1500and 3000 degrees Kelvin is shown below. Thepeak wavelengthsλmax are at 9.7, 1.93 and 0.97µm as given by the Wien’s law.

Blackbody radiation.

blackout, radio Also called fade-out orshort-wave fadeout,a loss of short-wave com-munication along a specific propagation pathdue to prolonged fading of the radio signal.Blackout conditions occur whenever the lowestusable frequency (LUF) exceeds the maximumusable frequency (MUF) along a particular iono-spheric propagation path.

Radio blackouts are often caused by iono-spheric storms triggered by magnetic distur-bances, resulting in a dramatic increase in iono-spheric D-layer absorption. These storms maybuild slowly and last up to a week. Sudden iono-spheric disturbances (SIDs) cause shorter black-outs, erupting very quickly and usually lastingless than an hour. They are precipitated by

solar flare activity causing a reduction of F2-layer MUF and increase in D-layer absorption.Polar-cap absorption is caused by high-energysolar protons in high-latitude regions, resultingin blackouts lasting from a few hours to severaldays.

blanking In radar, turning off the receiver ortransmitter to reduce interference from a partic-ular direction or during a particular time. Targetinformation can be lost if it arrives during theblanking period, or originates from the blank-ing direction.

In communications, silencing a receiver dur-ing a short period to reduce impulse noise. Theperiod is chosen so that the loss of informationdoes not significantly degrade the received sig-nal.

In television, the use of a pulse waveform torender the return trace of the raster scan invisi-ble.

blazing, of grating The technique of shapingthe grooves of a ruled grating so that the maxi-mum of the diffraction envelope due to the widthof each groove coincides with the desired orderof diffraction of the grating. As an example, agrating with the reflecting grooves making theblaze angleθB with the grating surface wouldhave the grating equation

a (sin θi + sin θm) = mλ ; θm = 2 θB − θi .

Hereθi, θm andθB are the angles of incidencewith respect to the normal to the grating sur-face, angle of diffraction in themth order, andthe blaze angle, respectively. Typically spec-trographs are designed so that eitherθB = θi

(Littrow mount) orθi = 0 (Normal mount).

Blazing.


blindness, color The inability of the eye todistinguish different colors. Due to genetic fac-tors or disease, about 3% of the human popula-tion does not have one or more types of cones,which have three different absorption spectradue to the three pigments present in them. Ifonly one type of cone is present, then the per-son is amonochromatand can see only blackand white. This occurs in 0.003% of the popula-tion. If two types of cones are present (about 5%males and 0.4% females) the person is aDichro-mat. The most common among these are thosewho lack green-sensitive cones. They cannotdistinguish red, yellow, and yellow-green. Theyare calledDeuteranopes.

blind spot A small region of the retina, typi-cally oval in shape, approximately 7.5 along itsvertical axis and 5.5 along the horizontal axis,with its center located approximately 15.5 tothe temporal side of the visual field. This cor-responds to the point of exit of the optic nerveand is insensitive to light stimulation becauseit is devoid of photoreceptors (rods and cones).It is also called thephysiological blind spotorMariotte’s spot.

blinking Alarm function in loran (long-rangeradio navigation) indicating the loss of signalintegrity. Users are warned within one minute ofsignal loss. For aviation applications, an alert isgiven when the signal-to-noise ratio drops below−6 dB.

Bloch-Gruneisen formula The Bloch-Gruneisen formula is an approximate formulafor the resistivity of a metal due to electron-phonon scattering. The equation is

ρphonons= AT

ΘD

5

∫ ΘD/T

0

dxx5

(ex − 1)(1− e−x)

for temperatures below the Debye temperature,whereΘD is the Debye temperature andA is amaterial-dependent constant. This approxima-tion is valid for a free-electron model of a Debyesolid if Umklapp processes are negligible.

Bloch’s equations Bloch’s equations are themacroscopic equations of motion for a spin sys-

tem in a magnetic field. For homogeneous,isotropic systems, Bloch’s equations take theform

dM/dt = γM ×B + relaxation terms

whereM is the magnetization of the sample,γ is the gyromagnetic ratio,B is the appliedmagnetic field, and the relaxation terms will bediscussed below. If the external field is constantand in thez direction, in equilibrium, the Blochequations becomeMx = 0,My = 0,Mz =χ0B0 whereχ0 is the magnetic susceptibility.

The relaxation terms take into account thespin-spin and spin-lattice interactions. If a sys-tem of spins, initially unmagnetized, is placed ina magnetic field, the magnetization approachesa new equilibrium value,M0. Since this magne-tization is in the same direction as the externalfield, it is thelongitudinal magnetization.Thisrelaxation to equilibrium takes place as energyflows from the spin system to the lattice sys-tem and is therefore known asspin-lattice relax-ation. The spin-lattice relaxation time,T1, mustbe included in the Bloch equations in order todescribe these non-equilibrium processes. An-other relaxation mechanism is present for trans-verse components of the magnetization. Thespin-spin relaxation time,T2, is a measure of thephase coherence of the spins. If, for example,a given spin, spinα, is in a local magnetic fieldBα and another spin, spinβ, is in local mag-netic fieldBβ , these spins will precess at differ-ent frequencies. After some time, the magneticmoments of spinsαandβwill have different ori-entations which cause their magnetic momentsto cancel each other out. At this point, thesespins no longer add to the total magnetization.

Including relaxation effects in Bloch’s equa-tions, we obtain

dMx/dt = γ(M ×B)x −Mx/T2

dMy/dt = γ(M ×B)y −My/T2

dMz/dt = γ(M ×B)z + (M0 −Mz)/T1 .

Note that Bloch’s equations are only approx-imate equations. No effort is made to determinethe exact interactions in the system, which areonly included viaT1 andT2. As a result, theBloch equations must be modified to describeaccurately magnetic resonance in solids.


In nuclear magnetic resonance, an rf field isapplied perpendicular to the static field. If the rffield is at a frequencyω = ω0 whereω0 = γ B0,the spins absorb energy from the rf field reso-nantly, thereby producingnuclear magnetic res-onance. Nuclear magnetic resonance has be-come a powerful tool in chemistry, biology, andmedicine.

Bloch’s law A relation between the fractionalchange in magnetization of a material and abso-lute temperature based on spin wave interactionand scattering in the material. Felix Bloch wasable to show that the fraction change in magne-tization should be proportional toT 3/2 whereTis the absolute temperature.

Bloch walls In 1931 Bloch showed theoreti-cally that the boundary between magnetic do-mains is not sharp on an atomic scale but isspread over certain thickness wherein the direc-tion of spins changes gradually from one domainto the next. This layer is usually called a domainwall or Bloch wall.

blood cell analysis, electrical impedancemethod A sample of blood is placed betweentwo apposing conducting plates across which anAC voltage is applied. The impedance is cal-culated from the response as a function of fre-quency and phase angle. From this informationstructure and content function of the blood maybe inferred. Also referred to asimpedance spec-troscopy.

blood cell analysis, hydrodynamic methodBlood flow through a restriction creates a pres-sure differential across the flow and a velocitydifferential along the flow. From this informa-tion blood density, viscosity, and volume maybe inferred.

blood cell analysis, photoelectric methodIn this technique, a diluted blood specimen ispassed through a laser beam. Each blood cellscatters the light; the amount of scattering andthe intensity of the scattered light reaching thephotodetector yields volume, optical density,and number of blood cells.

blood flow The movement of blood past agiven point. The motion of blood in the cardio-vascular system.

blood flow measurement, Doppler methodThe Doppler effect is the shift in frequency ofsound upon reflection by a moving object. If thesound is reflected by an object moving towardyou, the frequency increases in proportion to theobject’s speed. If the sound is reflected by anobject moving away from you, the frequencydecreases in proportion to the object’s speed. Inthe Doppler method, a pulse of sound is passedinto an artery and the frequency of the reflectedsound is measured, yielding a plot of blood flowspeed as a function of time and location. Forexample, this method is currently used to detectconstrictions in the carotid arteries.

blood flow measurement, electromagnetic in-duction method Blood is allowed to flow inan artery through a magnetic field. Because theblood plasma is ionic, the Faraday effect causesa transverse electric potential to appear acrossthe artery, the strength of which is proportionalto the flow rate of the blood.

blood flow measurement, fiber-optic methodA fiber optic probe is inserted into the arteryallowing direct measurement of blood flow bycount of the scattered light reflected back intothe fiber-optic probe.

blood flow measurement, ultrasonic methodShort pulses of ultrasound (8 MHz) are appliedto the skin via a transducer. The echo signalsare picked up acoustically and the Doppler shiftyields the blood flow rate.

blood pressure The pressure, relative to at-mospheric, in the arteries of the circulatory sys-tem. The larger pressure, systolic, occurs whilethe heart is pumping and the lower pressure, di-astolic, occurs while the heart is at rest. The typ-ical range for humans at heart level is about 130mm Hg to 80 mm Hg. However, the blood pres-sure decreases with height, and is much largerat the feet than at the head of a standing person.Blood pressure also depends on mood, exercise,chemical intake, and conditioning.


blood viscosity The resistance to flow ofblood. A function of blood content, artery walllining, and the presence of clotting factor whichresults in a marked increase in blood viscosity.

blue color of sky When sun light traverses ina region of clear sky, the elastic or Rayleigh scat-tered light from air molecules appears blue whenviewed in the lateral direction. This effect is dueto the spectral dependence of scattered intensity(which is proportional to the fourth power ofthe frequency of light) and the sensitivity of thehuman eye. More violet and blue photons arescattered than red but the eye is less sensitiveto violet. The scattered light is also partiallypolarized in the vertical direction, the degree ofpolarization being maximum when the directionof the sun light in the region of the sky and theviewing direction are perpendicular.

blue light, biological action Blue light, be-ing closest to ultraviolet radiation, appears tomore strongly affect biological action than theother visible wavelengths. Blue light appears toset the circadian clock in mammals, stimulatescell proliferation, and appears to activate someof the transferase in cells. The mechanism ap-pears to be proton transfer kinetics due to theabsorption energy of the blue light.

Bode plot Complex quantities that may varywith frequency can be visualized by simultane-ously graphing a measure of the amplitude ofthe quantity (either directly or in decibels fromthe minimum or maximum amplitude) vs. fre-quency and the phase (measured in radians ordegrees). Such a plot is termed aBode plot.Fora complex valueZ = |Z| exp iφ, a Bode plotshows|Z| andφ on separate traces as a functionof frequency.

bolometer A thermal detector of infrared ra-diation. The detection mechanism is the changein temperature produced by the absorption ofincident radiation. It consists of a thin black-ened slab whose impedance is temperature de-pendent. It can be used either in a DC (for steadysignals) or AC (for periodic signals such as thosefrom a pulsed laser or a chopped light beam)mode. The range of wavelengths and operatingtemperatures also vary. For some commonly

A Bode plot.

known bolometers.Seebolometer, thermistor;bolometer, free electron; bolometer, low tem-perature.

bolometer, free electron The mobility offree carriers in a semiconductor (e.g., indiumantimonide) increases via absorption of the in-cident radiation leading to the bolometer effect.These detectors operate at liquid helium temper-atures and have two to three orders of magnitudesuperiorD star value compared to thermistorbolometers. The response time is in microsec-onds and the spectral range is in the mid- and farinfrared.

bolometer, low temperature Doped siliconor germanium bolometers operate over a verybroad spectral range (1.7 to 1000µm) with Dstar values of 1013 cm. Hz1/2/W at 10 Hz. Op-erating temperature is 2 K. The time constant isinversely dependent on the thermal conductancewhich is typically 1µW/K.

bolometer, thermistor The bolometer ele-ment is made of thin flakes (10µm thick) ofpolycrystalline oxides of Mn, Ni, and Co whoseresistance can change by several percent perchange of temperature of 1 K. The spectral re-sponse is in the mid-infrared and depends oncoating. The time constant is in the millisecondrange with the spectralD star values of 109 cm.Hz1/2/W. The detector operates at room temper-ature.

boolean algebra A set of rules for the formalrepresentation of set or logic relationships. Forinstance, the union of two setsA andB can be


represented asA ∪ B, while the intersection isA ∩B. The empty set isA ∩ A whereA is thecomplement ofA. Algebraic relationships andrules such as commutation are defined. Whenapplied to logic, the empty set is analogous tofalse and the rules for formal logic can be statedin symbolic form, e.g., by replacing AND withthe∩ operator.

Common use of the term boolean algebra in-cludes logical formalisms and some logic circuitdesign rules.

Bose-Einstein condensation (BEC) As thetemperature of a system of N bosons is low-ered, more and more of the particles occupya single quantum mechanical state, the groundstate. Bose-Einstein condensation, the pres-ence of macroscopic numbers of particles in theground state, is a result of Bose-Einstein statis-tics only and will occur even in the absence ofinteractions. If the phase space density of a sys-tem of non-interacting bosons reaches

nc = 2.612/Λ3

where Λ is the de Broglie wavelength,(2π2/mkBT )1/2, Bose-Einstein condensationtakes place. If this equation is solved for the crit-ical temperature, it is found to be

TBEC = 115/(V

2/3M M

)where VM is the molar volume (cm3/mol)and M is the molecular weight. The frac-tion of particles in the ground state is(1 −−(T/TBEC)3/2). Superfluid 4He and exci-tonic superfluids are both physical realizationsof Bose-Einstein condensation, but the high den-sities involved result in highly interacting sys-tems not easily described by weakly interactingBEC models.

BEC was observed in dilute gases of alkaliatoms in 1995, and great progress has been madesince then in a variety of studies. Currently,BECs consisting of tens of millions of atomscan be made routinely.

The techniques used to create a BEC vary,but most have several common features. Thesource of atoms is first heated in an oven, boilingatoms off the surface. These liberated atoms arethen slowed (and thereby cooled) during loading

into an optical or magneto-optical trap. Such atrap uses a set of lasers and/or a non-uniformmagnetic field to confine the atoms. The mag-netic fields are (usually) arranged such that onlyatoms in a particular hyperfine state are trapped,all others are rejected from the trap. These atomsare cooled further by “evaporation” of the highenergy atoms. This is accomplished by inducingZeeman transitions in the high energy atoms byusing an rf field tuned to put the most energeticatoms into states that are not trapped. Eventu-ally, the remaining atoms cross the critical phasespace density, resulting in a BEC.

For the alkali gases, the transition tempera-tures are of the order of 0.1 – 1µK (but re-call that the transition temperature is connectedto the density of atoms, so a denser gas resultsin a higherTBEC). Present and future studiesof dilute gas BECs will probe for effects sim-ilar to those found in superfluid helium (e.g.,the Josephson effect) and other new possibili-ties (e.g., an atom laser).See alsohelium-4,superfluid.

Bose-Stoner hypothesis Stoner theory is atheory of metallic ferromagnetism. Bose exci-tations are spin waves in these systems.

Bouger’s law Describes the behavior of lighttransmission through an optical medium as

log10(τ)/d = constant

whereτ is the internal transmittance,d is thethickness of the medium, and the constant de-pends on the optical properties of the material.

boundary conditions Conditions that are im-posed on the pressure and the fluid velocity at aninterface of two media in fluid-dynamics. Thepressure and the component of the fluid veloc-ity normal to an interface must be continuousacross the interface. No conditions are imposedon the temperature. Continuity of pressure andthe normal component of the fluid velocity resultin relationships for acoustic pressure and fluidvelocity at both sides of an interface, which arealso calledboundary conditions.

boundary conditions (magnetic field)When an electromagnetic wave is incident onthe boundary between two dielectric media such


that the electric field is perpendicular to theplane containing the media (the transverse elec-tric mode), then the boundary conditions thathave to be applied to the magnetic field compo-nent require that the net magnetic fields on bothsides of the boundary be equal.

boundary resistance Any time two materialsare joined, a resistance (thermal and/or electri-cal) will occur at the joint. This resistance isdue to imperfections in the joint and the differ-ing properties of the two materials. In joints be-tween two metals, electrons are scattered off theinterface between the two materials; the samecan be said for phonons in the case of two dielec-tric media. Thermal boundary resistances are amajor concern in low temperature experimentsas they can easily limit the sample temperature.

Whenever such a resistance exists, there willbe a step in the temperature across the interfaceaccording to∆T = QRTh whereQ is the heatincident on the surface andRTh is the thermalboundary resistance. In metals, thermal bound-ary resistance can be minimized by minimizingthe electronic boundary resistance. This can bedone by maximizing the actual area of contactand ensuring clean surfaces free of oxide layers.When two metals are pressed together lightly,as little as one part in a million of the area willactually be in contact. Therefore, pressure mustbe exerted to improve contact.

Another technique is to weld the two piecestogether, but only if the metals do not producean alloy with large thermal resistance, of course.To guarantee clean, oxide-free surfaces for pressjoints, the metals are often gold-plated. It is alsocommon practice to use the thermal contractionof materials to good advantage when designingthe parts to be joined. When these techniquesare combined, it is possible to get 10–100nΩ ofboundary resistance between two metals.

For non-metals, heat is conducted primarilyby phonons, and so transmission of phononsacross the boundary is of utmost importance. Inthis case, however, there is not as much for theexperimentalist to do except choose materialswisely and maximize contact area. At the inter-face between dielectrics, phonons scatter off thesurface according to acoustic mismatch theory.In complete analogy with optics, a phonon ismuch more likely to be scattered when crossing

the boundary between two very dissimilar ma-terials than if the sound speeds are nearly equal.In general, it is better to plan not to rely onheat conduction through dielectrics unless ab-solutely necessary.See alsoKapitza boundaryresistance.

boundary waves Also known as surfacewaves. When a ray of light is incident at theinterface of two media at an angle larger thanthe critical angle, it is totally internally reflected.However, there is a tangential component of theelectric field at the boundary of the interface.The amplitude of the boundary waves decays ex-ponentially with distance. This field can couplewith another nearby medium of higher refractiveindex leading tofrustrated total internal reflec-tion. Seecritical angle.

bound charge Bound charges are chargesdue to the polarization of the material. Thereare two types of bound charges:

1. surface bound charge,σb, which is the vec-tor product of polarization and surface normalunit vector; and

2. volume bound charge,ρb, which equals thenegative of the divergence of polarization.

bow wave A wave occurring in front of a shipin motion.

boy’s method A method of measuring therefractive indexn of the material of a lens. Theradii of curvaturer ands of the two surfaces ofthe lens, and the focal lengthf are measured bydetermining the distances at which an object iscoincident with its image produced after reflec-tion from the respective curved surface of thelens, and after putting a plane mirror behind thelens. The relation1/f = (n − 1)(1/r + 1/s)then gives the desired refractive index.

Bragg’s law The diffraction of a beam of X-rays by the atomic planes of a crystal results inbright spots obeying the Bragg’s law2d sin θ =mλ whered is the spacing of atomic planes,θthe angle of the incident beam from the planes,andλ is the wavelength of radiation. The inte-germ refers to the order of diffraction. Differentsets of atomic planes in a crystal diffract X-raysat different angles, as shown in the figure below,


leading to a pattern of bright spots. Analysis ofsuch data can yield information of the crystalstructure. Other applications of Bragg’s law in-clude the scattering of light by a periodic refrac-tive index grating generated by acoustic wavesin crystals. A special case of such an acousto-optic effect is Brillouin scattering.

Bragg’s law.

Bragg-Williams approximation TheBragg-Williams (B-W) approximation is a zeroorder mean field approximation often used tostudy the Ising model, binary alloys, and othersimilar systems. The primary assumption in theB-W theory is that spatial correlations betweennearby spins (or lattice sites) are unimportant.The Bragg-Williams approximation is equiva-lent to the Weiss molecular field model. It mostoften produces qualitatively correct behavior,but fails when fluctuations and long-rangecorrelations become important — in the criticalregime near phase transitions, for example.

Braun tube Cathode-ray tube (CRT). It wasinvented by Braun (1850–1918) and was origi-nally calledBraun tube.

Bremsstrahlung A continuous spectrum ofX-rays (photon energies in the 100 to 100,000eV ). A typical X-ray tube consists of a hot cath-ode and a rotating anode made of metals suchas copper, molybdenum or tungsten. Thermallyemitted electrons from the cathode are acceler-ated toward the anode by an applied voltage (ofthe order of kilovolts) between them. As theelectrons colliding with the target nuclei are de-celerated, X-rays are emitted. The minimumwavelength is inversely proportional to the ap-plied voltage. At higher voltages, the electrons

colliding with the anode will have sufficient en-ergy to eject the electrons of the target atomfrom their inner shells. These ejected electronsrelax to the available empty shells giving riseto X-rays of discrete energies. The sharp linespectrum of the so-calledcharacteristicX-rayswill be superposed on the continuous spectrumcalled the Bremsstrahlung.

Brewster angle The angle of incidence (mea-sured with respect to the normal) of a ray oflight traveling from a medium of refractive in-dex n1 to that ofn2, so that theTM (trans-verse magnetic) orp-polarized light, with theelectric vector parallel to the plane of incidence,has zero reflection. This angle is also calledpolarizing anglebecause the reflected light iscompletelys-polarized orTE (transverse elec-tric) with the electric vector perpendicular to theplane of incidence. The transmitted light will bepartiallyTM polarized. In terms of the refrac-tive indices, the Brewster angle,θB , is given byθB = tan−1(n2

n1), wheren1 andn2 correspond

to incident and transmitted media, respectively.For the angle of incidence atθB , the reflectedand transmitted beams will be at right angles.

Brewster angle.

The figure below shows the reflectance oflight for TE andTM polarizations as a func-tion of the angle of incidenceθi. The figure (a)is drawn for light traveling from air (n1 = 1) toglass (n2 = 1.52). The direction is reversed infigure (b).

Brewster’s fringes Two plane-parallelFabry-Perot plates with exactly the same thick-


Brewster angle.

ness, or the thickness of one being an exact mul-tiple of the other, are inclined at an angle of 1to 2 and the interference due to white light isobserved. Straight line fringes calledBrewster’sfringesappear if the ratio of interferometer spac-ings is an exact integer. These measurements areuseful for accurate calibration of length such asthe determination of the standard meter.

Brewster’s law Refers to the state of polar-ization of reflected and transmitted light fromone optical medium to other.SeeBrewster an-gle.

Brewster window Suppose a beam ofp orTM polarized light beam is incident on a mate-rial, in the form of a plate with parallel faces: atthe Brewster Angleit will be transmitted entirelywith no reflection losses. Such perfect windowsare used extensively in lasers.SeeBrewster an-gle.

brightness Also calledluminosityof an ob-ject; brightness is the power per unit solid angleper unit projected area emitted or scattered. Theunits used in radiometry are watt per steradianper meter square.

broadcast Broadcast refers to a one-waytransmission of information to the general pub-lic tuned to the particular transmission channelwithin a given geographical area or to the gen-eral public in a given user domain, with thereceivers providing no acknowledgment of re-ceipt to the transmitter. The broadcast chan-nel thus consists of one transmitter but manyreceivers. Typical uses of broadcast includescommercial television and radio, and traffic sta-tion communication to ships or aircrafts. Ra-dio broadcasting uses four different frequencybands: pan-European broadcasting in the long-wave frequencies between 150 and 290 KHz,

AM (amplitude modulation), broadcasting inthe medium-frequency range of 525 to 1700KHz, long-distance international broadcastingin several short-wave sub-bands lying between5950 KHz to 26.1 MHz, and FM (frequencymodulation) broadcasting in the very-high fre-quencies from 88 to 108 MHz. FM broadcastingoffers superior fidelity and robust reception overAM broadcasting.

broad-side on (magnets) The broad-side onposition is a point that lies on a line through thecenter of a magnet perpendicular to the magneticaxis.

bubbles, suppression of (low temperature)At temperatures above the superfluid transitionin liquid 4He, the thermal conductivity is suffi-ciently poor as to allow local heating and therebyallow bubble formation. The thermal conduc-tivity of superfluid helium is very high, compa-rable to that of a metal. As a result, the localheating necessary for bubble formation cannotoccur, and superfluid helium is “quiet” with nobubbles formed in the interior of the fluid. Thissuppression allows visual identification of thesuperfluid transition in quite dramatic fashion.See alsohelium-4, superfluid.

bug A bug may refer to an error in a com-puter program or a defect in an apparatus. A bugmay alternately refer to a miniature concealed orsecret electronic gadget for eavesdropping.

bus A bus refers to a single conductor or agroup of conductors for power or signal trans-mission. The signal bus transmits data, ad-dress and control commands to facilitate dataexchange among various components in a com-munication or a computer system. In a computersystem, a bus embodies a standardized circuitinterface.

byte A byte refers to a batch of binary dig-its (or bits) processed as one unit. Eight-bitbytes are commonly used to represent an al-phanumeric character or a control signal in theAmerican National Standard Code for Informa-tion Interchange (ASCII). Two or more bytesmake up one word.


Ccable A cable is a thick wire or a bundleof wires inside an insulated covering used fortransmitting electric or electronic signals.

cable model, cell membrane Intracell com-munication across intracellular membranes maybe modeled after the theory of transmission linesin electronic engineering (cable model). Studiessuggest that the transmission of voltage signalsacross membranes occurs with an effective cablelength on order of 100 micrometers.

cadmium cell The cadmium cell was devel-oped by W. Jungner in the late 1890s and early1900s. It is a rechargeable cell employing thefollowing chemical reaction:

AgO + Cd+ H2O −→ Cd (OH)2 + Ag .

The cadmium cell is still in common use today asa voltage reference, at 20C its voltage is 1.0186volts. Cadmium cells are noted for their highenergy and power density, but the high cost haslimited their applications.

cadmium red Compounds of color pigmentsranging from orange through red made of solidsolutions of the semiconductors cadmium sul-phide and cadmium selenide.

cadmium wavelength standard An inter-nationally agreed standard that in dry air at 15degrees Celsius and a pressure of 760 mm ofmercury the red line of cadmium has a wave-length of 6438.4696 Å.

calorimeter, adiabatic (1) A calorimeterthermally isolated from all heat sources and/orsinks not actively involved in the experiment.If the temperature of a sample inside an adia-batic calorimeter were measured while no heatwas being added to the sample, the temperaturewould remain constant indefinitely. Experimen-tally it is possible to create such calorimeterswith heat leaks of below 1 nW.

(2) This is a bomb calorimeter for which theinsulating jacket temperature is kept equal tothat of the bucket. With the jacket and the buckettemperatures equal, there is no heat transfer outof the bucket and the necessary corrections forisothermal systems are removed.

calorimeter, Nernst vacuum A Nernstcalorimeter consists of a sample inside a vac-uum can that is immersed in a liquid cryogen. Athermometer and heater are attached to the sam-ple — in practice, they may be the same item.The sample is cooled through the thermal con-duction of a gas admitted into the vacuum can.After reaching thermal equilibrium, the sampleis thermally isolated by evacuating the vacuumcan. Then, the temperature is recorded whileknown amounts of heat are added to the sample.

calorimetry, low temperature Low temper-ature calorimetry is fraught with complexitiesbeyond those found at higher temperatures. Atlow temperatures, most materials under studyhave very small heat capacities, making smallheat leaks and the heat capacity of the surround-ing materials very important. Much of mod-ern low temperature calorimetry is designed tomeasure the specific heat of very small (meso-scopic – microscopic) samples attached to a sub-strate. In many of these experiments, the sub-strate and surrounding apparatus have a heat ca-pacity which is much larger than that of the ac-tual sample. This addendum heat capacity mustbe measured very precisely for such experimentsto have any meaning whatsoever.

At low temperatures, the boundary resistancebetween the sample and its surroundings be-comes quite large, requiring care to thermallyanchor the sample properly. As at higher tem-peratures, in order to measure the specific heat,it is necessary to also understand all possiblesources of heat input into the system. The lowtemperature experiment is sensitive to incredi-bly minute heat leaks. It is common in ultra-low-temperature calorimetry to be sensitive tostray heat at the tens of pW (1pW = 10−12 W)level!

At low temperatures, it is also necessary toconsider a wide variety of sources for heat in-put, including heat leaks from residual atoms ina vacuum chamber and the black body radia-


tion from surfaces at higher temperatures. As inall low temperature experiments, it is necessaryto measure the temperature accurately and withhigh precision. In measurements to determinethe specific heat, it is also important that the ther-mometers do not deposit a substantial amountof heat into the sample. As mentioned earlier,sensitivity to very small heat sources makes thisproblem additionally important.See alsoheatswitches; Kapitza boundary resistance.

camera Any instrument used to form an im-age. The simplest camera is a pinhole camera(seecamera, pinhole). A typical camera con-sists of a positive lens on one side of a box forforming a real image on a photographic plate ora screen situated on the other side of the box.This image is later developed or printed to getthe final picture. For fast moving objects or fora handheld camera, shorter exposure time re-quires large aperture for the lens, thereby neces-sitating corrections for various aberrations. Thematerials of the lens and the plate depend on thewavelength of the radiation involved.

camera, aperture mechanism of Theaper-ture mechanism,which controls the throughputof light from the object entering the lens, canbe set to a number of settings called f-stops orf-number. The f-number is the ratio of the fo-cal length of the lens system to the diameter ofthe aperture. The sequence of f-stops denoted asf/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 andf/32 imply aperture diameters beginning withthe largest at f/1.4 and decreasing so that succes-sive stops have half the throughput of the formerone. To keep the amount of exposure fixed, ifthe f-stop is increased by one step the shuttertime should be doubled.Seecamera.

camera, depth of field of Thedepth of fieldis the range of object distances in focus withoutblur. The depth of field is inversely related tothe aperture diameter. In order to increase thedepth of field and maintain the same throughputone should increase both the f-stop number andthe exposure time.Seedepth of focus; depth offield; camera.

camera, field of view of Thefield of viewisthe angular width of the object that can fit on the

film. It depends on the film size and the focallength of the lens. For example, a typical 35mm camera (i.e., the width of film is 35 mm)using a lens of focal length 50 mm has a fieldof view of 45 degrees. A telephoto lens of 200mm focal length has 10 degrees. In contrast, a28 mm wide angle lens will have a field of viewof 75 degrees.Seecamera.

camera, lens of The lens systemusuallyconsists of multiple lenses corrected for variouskinds of aberrations.Seeaberration; camera.

camera obscura Also known as a pin-holecamera. The earliest form of a camera with alight-tight box and a small hole. Light from anobject enters through the hole and an invertedimage is produced on a screen.

camera, pinhole The simplest optical deviceconsisting of a light-tight box with a very smallhole (pinhole) on one side which is placed to-ward the object and a photographic plate or ascreen on the opposite side. There is an optimalaperture size for a given distance of the aperturefrom the plate. With long time exposure, excel-lent inverted images of the extended objects canthus be formed without distortion. This cam-era is very useful in capturing the architecturaldetails of buildings.

camera, shutter mechanism of Theshuttermechanismcontrols the duration of time the filmis exposed, which can range from a few secondsdown to a thousandth of a second in steps oftwo. A leaf shuttercontains metal blades thatswing open momentarily. Expensive camerascontainfocal plane shuttersthat slide past thefilm. The exposure time is a critical parameterin fast action photography.Seecamera.

camera, television Instruments for convert-ing audio-visual information to electric signalsthat are ultimately used for display on a tele-vision screen. The basic process is either col-lection and acceleration of electrons when lightfalls on the surface of photosensitive cathodes(image-orthicon tubes) or recording changes inthe electrical conductivity of a photoconductivelayer deposited on one side of a transparent con-ducting film (Vidicon tubes).


camera, types of Cameras can be classifiedinto three types according to how the image isviewed by the photographer.

1. The oldest kind is theView Camera,builtlike an accordion, in which a large film can beused to record the details of a large landscapeor a group of people. The photographer viewsthe image on a glass plate and adjusts the pa-rameters such as the focus and composition ofthe image. The glass plate is replaced by thefilm and the picture is exposed. The image isinverted on the viewing glass plate and can bedisconcerting to a novice photographer. The ad-vantage is that one can tilt the lens relative to theplane of the film moving the bellows thus cor-recting for distortions due to foreshortening anddepth of focus.

2. The second type is areflex camera,whichpermits the film in place all the time. A mirrorbetween the lens and the film reflects the imageon a viewing screen. The mirror is positionedsuch that the image position is identical on thefilm without the mirror as it is on the viewingscreen with it in place. A single lens reflex (orSLR) also contains a five-sided prism that cor-rects the inversion of the image from left to rightcaused by the mirror reflection. When an expo-sure is made, the mirror swings out of the waymomentarily to admit light on the film.

3. The third type is aviewfinder camerawitha separate optical system to view the image andanother to focus on the film. Due to their sim-plicity such cameras are inexpensive but sufferfrom parallax. The two images may not coin-cide exactly. A range finder that views the imagefrom two different angles can reduce this defect.Seecamera.

candela Unit for luminous intensity equal toone lumen per steradian. It is defined as the lu-minous intensity of one sixtieth of one squarecentimeter of the projected area of a black bodyradiator operating at the freezing point of plat-inum (2042 K). Abbreviation: cd.

candle, international Seecandela.

candle power Luminous intensity equal toone candela.

capacitance Property of an electric conduc-tor, or set of conductors, that is measured by theamount of electric charge that can be stored onit per unit change in electrical potential.

capacitance, cell membrane A cell mem-brane, due to impermeability or diffusion-limited ion flow, has unlike charges distributedon either side. This is the basic structure of a ca-pacitor; a physical separation of unlike chargesand hence a capacitance may be associated witha cell membrane, the capacitance being the ra-tio of charge to potential difference across themembrane.

capacitance, distributed The effective ca-pacitance of a circuit network that is physicallydistributed through the circuit or device. Forinstance, microstrip antennas have some capac-itance per unit length, as well as resistance andinductance per unit length. The proper calcu-lation of all infinitesimal impedances will yieldan effective reactance (possibly mostly capaci-tance) that equals the measured capacitance inthe whole device, but which is not localized toa specific portion of the circuit.

capacitance, interelectrode The (often un-wanted) capacitance between the electrodes ofa device.

capacitance, junction In a diode, the capac-itance across the diode junction is termed thejunction capacitance. For abrupt junctions, thiscapacitanceCj is inversely proportional to thedepletion depth, and therefore

Cj ∝ (Vd)−12

(for a reverse bias voltageVd).

capacitive discharge When two terminals ofa charged capacitor are connected through a con-ductor, the charge on the capacitor will be anni-hilated by a current through the conductor. Thisphenomenon is calledcapacitive discharge.Thetime constant of the discharge depends on the ca-pacitance of the capacitor and the resistance ofthe conductor. In the process, the energy storedin the capacitor is dissipated through the Jouleheating of the conductor.


capacitive reactance Capacitive reactance isdefined as the inverse of the product of angularfrequency and capacitance, orXc = 1/ωC =1/(2πfC).

capacitor, blocking A capacitor placed in acircuit for the purpose of blocking a DC offsetvoltage from another portion of the circuit.

capacitor, by-pass A capacitor placed in acircuit to allow the high frequency componentsto bypass a part of the circuit. This may be toeliminate unwanted signals to a power supply,direct the AC signal elsewhere, or to protect thebiasing voltage of a device from dropping belowthe useful level when many devices become ac-tive at once. This eliminates power spikes and“brownouts” from the current drain of many de-vices clocking at once in a synchronous circuit,and is a crucial design feature in digital circuits.

capacitors Capacitors are electric compo-nents that are made of two conductors embeddedin a dielectric medium. Capacitors are indis-pensable electric components in an electric cir-cuit. They can be used to store electric charges,to block direct current flow, and to pass alter-nate current. Capacitors can be combined withresistors to form anRC circuit.

capacitors in parallel When two or morecapacitors are connected in such a manner thatone terminal of each capacitor is connected tothe same common pointA, while the other ter-minal of each capacitor is connected to anothercommon pointB, as shown in the first diagram,this is calledcapacitors in parallel.When ca-pacitors are connected in parallel, the voltagedifference between the two terminals of each ca-pacitor is the same. The equivalent capacitanceof capacitors in parallel is equal to the sum ofeach individual capacitance:

C = C1 + C2 + C3 + . . . .

capacitors in series When two or more ca-pacitors are connected in such a manner that onecapacitor is connected to the next sequentially asshown in the next diagram, this is calledcapaci-tors in series.When capacitors are connected in

Capacitors in parallel.

series, the inverse of the equivalent capacitanceis equal to the sum of the inverse of capacitanceof each individual capacitor:

C−1 = C−11 + C−1

2 + C−13 + . . . .

Capacitors in series.

cardinal planes/points A thick lens/lens sys-tem can be described in terms of six cardinalpoints allowing graphical determination of im-ages for arbitrary objects. These six cardinalpoints on the axis of a thick lens consist of firstand second focal points, first and second prin-cipal points, and first and second nodal points.The corresponding planes normal to the axis atthis point are calledcardinal planes.

cardinal points The six cardinal points of anoptical system are a) two focal points (F1 andF2), b) two principal points (H1 andH2), and c)two nodal points (N1 andN2). As shown in thediagram, ray 1 originating from the focal pointF1 emerges parallel to the axis on the other side.The intersection of these two rays defines thefirst principal plane, which cuts the axis atH1.Ray 2, parallel to the axis, passes throughF2 onthe output side. The intersection of these tworays defines the second principal plane, whichcuts the axis atH2. An off-axis ray 3, passingthrough one nodal pointN1 emerges parallel toit through another (N2). Planes passing throughthe cardinal points normal to the axis are calledcardinal planes. The sketch below identifiesthese points and planes in an optical system.V1

andV2 are the input and output planes. The re-fractive indices in input, optical system and out


media aren3, n2 andn1, respectively. In termsof theABCD matrixelements, the distances tovarious cardinal points, as defined in the figurebelow, are

p = DC , q = −A

C , r =D−n3

n1C , s = 1−A

C , v =D−1

C , w =n3n1−A

C , f1 = p− r andf2 = q − s.

Cardinal points and cardinal planes.

Note that the distances obey the followingsign convention:p, r andv are taken positive ifthey are to the right ofV1 and negative other-wise. q, s andw are positive if they are to theright of V2. Similarly f1 andf2 are positive ifthey are to the right of their respective principalplanesH1 andH2 and negative if to the left. Asan example,H1,H2, N1 andN2 coincide at thecenter of a refracting sphere irrespective of itsradius of curvature and refractive index. How-ever,p = −q = R(2−n)

2(1−n) for a sphere of radiusR and refractive indexn. SeeABCD matrix.

carrier A carrier is the basic continu-ous waveform or pulse train on which theinformation-bearing signal is to be modulated inorder for many channels to share one transmis-sion medium. The particulars about the carrierare defined by the particular communicationssystem apart from the particular information-bearing signal to be transmitted. The carrier isoften a sinusoidal wave at a specific frequencyset orders-of-magnitude higher than the band-width of the information-bearing signal. Thecarrier may alternatively be a similarly high-frequency periodic pulse train or simply a con-stant direct-current (DC) voltage offset. A setof distinct carriers may be viewed as a strat-egy to partition the total available transmissionbandwidth into distinct transmission channels.

In order for an information-bearing signal tobe transmitted on any of these channels, theinformation-bearing signal modulates the car-rier corresponding to the channel to be used. Un-modulated versions of the carrier may or may notbe transmitted alongside the modulated carrier.If not, the carrier is said to have been suppressed.At the receiver, the information-bearing signalis decoupled from the carrier.

carrier binding The attachment mechanismfor the cotransporter binding to the transporterin carrier-mediated diffusion.

carrier frequency The carrier frequencyrefers to the frequency of an unmodulated sinu-soidal wave carrier or the pulse repetition rate ofa periodic pulse-train carrier. In frequency mod-ulation schemes, the carrier frequency is equalto the center frequency.

carrier level The carrier level refers to thepower level of the unmodulated carrier at a spe-cific position within the communication sys-tems; may be expressed in absolute terms inwatts or in relative terms, in reference to somesystem ground level, in decibels (dB).

carrier, majority, of current In doped semi-conductors, one polarity of current carrier willprovide the dominant charge transfer mecha-nism, usually by simply being the most plen-tiful. These types of carriers are the majoritycarrier.

For instance, in boron doped silicon (p-type),the boron is an electron acceptor. This allowsholes to be the majority carrier of current. Inn-type semiconductors, electrons are the majoritycarriers.See alsoacceptor; donor.

carrier mediated transport, macroscopicmodel General description of the transportmodel using free energy, kinetic, and entropicmodelling for the prediction and the descriptionof the rate constants.

carrier mediated transport, microscopicmodel Detailed molecular modelling of thestoichiometry and energy analysis of the carriertransport mechanism.


carrier mediated transport, steady stateprocess Transport of biomolecules through amembrane restriction not normally permeable tothe biomolecules. The biomolecule reacts witha transport moleculeA to form transportablemoleculeB which passes through the mem-brane.B is then decomposed to yieldA and theoriginal biomolecule.A diffuses back throughthe membrane to start the process again.

carrier, minority, of current The minoritytype of charge carrier in a semiconductor. Forexample, inp-type boron doped silicon, mostcharge transfer is through hole migration. How-ever, some electrons do diffuse in the crystal.These electrons are minority carriers. Inn-typesemiconductors, the holes are the minority car-riers. Seecarrier, majority, of current.

carrier models (cell) Modelling of the ac-tive transport of biomolecules across mem-branes upon reversible attachment to a trans-porter molecule.

carrier power The average power of an un-modulated sinusoidal carrier over one sinusoidalperiod. The carrier power may deviate fromthe nominal value when flawed modulation pro-duces unequal envelope amplitudes on the pos-itive and negative sides. This phenomenon iscalledcarrier shift.

carriers (cell) A molecule to which an-other molecule may become reversibly attachedfor transport through a membrane, in effect, amolecule that can be reduced by attachment tothe second molecule and after transport may bere-oxidized.

carrier-to-noise ratio The carrier-to-noiseratio refers to the ratio of carrier power to noisepower at the same position within the commu-nication system. The carrier-to-noise ratio istypically expressed in decibels (dB).

carrier wave A wave in which amplitude,frequency and/or phase are varied in time to pro-duce modulated oscillations (signals). The fre-quency of the carrier wave is much greater thanthose of amplitude, frequency or phase modula-tions.

CARRY, half-adder The flag in binary ad-dition signifying the carry operation.

A half-adder is a binary logic circuit thatgives two outputs for two binary inputs. Oneoutput S (for Sum without carry) is, in the sin-gle binary digit case, an exclusive OR operationof the inputs. The other output, C (for carry) istrue if S is not the binary sum. In the simple bi-nary digit case, this is the AND operation of theinputs. For this case, given single binary digitinputs A and B, the S and C outputs are

A 0 0 1 1B 0 1 0 1C 0 0 0 1S 0 1 1 0

Since in general the carry is only a flag,two half-adders (plus some logic operations) areneeded for true binary addition.

cartesian surface Refracting or reflectingsurfaces that form perfect images, named afterRené Descartes. Each object point requires itsown surface. In case of reflection, such surfacesare invariably conic sections.

Cary-Foster bridge Cary-Foster bridge isused to measure the small difference betweentwo nearly equal resistances. The bridge is ofthe slide-wire type and the resistance of the slidewire per unit length,r, is accurately known.Rx

andRs are to be compared. A balance is firstsecured with the contactC at a distancea1 fromtheD. ThenRx andRs are interchanged andanother balance obtained withC at a distancea2 fromD. It can be shown that.

Rx −Rs = (a1 − a2) r .

Cassegrain telescope In this design of thereflecting telescope, the light from the primarymirror is incident on a secondary mirror shapedas a convex hyperboloid. The light reflected bythe secondary mirror passes through an aperturein the primary mirror to secondary focus,fs, forviewing. The primaryfp and secondaryfs focalpoints are adjusted to coincide with the foci ofthe hyperboloid which results in a distortion freeimage.


Cary-Foster bridge.

Cassegrain telescope.

cataracts A disorder that causes pockets ofcloudy or opaque discoloration of the lens tis-sue of the eye; a surgery involves removal ofthe lens and replacing it with a plastic implant.Often the membranes that hold the implant be-come opaque and a corrective procedure knownasposterior capsulotomyuses a high power laserto rupture the membrane and restore vision.

catheters, blood pressure measurement Acatheter is any instrument designed for the freeflow passage of fluid into or out of the body.When used for blood pressure measurement thecatheter allows the blood to be in direct con-tact with a manometer thus allowing direct bloodpressure measurement.

cathode The electrode that emits electronsinto a space, medium or device. May also beviewed as the path for positive current to leavethe medium.

cathode ray oscilloscope A device based ona CRT that allows visualization of input signal

voltage waveforms. There are two main modesof operating the device.

1. One mode is a two-input mode where thexcoordinate of the electron beam is proportionalto the first input, while they coordinate is pro-portional to the second.

2. The other mode uses a synthesized volt-age for thex input signal that is proportional totime (modulo the repeat time). When they po-sition is proportional to the input voltage, a rep-resentation of the waveform (voltage vs. time)is traced out on the screen. If the signal wave-form is periodic, triggering circuitry allows thephase matching of successive traces, producinga sustained trace of the waveform.

cathode ray tube An evacuated tube or bulbin which a voltage difference of separated con-ductors produces accelerated electrons (cathoderays). These electrons may be used in the pro-duction of X-rays, for physical measurements,or other purposes. However, the most promi-nent use of the cathode ray tube (CRT) is thefocusing of the electron beam in a phosphores-cent surface for the displaying of information intwo dimensions.

Most television screens, computer monitorscreens, and oscilloscope displays are the phos-phorescent end of a CRT. The x and y posi-tion the beam strikes is controlled by a mag-netic (or sometimes electric) field perpendicu-lar to the electron beam direction. The intensitymay be varied by control circuitry in the elec-tron gun producing the beam. This electron gunalso has the cathode/heater circuit, focusing el-ements and the accelerator system.

catoptrics Optics of reflecting surfaces.Telescopes and microscopes built entirely of re-flecting optics are calledcatoptric systems.Incontrast, systems with a combination of lensesare calleddioptric systems.Combinations oflenses and mirrors arecatadioptric systems.

Cauchy dispersion formula The refractiveindex of transparent material is dependent onthe wavelengths. When the refractive index isplotted as a function of the square of the wave-lengths, the resulting curve is known as the dis-persion curve which appears to be asymptote inthe UV region and is somewhat linear in the


near infra red (up to 1µm). This is callednor-mal dispersion. When the refractive mediumhas characteristic excitation that absorbs light ofwavelengths within the range of the dispersioncurve, the curve in general will be monotoni-cally decreasing, but will have a positive slopein the wavelength region of the absorption. Thisis calledanomalous dispersion.

An empirical relation that givesn as a func-tion of λ for normal dispersion is called theCauchy dispersion formula:

n = a+ bλ2 +c

λ2 − 0.028+

d

(λ2 − 0.028)2

wherea, b, c andd are empirical constants. Amore general expression is

n(λ) = a+b

λ2+

c

λ4+ . . . .

Often, the first two terms in the above expressionare sufficient to provide a reasonable fit and if wehave experimental knowledge ofn at 2 distinctwavelengths, then the constants can be deter-mined. The dispersion is defined asdn

dλ , and isapproximately equal to−2b

λ3 .

caustic curve The geometrical envelope ofthe meridian section of a bundle of refracted orreflected rays. The points of intersections ofpairs of consecutive rays lying in the plane ofa meridian section of a refracting or reflectingspherical surface form a curve lying symmet-rically above and below the optical axis, if theincident bundle is symmetrical with respect tothe optical axis. This plane curve is called thecaustic curveof the meridian rays. The twobranches on the opposite side unite in a doublepoint or cusp at the point on the axis where theparaxial rays intersect, so that the axis is tangentto both branches at this point. Each refracted orreflected ray in the meridian plane touches thecaustic curve.

cavitation Formation and collapse of cavitiesand bubbles in liquids, filled with gas and vapor.Cavities and bubbles may be formed by severalmechanisms: due to working pumps or rotatingturbines and ship propellers (hydrodynamic cav-itation); due to intense sound radiated into a liq-uid (acoustic cavitation); and due to laser beams

and elementary particles propagation through aliquid. In the cases of hydrodynamic and acous-tic cavitation, formation of cavities and bubblesoccurs at points where the local pressure is be-low a threshold that allow so-called cavitationnuclei (tiny bubbles filled with gas or vapor) togrow. Collapse of cavities and bubbles produceintense noise. This collapse can also destroymaterials of different kinds, for example, shippropellers.

cavity dumper Energy can be built up in alaser cavity for a length of time and deflectedoutside the cavity to get a sudden burst of en-ergy. Such a device is called a cavity dumper.An acousto-optic deflector placed inside a lasercavity can spoil the alignment temporarily whenturned on and dump the energy.Seeacousto-optic deflector.

cavity modes of a laser A laser resonator cansustain two types of resonant modes of oscilla-tion depending on the separation and curvatureof the mirrors. These are (a)axial or longitudi-nal modesand (b)transverse modes.

If L is the length of the laser cavity resonator,the axial mode frequencies arefm = m

(c

2L

)wherem is an integer (usually very large) andc is the speed of light. The separation betweensuccessive modes is the free spectral range ofthe resonator, and a typical laser transition lineis broad enough to accommodate several modes.A single axial mode can be sustained at the ex-pense of others by inserting an etalon of appro-priate length in the cavity.

The transverse modes, denoted by TEMmn

(transverse electric magnetic;m andn are in-tegers), have characteristic intensity patterns inthe plane normal to the beam direction. Thelowest order TEM00 mode has a Gaussian pro-file of intensity over the cross section. Some ofthe other less desirable patterns, sketched below,have multiple spots. The electric field directionsare shown by arrows.

cavity resonance Resonant vibrations in acavity. Cavity resonance occurs when the fre-quency of a sound wave incident on a cavity isequal to one of its natural frequencies. In thiscase, the ratio of the acoustic pressure ampli-tude inside the cavity to that of the incident wave


Cavity modes of a laser.

reaches its maximum. Cavity resonance is usedin acoustic devices to amplify specific frequen-cies of a complex sound. A resonant cavity iscalled aresonator.

CCD Charge coupled device, a planar de-vice for holding and moving charge in two di-mensions. The charge is managed by potentialwells controlled by pixel electrodes over (or un-der) the area of the well. These charge bins canbe multiplexed out to an analog to digital con-verter or any other application.

CCDs are generally used in optics (likevideo cameras, telescope readout electronics,or infrared imagers). However, other uses arealso possible, such as charged particle radiationtracking detectors.

cell An electric device that converts chemi-cal energy into electric energy. There are manydifferent types of cell depending on the materi-als or type of chemical reaction employed. Acell consists of a positive and a negative elec-trode immersed in a chemical solution (elec-trolyte). Cells are classified into two ma-jor types: primary (non-rechargeable) and sec-ondary (rechargeable). In both types of cells,the electric energy released is derived from thechemical reaction that takes place between theelectrodes and in the electrolyte.

centered optical system An optical systemwhere all surfaces are rotationally symmetricabout a common axis.

centrifugation Separation of molecular orparticle species by placement in a rapidly rotat-ing environment. The particles with the greatercentripetal force flow to different depths in thegel medium. The centripetal force being equalto mω2r whereω is the angular speed of thecentrifuge,r is the radius from the center of ro-tation, andm is the mass of the particle.

centrifugation, isopycnic Centrifugation ofa mixture when the separation is based on den-sity, not mass difference.

centrifuges Instruments consisting of tubeholders distributed in a circular array about acommon axis of rotation and capable of achiev-ing very high frequencies of revolution. Thisgives rise to a strong centripetal force (some-times referred to by the non-inertial term cen-trifugal force) yielding separation of mixturesin solution.

ceramic magnets These are ferrimag-nets composed of the hard magnetic materialBaO.6Fe2O3.

Cerenkov radiation Radiation emitted bycharged particles traveling in a medium at aspeed faster than the speed of light in thatmedium. The wavefront of such a shock wavewill be in the shape of a cone in three dimensionswith the apex at the source. The half angleα is

given byα = sin−1(

VVs

)whereV andVs are

the speeds of light and that of the charged par-ticle, respectively, in the medium in question.One can observe a blue shimmer of Cerenkovradiation in nuclear reactors with the core im-mersed in a pool of water. The speed of chargednuclear fragments can easily exceed the speedof light in water which is about two thirds thespeed of light in vacuum.

channel A channel represents what separatesthe transmitter from the receiver in a communi-cation channel. The channel embodies a com-munication connection for the transfer of infor-mation signals from the data source to the datasink. The channel may be unidirectional or bi-directional in how the information may flow. Achannel may correspond to one particular car-rier frequency in a frequency-division multiplexcommunication system, a particular time-slot ina time-division multiplex system, or a particu-lar spreading code in a code-division multiplexsystem. The channel may distort the transmittedsignal if the transmitted signal is altered by otherthan a real-valued constant multiplicative factorand/or a constant time delay, thereby modifyingthe signal shape.


channel (cell) A corridor through the lipidbilayer of the plasma membrane often created bymacromolecular proteins. This channel allowsan imbalance of electrical potential across themembrane resulting in the assisted flow of ionsthrough the plasma membrane.

channel bank A channel bank embodies thatpart of a carrier-multiplex terminal that multi-plexes a set of channels onto a higher frequencyband or demultiplexes a higher frequency bandinto distinct channels.

channel capacity The channel capacity of achannel refers to that channel’s theoretical max-imum information transfer rate, typically de-noted asC. If the information rate of the in-formation signal (typically symbolized byR)remains below the channel capacity, then arbi-trarily small error probability may be attained bysuitable signal coding. Otherwise, transmissionerror is unavoidable, regardless of any coding.For a bandlimited channel (with a bandwidth ofB Hz) affected by additive white Gaussian noisewith signal-to-noise ratio atS/N , the channelcapacity (C) equalsC = Blog2(1 + S/N) bitsper second. Channel capacity rises with in-creasing bandwidth or improved signal-to-noiseratio. The bandwidth and the signal-to-noiseratio thus become two design variables to betraded off each other for any particular com-munication system and application. For exam-ple, limited power resources onboard a spacesatellite means wider bandwidths may be usedfor a lower signal-to-noise requirement. It isfalse that channel capacity would become un-limited as channel bandwidth grows towards in-finity. This is because the wider the bandwidth,the more channel noise there would be whilesignal power remains constant, thereby decreas-ing the signal-to-power ratio. If noise is com-pletely absent,S/N equals infinity, andC be-comes unlimited regardless of channel band-width. If the additive channel noise is other thanwhite Gaussian, then channel capacity may ex-ceed that given by the above expression.

channel capacity and distortion Channeldistortion occurs whenever the transmitted sig-nal is altered other than by a real-valued constantmultiplicative factor and/or a constant time de-

lay, thereby modifying the signal shape. The in-formation signal may also be distorted by signaltransients during modulation; such distortion,called characteristic distortion, depends in parton the particular information signal and carriersignal concerned.

channel capacity and interference For aGaussian interference channel with power con-straint, the channel’s capacity region in the pres-ence of very strong interference is surprisinglyequivalent to the case when there exists no in-terference.

channel capacity and rate distortion If thesource information rateR exceeds the channelcapacityC, then distortion must necessarily oc-cur, regardless of the type of source coding andchannel coding used to process the source datastream. The distortion incurred in representinga source alphabetcX by a reproduction alpha-bet ˆcX is measured by the distortion functionDof that particular source alphabet-representationalphabet set. The rate distortion region of asource is defined as the closure of the set ofachievable rate distortion pairs(R,D). The ratedistortion function,R(D), is defined as the in-fimum of ratesR such that(R,D) lies in therate distortion region of the source for a givendistortionD.

channel capacity and self-information Theself-informationI(X = xn) of the eventX =xn (that is, symbolX is valued atxn) is definedas:

I (xn) = log2

1P (xn)

.

The self-information, when averaged over allIgives the entropyH(X) of a source:

H(X) =N∑

n=1

P (xi) I (xn) .

Both I(xn) andH(xn), as defined above, haveunits of bits per symbol. If the source’s symbolrate equalsR symbols per second, thenH(X)R≤ C or transmission error must occur despiteany encoding.

channel capacity, energy per bit To suc-cessfully transmit information from an informa-tion source with information rateR through a


channel of capacity ofC bits per second, band-width B Hz and additive Gaussian noise levelNo, the minimum signal power level in energyper bit equals:

Eb =NoB

R

(2

CB − 1

) ∫ T

0s(t)2dtK

.

channel kinetics (cell) The study of the mo-tion of ions and particles in membrane channels.

channel noise level The channel noise levelequals the noise power in the channel, typi-cally expressed in decibels (dB). The channelnoise level may also be defined relative to thenoise power at some reference point. Channelnoise may arise from thermal noise, intermod-ulation products, adjacent channel crosstalk, orother unwanted interference. The channel noisepower density measures the noise level per hertzof channel bandwidth.

Chapman-Kolmogorov relation A predic-tive mathematical relation between system pa-rameters subject to random, or chaotic, noisegeneration.

character A character embodies a uniquelydefined cluster of consecutive bits representingan alphanumeric mark (such asA, 9), a non-printable control token (such as theescapechar-acter or thecarriage-returncharacter). A fi-nite ordered set of distinct characters constitutea character set. Eight-bit bytes are commonlyused to represent a character in the AmericanNational Standard Code for Information Inter-change (ASCII) character set. The charactermean entropy of a particular information source,in units of shannon per character, measures theinformation-content of that source.

character generator The character genera-tor embodies a device used to control a displaywriter to display any character graphically onthe display device’s display surface (such as thedisplay screen of a cathode-ray tube).

characteristic curve A plot showing the rela-tionship of two variables, which may show use-ful information about a device. For instance,current vs. voltage for a diode, collector current

vs. base voltage for a transistor with a specifiedemitter voltage, or the capacitance vs. reversebias voltage for a diode junction.

characteristic function The characteristicfunctionC(f) of a random variablexwith prob-ability distributionp(x) is defined as:

C(f) =∫ ∞

−∞p(x)ej2πfxdx .

In other words, the characteristic functionC(f)embodies the Fourier transform ofp(x). Thecharacteristic function facilitates the determina-tion of the moments (i.e., expected values ofpowers) of the random variable as follows:

E xn = (j2π)−n dnC(f)dfn

|f=0 .

Moreover, the characteristic functionCx+y(f)of a sum of two random variables (x and y)equals the product of the two random variables’respective characteristic functions (Cx(f) andCy(f)):

Cx+y(f) = Cx(f)Cy(f) .

charge coupled device Consists of a two-dimensional array of photo diodes on a sub-strate. The incident radiation produces elec-tric charges that are stored in a potential wellcreated by the gate voltage. The stored chargein each pixel is proportional to the irradiancewhich is later scanned and read out along rowsof such devices referred to as CCD. The chargeis cleared and the device is reset for the nextmeasurement. By a sequential reading of thestored charge a two-dimensional image is con-structed. Such CCD arrays are used in detectorsin telescopes and spectrographs, television andvideo cameras.See alsoCCD.

charge density Charge per unit volume iscalled charge density. Charge density is an im-portant physical quantity that appears in manyphysical laws. For example, Gauss law statesthat the divergence of the electric field is equalto 4π times charge density.

charge, diffusion Whenp-type andn-typesemiconductors join together to form a diode,


some electrons fromn-type semiconductors willmigrate intop-type semiconductors, while someholes fromp-type semiconductors will migrateinto n-type semiconductors. These minoritycharge carriers are called diffusion charges.

charge, electric A fundamental characteris-tic of matter. All fundamental particles can beeither positively charged, negatively charged, orneutral. Charge is quantized, which means thatcharges come in multiples of1.602 × 10−19

Coulomb, the amount equal to the magnitudeof the charge of one proton, or one electron.

charge injection device This is similar to acharge coupled device (CCD) with the excep-tion of the method of processing of the photo-induced charge. The charge is injected to theunderlying substrate semiconductor.Seechargecoupled device.

charge transfer device Any device capableof storing signal charge, and transferring thatcharge from one capacitor to the next at someclock rate. A CCD is a specific type of chargetransfer device, which uses field-produced en-ergy wells without discrete capacitors.

charging by friction When two differentsubstances rub against each other, one may giveup its electrons while the other may gain elec-trons. Once they are separated, one carries pos-itive charges while the other carries negativecharges. This process is calledcharging by fric-tion.

charging/discharging capacitors When abattery is connected to a capacitor, the positivecharge flows into one terminal of the capacitorwhile the negative charge flows into the otherterminal until the potential difference betweenthe terminals reaches the emf of the battery. Thisis called thecharging of a capacitor.In this pro-cess, energy is converted from chemical energystored in the battery to the electric energy storedinside the capacitor,ε = 1/2QV = 1/2CV 2

. Discharging a capacitor is the reverse pro-cess of charging a capacitor. When a resistor isconnected to the two terminals of a charged ca-pacitor, a current will flow through the resistor.This process is called thedischarging of a ca-

pacitor. The energy stored in the capacitor willbe dissipated through Joule heating.

chemiluminescence The luminescence, oremission of light, due to chemical reactions tak-ing place. If the chemical reactions are of anorganic nature it is also referred to asbiolumi-nescence.

Child-Langmuir Law The relationship be-tween current in a tube (thermionic diode) andthe applied voltage and distance. For the cath-ode currentI limited by space charge

I = GV32

whereV is the applied anode voltage andGis termed the perveance. In general the per-veance depends on the geometry, and, for in-stance, may be inversely proportional to the dis-tance between cathode and anode.

chip A chip may refer to an integrated circuitor to one basic time unit between signaling tran-sitions for one digit such as in a pseudo-random(PN) sequence used in code-division multipleaccess (CDMA) communications.

chirality A property of left/right, or mirror,asymmetry of a molecule. The molecule is dis-tinguishable in such a way that it cannot be su-perimposed on its mirror image. Chirality is acause of asymmetric optical scattering.

chirp A short, high pitched sound.

choke An inductor in some specific applica-tion is called a choke. It consists of many turnsof wire wound on a support.

cholesteric crystals These are organic com-pounds that can flow like liquids yet maintaintheir molecular orientations. Such liquid crys-tals have a helical structure and exhibit verylarge optical rotatory powers. The polarizationaxis of an incident beam can rotate by as muchas 40,000 degrees per mm of the liquid crystal.The pitch of the screw-like molecular structure ismuch smaller than that in crystals such as quartz.

chopper A simple switching circuit that opensand closes the primary circuit at some clocking


rate. The time open need not be any set fractionof the period.

chromatic resolving power mR The termused to denote the limit of resolution betweentwo neighboring wavelengths as observed in adiffraction grating withN slits. Application ofthe Rayleigh criterion yields

R = mN ,

for themth order of the spectrum.

chromatography A percolation procedurewhere particle, chemical, or pigment separationis achieved by passing the substance through atwo phase (usually liquid/solid) system in theearth’s gravitational field.

chromatography, exclusion Chromatogra-phy in which the gel phase is a stationary phaseof controlled pore size. Hence, molecules areseparated as a function of their size and theirshape (exclusion).

cineradiography A motion picture radio-graph, or in today’s terms a video radiographof a moving biological organ such as the heart,the lungs, or blood flow through a constriction.Another related process is cinefluorography.

cipher A cipher represents a cryptographicsystem transposing or displacing alphanumericcharacters in a plain text with other alphanu-meric characters in accordance with predeter-mined code. A cipher may also refer to such acryptosystem or a message written or transmit-ted in such a cryptosystem.

ciphony Ciphony (or cyphony) refers to theuse of ciphering for telecommunication signals,typically for confidentiality.

circle of least confusion When a circular ob-ject is imaged by a sphero-cylindrical or cylin-drical lens, the circle of least confusion is thecircular section found in between the two im-ages forming the Conoid of Sturm. It is dioptri-cally halfway between the two images and hasthe smallest cross-section of the images.

circuit A circuit consists of various elec-tric/electronic components connected togetherto perform a specific task. It could be as simpleas a flash light that consists of batteries, resis-tor, and a switch, or it could be as complex as acomputer motherboard. When a circuit is builtmonolithically on a single semiconductor wafer,it is called an integrated circuit,IC.

circuit, active A circuit that contains one ormore active elements or devices. These devicesdo not merely store or dissipate energy, but mayintroduce energy into the primary circuit.Seeactive device.

circuit, astable A circuit that oscillates be-tween unstable states of the device. Devicesthat alternate between the states at a regular fre-quency can be used as a clock for synchronousswitching circuitry.

circuit, bipolar A circuit including a bipo-lar element or device.Seetransistor, junction;bipolar code.

circuit, bootstrap An amplifier circuit usingresistors to lower the effective input impedance.

An AC amplifier and a bootstrap version of the same

circuit.

circuit breaker A switch that automati-cally disconnects the circuit when the currentflowthrough becomes larger than a preset value.

circuit, clipping A circuit that limits themaximum or minimum voltage level of a sig-nal that traverses it.

circuit, logic A switching circuit in which thequantized states represent logical states. This


allows the definition of circuit elements anal-ogous to logical operations (AND, OR, NOT,etc.). These can also be seen as logical ones andzeros for use in any type of binary numberingsystem.

All computer circuits are logical circuits. Anexample of an actual (in this case binary) logiccircuit type is the standard TTL, for bipolartransistor-transistor logic, where an output aboutthe high voltage represents true or logical one,and no voltage (or below the low cutoff) repre-sents false or logical zero. Additionally, othervalues may be chosen to represent the logicalstates, including current and optical intensity.

circuit, master-slave A segmented circuitin which one portion (the master) provides thetime base or other specification for the othersegments (slaves). For instance, a master clockmay provide timing signals, to which the slaveclocks must synchronize, providing the mastertime base to the slave circuit instead of an inde-pendent clock pulse.

circuit, parallel tee A bridge circuit contain-ing at least five known impedances, with a sixthunknown impedance element from one diagonalto common. Balancing the known impedancesallows for the calculation of the unknown value.

So named because the oscillating signal sentthrough the other diagonal can be seen as trav-elling through two T-circuits in parallel.

A parallel tee bridge circuit.

circuit, passive A circuit that contains onlypassive elements, storing and releasing energyapplied to the circuit.Seepassive device, activedevice.

circuit, switching Early switching circuitswere devices that selected which additional cir-cuits to complete. The limited number of (quan-tized) selections possible, along with the historyof the development of digital circuitry, has al-lowed an extension of the term to include anycircuit that handles quantized (or digital) infor-mation or outcomes.

Relay circuits and phone dialing systems areswitching circuits, but so are digital computers.Since the success of digital computing, mostswitching circuits are purely binary.

There are two main types of switching cir-cuits. Combinational, in which the inputsuniquely define the switch/output state, andstate-dependent (or sequential), where the inter-nal state(s) or memory of a device may be set,changing the response to inputs.

circuit, synchronous A logic switching cir-cuit in which all the state changes occur onsystem clock pulses (at the same time or syn-chronously).

circular intensity differential scatteringOptical spectroscopy using the scattering of cir-cularly polarized light to elucidate structure andfunction of optically active materials. A non-destructive investigation.

circular polarizer A device that can pro-duce or analyze circularly polarized light. Whenunpolarized light passes through a linear polar-izer followed by a quarter wave plate (90 degreephase retarder) with their principal axes at 45degrees from one another, the emerging lightis circularly polarized. The handedness (left orright) depends on whether the transmission axisof the polarizer makes+45 or −45 degrees tothe fast axis of the retarder.

cisternography A specific investigative toolusing radioactive contrast imaging (roentgenog-raphy) for visualization of the subarachnoidspaces containing cerebralspinal fluid.


citizen’s band (CB) The citizen’s band (orCB) represents a frequency band in the electro-magnetic spectrum allocated by governments inthe U.S., Canada, Germany and other countriesfor short-range voice communications amongprivate individuals. In the U.S., CB occupiesfrom 26.965 to 27.225 MHz and from 460 to470 MHz. Due to government regulations onCB radio transmitter power and receiver antennaheights, CB radio in the U.S. may typically reachup to 15 miles when used at a mobile or up to30 miles at a fixed location.

cladding, fiber optic Fiber optic materialsare usually encased in a material of lower refrac-tive index than that of the fiber. This claddingmaterial prevents degradation of light in the fiberby protecting it from surface scratches, dust,moisture, etc. Another important function ofthe cladding material is to prevent the frustratedtotal internal reflection from occurring. Underthe condition of total internal reflection, a longi-tudinal electric field exists at the surface of thefiber and the electromagnetic energy, whose am-plitude exponentially drops, penetrates acrossthe boundary. It can couple with the externalmedium leading to leakage and cross-talk of thesignal. The maximum acceptance angle,θm, oflight that can transmit through an optical fiberof indexn1 enclosed in a cladding material of

index n2 is θm = sin−1

√(n2

1−n22)

n0. The re-

fractive index of the surrounding medium isn0.

Cladding, fiber optics.

clamping diode The diode used in a clampcircuit, which is capable of adding a DC offsetto an input signal.

coagulation The transformation of a liquid toa gel via chemical reaction rather than by evap-oration. For example the coagulation of bloodupon exposure to air.

Clamping diode.

coaxial cable A circular cross-section trans-mission line made up of a central conductor sur-rounded by a dielectric layer which in turn iscovered by a metallic shield. An insulated coat-ing or outer layer is often used to protect theshield conductor.

cochlea A section of the inner ear lookinglike a spiral canal consisting of two and a halfturns around the modiolus and containing theorgan of Corti.

code A code represents an unambiguous setof explicit rules to represent information. Acode set means the set of all possible code val-ues allowed by the code. A code book embodiesa systematic ordering of the set of all codes ina particular coding system and the respectivecharacters they encode. Encoding refers to theconversion of information via such a set of rulesinto a specific code value. The source encodermaps the original data from one alphabet set toanother alphabet set, which may or may not beidentical as the first set of alphabets. Decod-ing refers to the recovery of information given aparticular code value.

code division multiple access (CDMA)Code division multiple access (CDMA) repre-sents a multi-user transmission coding schemewherein each user is assigned its unique pseudo-random signature spreading code sequence atthe transmitter to modulate (i.e., to spread) itsinformation-bearing message bits and at the re-ceiver to demodulate (i.e., to de-spread) the re-ceived digits back into the information-bearingmessage bits. Each user’s information-bearing


signal is recovered at the receiver by correlatingthe received signal with that user’s unique signa-ture sequence. All system users simultaneouslyshare the same frequency band but are mutuallydistinguishable by their respective unique signa-ture sequences. Different users in a CDMA sys-tem are typically assigned orthogonal or near-orthogonal signature spreading code sequence,such that the transmitted digits from differentusers have no or little interference against eachother at the receiver after de-spreading. CDMArepresents a spread spectrum transmission tech-nique because the bandwidth of the transmit-ted signal is spread to become much wider thanthe bandwidth of the original message signal.The spreading may involve amplitude modu-lation (as in direct sequence CDMA) or fre-quency modulation (as in frequency hoppingCDMA). The aforementioned spreading processdisperses signal energy over a very wide fre-quency spectrum, rendering it very difficult forhostile jammers to detect or to jam the transmit-ted CDMA signal.

coercive force (Seehysteresis.) Coerciveforce is the value of the magnetic field to beapplied to magnetic materials that exhibit hys-teresis in order to reduce the intensity of its mag-netization to zero.

coercivity This is a measure of the ease withwhich materials may be magnetized. Materialswith a high coercivity are difficult to magnetize.

coherence The phase correlation betweentwo distinct parts in time or space of the radiationfield of electromagnetic radiation. If the corre-lation is due to the same field, it is described asself coherence;the correlation between two dif-ferent fields is calledmutual coherence.This isthe property that enables interference betweentwo fields resulting in a pattern of bright anddark fringes. The fringe visibility i.e., contrastbetween successive bright and dark fringes, is ameasure of thedegree of coherence.The phasecorrelation between the wave functions of sub-atomic particles (e.g., neutrons, electrons) canalso exhibit coherence and interference.

coherence, acoustic Correlated in spaceand/or time behavior of two or more acous-

tic waves (oscillations); correlated behavior ofparts of a single wave, taken at different spatialpoints or at different time moments. Acousticcoherence is similar to optic coherence. Acous-tic waves (oscillations) are coherent only if thedifference between their phases is changed de-terministically, not randomly. Random fluctua-tions in phases and amplitudes of acoustic wavesoccur, for example, when they propagate in aturbulent or random medium (in a turbulent at-mosphere, ocean, etc.). One of the quantitativemeasures of coherence in a turbulent or randommedium is the coherence function of a soundpressure fieldp, determined byΓ(R,R′, t, t′)= 〈p(R, t) p∗(R′, t′)〉. Here,R is the Carte-sian coordinates,t is time,〈〉 denotes ensembleaverage, and the asterisk denotes complex con-jugation. Γ is 1 if R = R′ and t = t′, anddecreases if the difference|R−R′| (or |t− t′|)is increased.

coherence, degree of The normalized cor-relation function,γ12, between two radiationfieldsE1(t) andE2(t+∆t) as a function of∆tdepends on the time variation of the phase differ-ence between the two fields. For two fields withequal amplitudes the interference fringe visibil-ity, V , is

V =Imax − Imin

Imax + Imin= |γ12| .

The visibility, V , which is a measure of the de-gree of coherence, is zero for complete incoher-ence and one for complete coherence. In generalit is less than one for partially coherent radiationfields and decreases as∆t increases.Seecorre-lation function.

coherence length The temporal or transversecoherence length is the average length of wave-trains separated by abrupt changes in phase ina quasi monochromatic source. It is related tothe coherence time,τ , by ` = cτ = c

∆f . c isthe speed of light and∆f is the bandwidth ofthe radiation field. The coherence lengths canrange from a few centimeters in gas-dischargelamps to several kilometers in lasers. The spa-tial or longitudinal coherence length in the lat-eral plane is a measure of the phase correlationbetween different parts of an extended sourceof radiation. Extended sources can be rendered


spatially coherent by limiting their wavefrontwith apertures and masks. The spatial coher-ence length is (λ/θ) for slit-like sources emit-ting radiation of wavelengthλ and subtendingan angleθ. For sources with circular geometryspatial coherence length is (1.22λ/θ).

coherence, partial A radiation field is par-tially coherent if the normalized correlationfunction at different points in time or space isbetween zero and one. The two extremes cor-respond to complete incoherence and completecoherence, respectively.Seecoherence, degreeof and coherence.

coherence time Quasi monochromatic radia-tion fields contain harmonic wavetrains of finiteduration separated by others by abrupt changesin phase. The average lifetime of the wavetrainsis called the coherence time. By Fourier analy-sis it can be shown that the coherence time is thereciprocal of the frequency spread of the field.Seecoherence; coherence length.

coherent bundles Fiber optic assemblies inwhich the fibers are assembled in the same re-lationship at both ends. The signals entering atone end emerge at the other end with their mu-tual coherence in tact.

coherent sources Sources of radiation fieldwith temporal and spatial phase correlation.The temporal coherence is obtained by highlymonochromatic fields such as those from lasersand masers. The temporal or transverse coher-ence length is c

∆f wherec is the speed of lightand∆f is the bandwidth of the quasi monochro-matic radiation. The spatial coherence in thelateral plane is obtained by a point source. Ex-tended sources can be rendered spatially coher-ent by limiting their wavefront with aperturesand masks. The spatial coherence length is

(λθ

)for slit-like sources emitting radiation of wave-lengthλ and subtending an angleθ. For sourceswith circular geometry spatial coherence lengthis(1.22 λ

θ

).

coil foil A coil foil is a group of wires, lam-inated side by side, forming a close-packed flatarray of wires. A coil foil is made by winding awire around a cylinder, diameterd, several times

with each turn of wire wrapped tightly againstthe previous one – making a solenoid with onelayer of wires. After the needed number of turns(N ) are wound, the coil is glued together using avarnish or epoxy and then cut lengthwise. ThisproducesN wires,πd long, varnished togetherside by side. Such bundles of wire are often usedin any cryogenic apparatus where the wires needto be wrapped around a post for heat sinking pur-poses.

coincidence circuit An electric circuit thatgives an output only if two input-signals appearsimultaneously or within a specific time intervalof each other.

coincidence method The method for extract-ing information about different wavelengths oflight emitted by a source using a Fabry-Perotinterferometer. Suppose, in some given direc-tion, maxima of two wavelengths coincide fora given separation of the plates. The plate sep-aration is slowly increased till the two maximacoincide again in the same direction. One countsthe number of fringes that pass a given mark inthe field of view of the instrument, thereby deter-mining the ratio of the two wavelengths. If oneof the wavelengths is known, then the differencefrom the second wavelength can be calculated.

cold inactivation A hardening and functionceasing of a biological organism subject to pro-longed cold.

cold plate A cold plate is a metal plate in-side a cryostat that is anchored at a well-knowntemperature. In many cases, the cold plate tem-perature can be varied in a controlled manner.

cold resistance A resistance to hardeningand/or system shutdown of a biological organ-ism subject to prolonged cold.

cold shock Shock induced in a biologicalorganism due to prolonged cold; includes con-striction of blood vessels, contraction of invol-untary muscles, reduced blood flow, and markedpallor.


cold stress Negative effects on the function-ing and efficacy of a biological organism due toexperiencing prolonged cold.

collector In a bipolar transistor, the collec-tor is one of the bulk regions on the outside ofthe bi-junction laminate of the device. In mostamplifier applications, the collector is the pathfor the output signal.Seetransistor, junction;bipolar code; amplifier, bipolar.

collimator Any device that produces parallelrays for an optical system. The emergent beamis then calledcollimated.The simplest collima-tor is a device with a small aperture or pinholeat the principal focus of a well-corrected convexlens. For a beam of charged particles, a colli-mator is a heavy metal tube used to restrict thesolid angle of the emergent beam. Also, a termused for a small telescope attached to a largetelescope to set the line of sight.

collinear transformation The relationshipbetween the object and image within the paraxialregion, since for any plane object perpendicularto the optic axis, a plane image perpendicularto the axis is formed. The image can act as anobject for the next system. Thus, one speaks of acollinear transformation between the object andthe final image formed by an optical system.

color In general, electromagnetic radiationof different wavelengths can be said to have dif-ferent colors. However, this term has a specialmeaning for those wavelengths in the 400-700nm range where human visual perception candistinguish the amount of radiation of differ-ent wavelengths reflected by objects. The ta-ble shows approximate ranges of wavelengths ofelectromagnetic radiation and the correspondingnames of colors.

Wavelength (nm) Color380–430 Violet430–500 Blue500–520 Cyan520–560 Green560–590 Yellow590–630 Orange630–740 Red

The perception of color is often described interms of three attributes:hue, saturationand

brightness.These three form the coordinates foreach color in a three-dimensionalcolor space.The attributes of human visual perception thatgive rise to color names, such as blue, green,yellow, purple, etc., is called thehue. Satu-ration refers to how much a color differs fromwhite. Brightnessis the perceived intensity oflight. Black through different grays to brightare called achromatic colors. They lack hue andare characterized by brightness alone. As an ex-ample, the bright sun seen at noon has a yellowhue which is unsaturated whereas the dim redhue at sunset is strongly saturated.Seecolors,primary.

color code A method of representing circuitelement parameters by bands of color. Usually,the first two color bands represent the first twosignificant digits of the value of interest. Thethird band is an exponent for the tens multiplier,and the fourth is the tolerance. The digits 0through 9 are represented by the colors:

digit color0 black1 brown2 red3 orange4 yellow5 green6 blue7 violet8 gray9 white

In general, the value represented is(10d1 +d2)×10d3 wheredk is thekth digit. Special col-ors for the tolerances are gold (±5%) and silver(±10%), and if no fourth band exists±20%. Ifthe third band is gold use (−1) for the digit inthe multiplier, and for silver use (−2). The unitsare natural units for real devices, i.e., ohms forresistors, picofarads for capacitors.

color, complementary Any two colors thatproduce white when added together are calledcomplementary. The complement of a primarycolor is called secondary. Red (R) and cyan (C)are complementary; so are green (G) and ma-genta (M ). Blue (B) and yellow (Y ) are anotherpair. Seecolors, primary.


color, desaturated Color saturation refers tothe hue referenced to white light. For example,a desaturated hue in white background makesit indistinguishable. Desaturation of colors inrocks and dry wood surfaces comes from thepartial specular reflection by tiny facets in con-trast to the diffuse reflection from the texture ofthe surface. For example, moistening a surfacedarkens it and provides more saturated colors.Glossy surfaces are more desaturated when ob-served at angles at which they reflect light spec-ularly; otherwise they are more highly saturatedthan matte surfaces.Seecolor.

colorimetry The Commission of Interna-tionale d’Eclairage (CIE) has determined an ob-jective system of colorimetry in which a col-ored object illuminated by a standard source willhave a spectral power distribution (SPD) fromwhich the three color coordinates — luminance,which determinesbrightness,and chromaticity,which determineshueandsaturation— are de-rived. The field of color specifications in termsof these color matching functions valid for ob-servers with normal color vision is termed col-orimetry. Seecolor match.

color match A specific color in considerationcan be obtained by an additive mixture of threeprimary colors in appropriate values known astristimulus values. The color-matching func-tions are the tristimulus values as a function ofwavelength. The normalized (i.e.,x+y+z = 1)tristimulus values are called the trichromatic co-efficients or chromaticity coordinates. The colormatch for a standard observer were determinedby CIE, an international organization of col-orimetry in 1931 and a supplement was pub-lished in 1964. It can be found in handbookssuch as theCRC Handbook of Chemistry andPhysics. The graph below shows the trichro-matic coefficients as a function of wavelength.Seecolor, colorimetry.

color printing The process involves severalsteps that can be described as follows: The pic-ture is photographed through a fine mesh screenusing a blue filter. The screen is rotated by ap-proximately 30 and is photographed througha red filter. The screen is rotated again and theprocess repeated with a green filter. Three plates

Trichromatic coefficients.

are made by exposing the film to ultraviolet lightand etching away the exposed areas. Inks of ex-act complementary colors of the filters used foreach plate (yellow, cyan and magenta, respec-tively) are laid down on the unexposed areas.To reconstruct the picture, the three plates areprinted on the same sheet. In some cases, afourth print using black ink is made to enhancethe color contrast. The degree of overlap of thedots of the three colors produces different shadesof colors. The quality of reproduction is deter-mined by the number of inks used, the degreeto which the inks and their corresponding filtersare complementary and the number of dots perinch.

colors, primary The additive primaries arered (R), green (G) and blue (B). These col-ors have very little overlap and a suitable ad-ditive mixture of them can produce all othercolors including white (W ). The subtractiveprimariesare cyan (C), magenta (M ) and yel-low (Y ) which essentially remove red, green andblue colors from white light. One can obtain red,for example, by using yellow and magenta fil-ters in front of a projector producing white light.If the brightness of each color is the same, thefollowing relations hold good:G + R = Y ;B +R = M ; B +G = C; B +G+R = W .


color stimulus Radiant flux of a given spec-tral composition that produces a sensation ofcolor. Seecolor match.

color, surface It is the common primary colorof the light falling on the surface and the colorreflected by the surface under illumination ofwhite light. For example, if a surface appearsyellow (Y ) under white light, and is illuminatedby cyan (C), the color of the surface is green(G). SinceY = G + R andC = B + G , thecommon color is green (G).

color temperature The color temperature ofa specimen is the temperature of the blackbodywhose spectrum is closest to that of the speci-men. For example, the spectral distribution ofthe sun can be best fit by the blackbody spec-trum at 5500K. Hence one can specify the colortemperature of the sun as 5500K.Seeblackbodyradiation.

coma A distortion of the image due to obliqueand non-paraxial rays incident on a lens or mir-ror. It derives its name from the comet-like im-age of a point object located off-axis. Rays strik-ing different parts of the lens or mirror lead toimage points with differing magnification. Ifthe magnification for the outer rays is greaterthan that compared to the central rays, the comais said to be positive. If the reverse is true thecoma is termed negative. For positive coma thetail of the comet spreads away from the axis asshown in the figure below. This defect is cor-rected by satisfying theAbbe’s sine condition.

Coma.

combinational logic Operations that pro-duce the same result or output from a specificarray of inputs. Thus the output only dependson the input vector now, not at any time previ-

ously, nor on an internal state history of a device.For instance, logic operations like AND and ORare combinational. Flip-flop circuits, where aninput changes the state and output of the device,are not combinational devices, neither are com-puter CPUs or memory arrays.

common A connection to a common voltageof the circuit. Often called common ground orlocal ground, the common is not necessarily re-lated to earth ground at all. For instance, thecommon may be connected to the metal chassisof an instrument, allowing all voltages within thecircuit to be defined in reference to the chassis.However, if the chassis itself is not connectedto earth ground, this voltage level may take anyvoltage. Since the difference between circuitelements and common remain unchanged, thevoltage to common remains a good local refer-ence for many devices.

communication by balloon A balloon maybe used as a low-altitude satellite to relaytelecommunication signals over a wide geo-graphic region. One of the most famous prim-itive satellites, Echo I, launched on August 12,1960, was a plastic balloon thinly coated withaluminum. Echo I was a passive experimen-tal satellite for voice and data telecommuni-cations. Echo II, launched in 1964, embod-ied another metalized Mylar balloon function-ing as a passive communication satellite. High-altitude balloons, more easily launched than tra-ditional satellites and may stay aloft for yearsduring their lifetime, can function like very-low-altitude satellites to offer wireless cellularservice in a stratospheric telecommunicationsnetworks. For example, a communication bal-loon platform at 70,000 feet can cover about 625miles in diameter on the ground. There also ex-ist balloon-borne meteorological telemetry in-struments to relay data collected onboard theballoon to ground-based communications basestations.

communication processing unit (CPU) Themessage control and processing unit of a com-munication network switching center. CPU alsostands for thecentral processing unitof a digi-tal computer, which houses arithmetic and logic


processing electronic circuitry that carries outthe execution of instructions.

communication satellite A communicationsatellite, which may range from a few kilo-grams to several metric tons, embodies a space-based re-transmission and routing station orbit-ing the earth to link disparate geographic pointson earth. A communication satellite receivesa radio communication signal from one earth-based station, amplifies and processes this re-ceived signal, and then re-transmits it to anotherearth-based station. The function played by acommunications satellite, thus, resembles thatof a ground-based microwave repeater and rout-ing station but covers a much wider geographicalexpanse interconnecting many more mobile aswell as fixed nodes in the communication net-work.

Communications satellites often orbit theearth near the equator, thereby covering the mostdensely populated regions. A low earth orbit-ing satellite, with an orbit about 100 to 300miles in altitude and about 17,500 miles perhour in speed, can circle the earth in about 90minutes. A geostationary or synchronous satel-lite, in contrast, possesses an orbit 22,280 miles(or 35,860 km) high and synchronized with theearth’s own rotation, thus allowing the satelliteto remain roughly fixed above a particular geo-graphic point. Communication satellites need tocontinually adjust their orbit and pointing direc-tions to compensate for gravitational and otherinfluences from the sun, the moon, the earth,and other planets. The very high orbital speedof low earth orbit satellite is to avoid havingthe satellite pulled out of orbit by gravity, butgeostationary satellites can nonetheless main-tain their orbital position more readily than lowearth orbit satellites. Several satellites on com-plementary orbits may form a group to providecontinuous communication service over a widegeographic expanse on earth. The higher al-titude of geostationary satellites permit themto cover about a third of the earth at any onetime. As few as three synchronous satellites aresufficient to cover the entire earth continually.Thus, many fewer geostationary satellites thanlow-earth orbit satellites are needed to cover agiven geographic region on earth. Geostation-ary satellites suffer no Doppler effects and are

less affected by radiation, eclipse-induced ther-mal stresses and battery drain, or perturbationby the earth’s magnetic fields. However, thegeostationary satellite’s higher altitude requiresgreater transmission power and longer transmis-sion delays — significant disadvantages for real-time two-way voice and data communications.Both low-orbit and geostationary satellites arevulnerable to catastrophic failure, because thereessentially exists no economic way to repairthem after launch.

Other essential characteristics of a commu-nication satellite, besides the choice of its or-bit, are its antenna system, transponders, mech-anism for position and orientational control, linkbudget, and the mechanism that places the satel-lite into its orbit.

communication satellite, passive A passivecommunication satellite has no active transmis-sion power source onboard and thus only reflectsback, and does not re-process and re-transmitthe signals it receives. Hence, only a very mod-est proportion of the signal energy transmittedby the ground station or other satellites to thepassive communication satellite is forwarded,thereby restricting the communications capacityof the satellite network. While most early com-munication satellites were passive, essentiallyall current communication satellites are activesatellites.

commutative law An algebraic statementthat the order of two operations does not matter.If AB = BA, then the operatorsA andB aresaid to commute. Commutation for a single op-eration means that the order of the operands doesnot matter. Addition commutes, and the ANDand OR operators are commutative. Transla-tions do not commute with rotations, a fact thatmakes parallel parking possible.

commutator A device employed in a gener-ator or motor to provide (1) electric connectionbetween the rotating armature winding and thestationary terminal, (2) a mean to reverse thecurrent.

comparator Comparators, like operationalamplifiers, are high gain difference amplifiers.The schematic symbol is shown below. It is


a device that ideally provides one of two fixedoutput levels depending on the relative valuesof its input levels,V+ andV−. Specifically, ifV+ > V− then the outputVout will be somefixed DC value, sayV1. On the other hand, ifV+ < V− thenVout = V2. Usually,V1 andV2

are the positive and negative saturation voltagesoperating the circuit.

Schematic symbol for a comparator.

WhenV− passes throughV+, Vout will there-fore make an abrupt change. These output volt-age characteristics are summarized in the pre-sented transfer function.

Comparator output Vout as a function of the input volt-

ages V+ and V−.

Comparators often form the basis of analogueto digital converters. They provide an input volt-age of one of two levels to a logic gate; i.e., oneof two logic levels. Thus the logic level trans-mitted depends on ifV+ < V− or if V+ > V−.

compass A device consisting of suspendedmagnetic material that rotates under the influ-ence of the earth’s magnetic field to point duemagnetic north or south.

compensation The shaping of an op-amp fre-quency response in order to obtain stable oper-ation.

Operational amplifiers (seeoperational am-plifier) generally have high frequency limita-tions. This is due to either frequency limitationsof the constituent active components or due tostray capacitance inherent in the construction ofthe circuit. Typically, when the input signal fre-quency is increased, the magnitude of the openloop gain will decrease. Also, the output signalwill suffer a phase shift as a function of fre-quency. If the open loop gain is greater thanone when the loop phase shift is2π radians, un-wanted oscillation of the amplifier can occur.This will also depend on the feedback networkin the amplifier circuit. By definition, an invert-ing amplifier already has a loop phase shift ofπ.

It may not be necessary to have an input sig-nal with high frequencies to cause an uncompen-sated op–amp to oscillate. If the gain is muchgreater than 1 where the phase shift is2π, theoscillation can grow out of noise in the amplifiercircuit.

Therefore, it is desirable to make the gain lessthan 1 when the phase shift is2π. Then it is guar-anteed that the amplifier will not oscillate and itis said to beunconditionally stable.This is ac-complished by adding non-essential frequencysensitive circuits to provide gain. Some op-amps are internally compensated; they are builtin. Others provide connections for externalcompensation circuits, such as a capacitor. Fre-quency compensation, however, does reduce thehigh-frequency gain of the amplifier.

compensator A plate, usually of variablethickness, to provide additional optical pathlength for a ray and for production and anal-ysis of elliptically polarized light. In a Babinetcompensator, for example, one uses two narrowangle quartz wedges, one fixed and the othermovable, with parallel refracting edges and hy-potenuses facing each other. The optical axesof the wedges are respectively parallel and per-pendicular to the refracting edges, so as to haveopposing effects on theE (extraordinary) andO (ordinary) rays passing through them. At anyone point, proper sliding of the lower wedgecan thus change the phase between the emergent


rays. In a Soleil compensator, with the additionof a parallel plate, the effective thickness of theplate is not dependent on the point of incidence.

complementary apertures Two aperturesare said to be complementary if the openingsand opaque sections in one are exactly reversedin the other. The figure below shows an exam-ple. SeeBabinet’s principle.

Complementary apertures.

complementary emitter follower Or aclass–B push pull amplifier stage. Uses twocomplementary transistors, e.g.,npn and pnp.Each transistor, considered separately, is used inan emitter follower scheme. An emitter followerstage has high input resistance and low outputresistance, hence it will increase the power levelof a signal. In the complementary emitter fol-lower, one transistor conducts for positive valuesof the input voltage, while the other complemen-tary transistor conducts for negative input volt-ages. This type of configuration is more efficientthan other amplifier schemes and has near zeropower loss at zero input signal. However, itsdisadvantage is that distortion may be greater,particularly crossover distortion.

This transistor connection scheme is also use-ful in improving the transient response in tran-sistor switches (e.g., logic gates). In single tran-sistor switches, the transient response can belimited by stray capacitance. Generally, the straycapacitance is easily charged through the lowimpedance of a turned–on transistor, but dis-charges more slowly when the transistor is off.By adding a second complementary transistorto be turned on when the other is off (and viceversa), any stray capacitance will be dischargedmore quickly.

complementary error function The errorfunction of an independent variablez is definedas

erf(z) =2√π

∫ ∞

0

e−z2dz

and represents the integrated probability of aGaussian probability function with unit standarddeviation. Thecomplementary error functionisdefined as

cerf(z) = 1− erf(z)

and is plotted in the following graph. The com-plementary error function, under certain bound-ary conditions, is the solution of the differentialequation describing diffusion. Thus it repre-sents the dopant density as a function of distancefrom the surface in a semiconductor doped bydiffusion process.

complex radius of curvature The radius ofcurvature of the wavefront of a Gaussian beam isoften expressed as a complex quantity in whichthe real part is related to the radius of curva-tureR and the imaginary part is related to thebeam waist (i.e., beam radius at the narrowestpart)w as shown:1q = 1

R +√−1 λ

πw2 . Hereqis the complex radius of curvature andλ is thewavelength.

compole This is an auxiliary pole arrangedbetween the main poles of a commutator in orderto produce an auxiliary flux to assist in commu-tation.

compressibility The property of a substanceto change its volume due to the application ofpressure. Compressibility is quantitatively de-fined as− 1

VdVdP , wheredV is the change in the

volumeV of a substance due to changedP inthe pressure. Compressibility depends on con-ditions under which the pressure is changed, andmay be adiabatic, isothermal, etc.

compression, acoustic An increase in thedensity of a medium at a point at which the pres-sure peak of a progressive sound wave arrives. Ahalf period later, this pressure peak is followedby the peak underpressure so that the density ofthe medium is rarefacted at this point. Becauseof compression and rarefaction of a medium at


a given point due to passage of sound, soundwaves are called compressional waves.

Compton effect First observed by A.H.Compton in 1922, this effect consists of thescattering of monoenergetic X-ray photons byweakly bound electrons in a metal target. Thescattered photon has a reduced energy that de-pends on the scattering angle. This process canbe treated as a collision of a photon with a freeelectron consistent with the conservation of en-ergy and momentum. The difference in wave-length,∆λ, between the scattered photon andthe incident photon is called the Compton shiftand is given by∆λ = h

m0 c (1 − cos θ). Hereh is the Planck’s constant (6.626 × 10−34 J.s),c, the speed of light (3× 108 m/s),m0, the restmass of the electron andθ, the angle betweenthe direction of the scattered photon to that ofthe incident photon.

computer A device consisting of hardwarefor the input, processing, and output of data orinformation. Input of raw data can be from a va-riety of sources, by human input via a keyboard,for example, or by remote kinematic sensingas in a robotic application. The processing ofdata is usually accomplished digitally with ascheme of gates, memory, processing units, etc.The computer also has a list of instructions, thesoftware, which details how the computer is togather and process the information. Thus, bysimply changing the software (instructions), thecomputer can perform a variety of tasks.

computer, analog A system that uses ana-log components such as operational amplifiersto perform specific computational tasks. Simpleoperations such as addition, subtraction, mul-tiplication, differentiation, and integration areeasily accomplished with operational amplifiercircuits. These can be combined into a larger,more sophisticated system to solve more diffi-cult problems. As an example, the differentialequation

d2V

dt2+ k1

dV

dt+ k2V = Vapp

ord2V

dt2= −k1

dV

dt− k2V + Vapp

can be solved with the use of some basic opera-tional amplifier circuits. The analysis of a blockdiagram of a circuit to perform this task starts byassuming the signald

2Vdt2 is available. It is inte-

grated twice with integrating circuits, each witha time constantRC = 1, to obtaindV

dt andV asa function of time. These are scaled and addedwith an applied signal as per the above equation.Since this sum is equal tod

2Vdt2 , it is sent back to

the first integrator, as initially assumed.

Initial values of dVdt andV at t = 0 can be

programmed into the circuit by charging the ca-pacitors of the respective integrators to the ap-propriate value. The solution of the above dif-ferential equation is obtained by recording theoutputV as a function of time.

computers, use in communication Comput-ers, defined as programmable electronic devicesthat perform high-speed mathematical, logicaloperations and that process and store informa-tion, are essential and ubiquitous components ofa communication system. Computers are usedto acquire, analyze, organize, store, disseminate,and present information.

On the physical level, computers are used assource encoders to condense raw informationinto an compact source code. They then serveas channel encoders to add in coding redundancyto render the message more robust against chan-nel distortions and noises. Computers also actas modulators to transform these digital chan-nel codes into continuous waveforms for con-tinuous channels. They then further multiplexvarious modulated signals to share one multiple-access communication system. The reverse ac-tions — channel demultiplexing, signal demod-ulating, channel decoding, and source decodingare also effected by computers at the receiver.On the network level, computers serve as rout-ing switches to process each transmitted mes-sage packet’s routing control signals to directsuch message packets from network node to net-work node toward their final network destina-tion. These computer switches may temporallystore the message packet if the next network linkis momentarily congested.

On the other hand, the various traditionalparts of a computer — a keyboard/screen forhuman/machine interface, computation process


and memory devices may be geographically dis-persed but linked via a local-area communica-tion network. For example, instead of provid-ing each user on the network one fully imple-mented computer, individually equipped soft-ware library, printers, modem, and storage disks,each user may instead have access to only a min-imally equipped work station linked to networkservers that have access to centrally adminis-tered software libraries, printers, modems, stor-age disks, or high-power corporate main-framecomputers. Different users can communicatewith each other and access each other’s data andfiles via this network.

concave grating A grating on a concave sur-face (rather than plane). The advantage of sucha grating is that the diffracted beams come to afocus without the need for additional focusingoptics. It is useful in spectrographs in which theentrance and exit slits are located at the radiusof curvature of the grating. The disadvantage isthe expense of fabrication.

concentrator A concentrator embodies afunctional unit in a communication system al-lowing a common path to handle more trans-mission sources than the number of currentlyaccessible channels within that path. A con-centrator can often handle numerous low-speedasynchronous channels with diverse transmis-sion speeds, coding and protocols using con-tention schemes with buffering, or one or morehigh-speed synchronous channels.

condensation, counterion The binding ofcounterions to the phosphate groups of nucleicacids in solution.

condensation in longitudinal wave Quan-titative measure of change in the density of amedium due to progressive sound wave. Thecondensations is mathematically defined ass =(%− %0)/%0, where%0 is theambientdensity ofa medium, and% is the instantaneous value ofthe density that is different from%0 due to prop-agation of a sound wave.

condenser (Abbe) A condenser obeying theAbbe sine condition.

conductance The real part of admittance of amaterial or a circuit. It is a measure of the abilityto conduct electricity of a material or a circuit.In the DC case, it is the reciprocal of resistance.

conductance, acoustic The real part of theacoustic admittance;i.e., if Y = G+ iB is theadmittance,G is the conductance. The imag-inary part of the admittance,B, is called thesusceptance.

conductance, membrane The ability of amembrane to pass flowing charge (electrical cur-rent) be it electrons or ions. Membrane conduc-tance is a function of frequency, with DC con-ductance on the order of10−2 to 10−5 siemens.Membrane conductance is often measured byimpedance spectroscopy.

conductance, state attribute Treating theconductivity of a subsystem (biological) as astate variable. That is, conductivity suffices tounambiguously identify the state of the biosys-tem.

conduction current Movement of electriccharge carriers constitutes a conduction current.The charge carriers can be electrons, protons,ions, or holes. Conduction current in a metal-lic wire is the result of electrons movement inthe conductor. The direction of the current isconventionally defined as the direction of posi-tive charge carriers. The MKS unit of current isampere.

conduction in metals In metals, valence elec-trons of atoms are free to move and are not asso-ciated with any particular atom; they are calledfree electrons. The free electron in the metalcan move freely inside the metal, which makesmetal a good conductor.

conduction, nerve Usually by ion flow wherethe modeling is a series of resistors (impedances)due to the conductivity jumps at the ion chan-nels. This is a very complicated process. Anintroduction may be found in K.S. Cole,Mem-branes, Ions, and Impulses,Univ. Calif. Press(1972), an older but useful introductory text.


conduction, saltatory Discontinuous con-duction of nerve impulses in myelinated nervefibers.

conductor, cytoplasm Cytoplasm is richer inpotassium ion concentration and poorer in cal-cium and sodium ion concentration as comparedwith the extracellular fluid. Hence the cyto-plasm has an “ionic pump” for transporting ionsacross it. This pump mechanism increases theconductivity of the cytoplasm relative to the ba-sic membrane conductivity. Hence, active trans-port is important in making cytoplasm a conduc-tor.

cones Cones are one of two kinds of pho-toreceptor cells found in the human retina. Itderives its name from its microscopic appear-ance. Cones are responsible for photopic vision.There are three distinct types of cones, eachhaving absorption spectra within peaks around440 nm, 530 nm and 560 nm. They are calledS-cones,M -cones andL-cones, respectively.S-cones (“blue” cones) refer to short wavelengthsensitive cones,M -cones (“green” cones) referto mid-wavelength sensitive cones, andL-cones(“red” cones) refer to long-wavelength sensitivecones. In the human eye, there are approxi-mately 8 million cones.

confocal microscopy A special kind of mi-croscopy that can image one plane at a timein a thick transparent medium. This is partic-ularly useful in imaging biological specimenswith features in different layers requiring con-trast in the axial direction as well as in the objectplane. Confocal microscopy takes advantage ofthe property of conjugate points in an opticalsystem. A point sourceS, the objectO, and apin hole in the image planeI are all made con-jugate or confocal to one another (see figure) sothat only the object is illuminated by the sourceand its image passes through the pin hole. An-other object in an adjacent plane is neither illu-minated nor is its image passed through the pinhole. A scanning confocal microscope imagesone object plane at a time when the specimen istranslated on the axis.

confocal resonator An optical resonator withtwo identical concave spherical mirrors sepa-

Confocal microscopy.

rated by a distance equal to their radius of cur-vature. The focal points will be coincident onthe axis midway between the mirrors. The sur-faces of constant phase match the curvature ofthe mirrors in the vicinity of mirrors while theyare planar at the focal point. The beam waistsat the position of mirrors and at the center of

the resonator are√

λdπ and

√λd2π , respectively.

Hereλ is the wavelength andd is the mirrorspacing.

conjugate distances/conjugate planesWhen imaging by a lens (or lens system) for ev-ery object point, there is an image point. By theprincipal of reversibility, one can consider theimage point to be imaged by the lens to the objectpoint. These two points, object and image, arethe conjugate points for the lens. The distancefrom the lens to these points are called conjugatedistances. Planes passing through these pointsare called conjugate planes. In an ideal opticalsystem, every ray from the object intercepted bythe lens also passes through the image.

conjugate points The point object and thecorresponding point image for an ideal opticalsystem. The rays from an object point take equaltime to converge to the image point. Accordingto Fermat’s principle, the reverse is also true.

conjugate rays The incident ray in the objectspace and the corresponding emergent ray in theimage space.

conjunction (logic) A fundamental booleanlogic operation over two or more logical vari-ables. Sometimes referred to asAND, it is thelogical product

A⊗B ≡ A AND B ,


where the variablesA andB are elements of theset1, 0 (or, equivalently, the set true, false). In other words, the conjunction of these twovariables is true ifand only ifA andB are true.

The postulated rules of the logical productare

0⊗A = 0

and1⊗A = A .

Also, for every variableA there exists the prod-uct inverse,NOT A ≡ A, such that

A⊗ A = 0 .

The logical product is commutative and associa-tive:

A⊗B = B ⊗A

A⊗ (B ⊗ C) = (A⊗B)⊗ C .

Given these rules, all possible values of theconjunctionY = A ⊗ B can be tabulated in a‘truth table’ and are presented in the accompa-nying table.

Conjunction (logical product) ofall possible values of thevariables A and B

A B Y = A⊗B

0 0 00 1 01 0 01 1 1

In implementing this operation in digitalelectronics, voltage signals are used to representthe logical variablesA andB following a pre-defined logic convention. A circuit, frequentlyreferred to as agate,is used to determine the con-junction, and the result is provided on an output,Y . The symbol representing such an electroniccircuit performing anAND operation is shownbelow. See alsodisjunction (logic).

conservation of charge The law of conser-vation of charge states that in any type of in-teraction, electric charges cannot be created ordestroyed. This is one of the most fundamentallaws of physics.

Symbol representing conjunction (AND) in digital

electronics.

consonance When two or more notes playedsimultaneously produce a harmonious and pleas-ant sound. Consonance occurs if the frequencyof tones are in simple ratios, i.e.,2 : 1, 3 : 2,4 : 3. These frequency ratios are called the oc-tave, the fifth, and the fourth. When two or moretones do not produce a harmonious and pleasantsound, they are said to be indissonance.

constant current source An electronic com-ponent that allows or forces a fixed amount ofcurrent to pass. Ideally, this current sourcemaintains the prescribed current flow regardlessof the voltage dropped across it. This can be con-trasted with a constantvoltagesource that main-tains the voltage dropped independent of the cur-rent flowing through the device. A schematicsymbol for aconstant current sourceis shownbelow as well as a realization based on a tran-sistor. This realization assumes the collector-emitter current of a transistor is equal to thebase–emitter current times the current gain, in-dependent of the collector-emitter voltage.

Schematic symbol for a constant current source and a

realization using a transistor.

constant deviation Arrangements, usuallywith the help of three prisms (e.g., Pellin-Brocaprism, Abbe prism), to render the emergent rayat a constant angle of deviation from the incidentray regardless of wavelength. Thus, in contrastto the ordinary arrangement for examining thespectra where the telescope is moved to view


the different parts of the spectrum, in the con-stant deviation configuration, the collimator andtelescope are fixed and the prism arrangement isrotated.

constant deviation prisms Prisms that havea constant angle of minimum deviation (whichcorresponds to maximum dispersion) for a rangeof wavelengths of light. They are used in spec-trographs in which the incident light directionis at a fixed angle to a photographic plate or aviewing telescope. Different wavelengths areselected by simply rotating the prism about anaxis normal to the plane of the beam. The an-gle of rotation can be calibrated to identify thewavelength. Most common constant deviationprisms are the Pellin-Broca and the Abbe prismsfor which the angle of deviation is 90 and 60 de-grees, respectively.

constringence Seeaberration, chromatic.

contact electrification The transfer of chargefrom one object to another (one molecule to an-other) due to physical contact between the twoobjects (molecules). The natural tendency oflike charges is to repel; hence the charge will re-distribute itself across two objects (molecules)if in contact with both simultaneously.

contact interaction Chemical, biological, ordynamic interaction due to physical contact be-tween the interacting species, as opposed to ac-tion at a distance (induced interactions).

contin. Standard abbreviation for the Latincintinue’tur meaning to let be continued.

continuity conditions, acoustic Conditionsgoverning change in acoustic pressure, displace-ment, fluid velocity and density across an inter-face between two media. An example of suchmedia are two homogeneous layers with dif-ferent values of the sound speed, density andmedium velocity, divided by a plane interface.Acoustic pressure, displacement and density arecontinuous across an interface. The componentof acoustic fluid velocity normal to a boundaryis continuous across this interface in motionlessmedia, but is in a general case discontinuous ifat least one of the two media is moving.

contrast For a system of fringes with themaxima and minima in irradiance denoted byImax andImin, respectively, the contrast (alsoknown asvisibility or modulation) is defined as

Imax − Imin

Imax + Imin.

convection current Convection current isproduced by the motion of unneutralized changein plasma.

convention (logic) An assignment of volt-age levels used to represent the two logical statestrue andfalsein the implementation of Booleanalgebraic computation with electronic signals.In practice, there is usually a range of allowedvoltage levels for each state with an unambigu-ous voltage range separating the two. There aretwo distinct conventions:

1. Positive logic convention: Voltage levelsthat represent logicalfalse are frequently de-noted byL (for low) and are in a range nearzero volts,

0 ≤ VL ≤ VLmax .

Similarly, voltage levels said to represent logicaltrue are denoted byH (for high) and are in arange greater than that forVL,

VH ≥ VHmax with VHmin > VLmax .

For example, the most frequently used logic con-vention in TTL (transistor-transistor logic) cir-cuits, has0 ≤ VL < 0.8 volts andVH ≥ 2volts and is sometimes referred to as a5-voltpositive logic system.These voltage ranges areillustrated in the accompanying line graph.

Voltage ranges corresponding to logical true and

false in the positive logic convention employed by

most TTL circuits.

2. Negative logic convention: Voltage levelsrepresentingfalseare denoted byH and are in


the range

VH min ≤ VH ≤ 0 .

Voltages representing logicaltrue are denotedbyL. These voltages are in the range

VL ≤ VL max with VL max < VH min .

Logic circuits based on ECL (emitter-coupledlogic), especially those of early discrete semi-conductor circuits, use a negative logic conven-tion. Modern ECL integrated circuits nominallyassume−0.9 ≤ VH ≤ 0 volts andVL ≤ −1.7volts as illustrated in the following line graph.

Voltage ranges assumed for logical true and falsein typical ECL circuits; an example of a negative logic

system.

Logic gates based on one logic conventioncan be made to operate using the opposite logicconvention by redefining 0 volts. However, thisinverts the definition oftrue andfalserelative toVH andVL and, therefore, the logical operationof the gate must be converted by applying theprinciple of duality.See alsoduality principle.

convention, sign of current and voltage Therelative sign of voltage difference between anytwo points in a circuit is determined by the rela-tive electrostatic potential energy felt by apos-itive test charge, e.g., a proton. An appliedvoltage difference in a circuit creates an elec-tric field. The electric field direction is deter-mined by the direction the positive test chargemoves under the influence of this field. As thetest charge moves in the direction of the appliedelectric field, it will lose potential energy. Thus,the positive test charge will tend to move awayfrom the point with greatest electrostatic poten-tial, the “+”, and move towards the point withlesser electrostatic potential, labeled “−”.

This can be compared with a ball rollingdown a hill under the influence of a gravita-tional field; it moves towards the point with

lesser gravitational potential energy. Of course,the direction of movement and relative poten-tial energy difference for anegativetest chargewill be opposite, but the convention assumes apositive test charge as above.

The sign or direction of current flow in a cir-cuit is determined by the direction that positivetest charges would flow under the influence ofthe applied electric field. In metals, it is wellknown that negatively charged electrons are themobile charge carriers and are hence responsiblefor current flow. However, the defined directionof current flow is still the direction that positivecharges will flow.

converging wave A wave in which ampli-tude and energy are increased with the distanceof propagation. Most sound sources emitdi-vergingwaves in which amplitude and energyare decreased with the distance of propagation.Sound waves can be converging only under cer-tain circumstances or conditions. For example,converging waves can be produced by using con-verging lenses and mirrors, can be radiated byspecially designed transducers, and can appearin an inhomogeneous medium due to randomfocusing.

converter A machine or device for chang-ing alternating current to direct current, or theconverse. If a conversion is made from DC toAC, the machine is called an invented converter,with an alternator DC generator combined inone machine having a single-field circuit. Con-verter losses consist of friction, resistance heatand core losses. The converter is frequentlycalled asynchronous converteror rotary con-verter.

converter, analog/digital Analog/digitalconverters are used to interface digital equip-ment such as computers with their binary op-erating scheme to real-world analog signals asfrom a sensor. Applications include digitalvoice communication where analog voice sig-nals are converted to digital information and re-constructed after transmission.

(1) A digital–to–analog converter(DAC)generally converts a given digital number intoa corresponding analog voltage level. This cor-respondence depends on the binary encoding


scheme used (e.g., sign-magnitude, 2’s comple-ment, etc), the resolution, and the voltage range.The resolution, or equivalently the smallest ef-fectual output change, of the DAC is determinedby the number of bits in the digital number. Forexample, if the converter can accept 8 bits ofdigital input data, the smallest change availableis 1

256 of the output voltage range. The volt-age range is the minimum and maximum outputvoltage levels.

For an ideal DAC, the output voltage is lin-early dependent on the input digital information.Each increment in digital numberδn will yieldan equal analog incrementδV . The slope of thislinear dependence is given by∆ = δn

δV . In thenon–ideal DAC, however, eachn may have anerrorε from the ideal linear dependence. Thesepoints are illustrated in the graph.

Example output voltages (solid squares) for a 3–bit

DAC. The line represents a least squares fit.

Two common types of digital–to–analogueconverters are briefly described below. In thedescription, the digital voltage levels are not ap-plied directly in the conversion scheme; rather,they are used to select between one of two setvoltage levels. This is accomplished with, forexample, a transistor switching network, the de-tails of which are not important in describing theconverter.

The weighted–resistorDAC applies the se-lected voltage levels through resistors whosevalue is inversely proportional to the numeri-cal significance of the corresponding digit. An

example of a converter based on this scheme isshown below.

An example of a 4–bit weighted resistor digital–to–

analog converter. The digital information chooses be-

tween one of two voltages, V0 and V1, corresponding

to logical false and true. Details of the switching

arrangement are not given.

The resistance effected by the least signifi-cant bit (LSB), bit 0, is given byR and the re-sistance associated with thenth bit isR/2n. Asmay be noted, this scheme requires a wide rangeof resistor values.

The R–2R Ladder only requires two dif-ferent resistor values,R and2R, but requirestwice as many resistors as the previous scheme.The digital information is converted by provid-ing inverse-proportionally weighted current di-vision for each of the corresponding bits. Thevoltage selected by the LSB has the most sig-nificant attenuation while the one controlled bythe most significant bit (MSB) has the least.

(2) An analog–to–digitalconverter (ADC)performs the opposite conversion of a DAC, i.e.,it converts an analog signal into a digital number.An ADC must be able to sample and hold theinput analog voltage long enough to determineits value. It must then quantize the value andrepresent it using some binary coding format.Quantization errors are introduced because ofthe limited amount of information that can beconveyed in the binary coding. Hence, a digitalvalue provided by an ADC actually representsa range of analog voltages as determined by thequantization error. The resolution of an ADC issimilar to that of a DAC: more digital bits implymore, and smaller, voltage ranges that can bediscerned.


An example of a 4–bit R–2R ladder digital–to–analog

converter. Again, digital information chooses between

one of two voltages: V1 and analog ground, in this

example.

The most elementary converter, and thefastest, is thecomparatorADC. It comprises2n−1 analog comparators, wheren is the num-ber of digital bits available to encode. There is aresistor chain that determines the reference volt-age of each comparator. The reference voltagesare usually set up such that the quantization erroris minimized for any input voltage. The outputsof the comparators drive a system of digital gatesthat encode the information into a usable binaryformat.

As an example, the figure below illustratesthe concept behind a 2–bit comparator ADC.With only 2 bits of digital information, thereare four voltage ranges to be considered by theconverter. To minimize the quantization error,the first and last cover a range ofV0/6 whilethe middle two ranges coverV0/3. The rela-tionship between input voltage range and outputdigital information is given in the table. Exten-sion of this scheme to output more digital bits,and hence better resolution, is straightforward.

The successive approximationconverter ispopular as it is relatively fast and can providegood resolution with less hardware than thecomparator type. It works by successivelyadding and comparing known voltages,Vi, withthe one to be measured. The values of the knownVi’s are determined by their corresponding bitsignificance. For ann–bit converter, the knownvoltages areV0/2 (associated with the MSB,V0/4,V0/8, . . . , andV0/2n (associated with the

An example 2–bit comparator type ADC.

Input voltage ranges for the indicated2–bit comparator-type ADC and itsoutput information

Binary InterpretedVin Encoding Value

5V0/6 < Vin < V0 11 V03V0/6 < Vin < 5V0/6 10 2V0/3V0/6 < Vin < 3V0/6 01 V0/3

0 < Vin < V0/6 00 0

LSB). To convert the input voltage, it is firstcompared withV0/2. If Vin > V0/2 then theMSB is set true, and the next smaller voltageV0/4 is added to the first. Otherwise, the MSBis set to false. The next most significant digit isdetermined by evaluatingVin > (V0/2 + V0/4)or Vin > V0/4, depending on the result of theMSB evaluation. This process is continued untilthe LSB is reached. To minimize the quantiza-tion error, the input voltage is shifted smaller byone–half the voltage associated with the LSB.

Input voltage ranges for an example2–bit successive approximation-typeADC and its output information

Binary InterpretedVin Encoding Value

5V0/8 < Vin < 7V0/8 11 3V0/43V0/8 < Vin < 5V0/8 10 V0/2V0/8 < Vin < 3V0/8 01 V0/4

0 < Vin < V0/8 00 0


Other common types of analog–to–digitalconverters are thecountinganddual slopecon-verters.

convolution Also known asfolding or su-perposition. Mathematically, convolution of afunctionf(x) with the functiong(x) results inanother functionh(X) such that

h(X) =∫f(x) g(X − x) dx .

The result can be extended to higher dimensionsand several variables.

convolutional code For every frame ofk con-secutive information symbols, aconvolutionalcodegenerates a resulting frame ofn consec-utive symbols (wheren > k) such that thesen encoded symbols’ values depend also on thepreviousm − 1 frames ofk information sym-bols. The convolutional encoder thus possessesmemory and stores themmost recent frames ofinformation symbols; when the encoder outputsa new frame ofn encoded symbols, the oldestframe ofk information symbols is discarded anda new frame ofk information symbols is input tothe encoder. The rate of the convolutional codeis defined ask/n and typically ranges from 1/4to 7/8. Then − k parity bits in each encodedframe provide a convolutional code error detec-tion as well as error correction capability. TheViterbi algorithm and the Fano algorithm repre-sent well-known convolutional decoding algo-rithms.

convolution theorem If two functionsf(x)and g(x) have their respective Fourier trans-formsF (k) andG(k), theconvolution theoremstates that the convolution off(x) with g(x)is equal to the Fourier transform of the prod-uct of F (k) andG(k). In some situations itis more convenient to carry out Fourier trans-forms than the convolution integral. Thecon-volution theoremallows circumvention of thisproblem. One important application of this the-orem is in signal smoothing. A spectrum canbe smoothed by convoluting it with a smooth-ing function. Instead the Fourier transform ofthe spectrum is obtained and multiplied withthe Fourier transform of the smoothing func-tion which is band limited. The noise at high

frequencies is thus eliminated in the spectrumwhich is smooth when a reverse transform isperformed.

cooling, magnetic A system of noninter-acting magnetic moments,µ, can be used as acoolant in low temperature experiments. Suchmoments in thermal equilibrium at temperatureT and magnetic fieldB will have an entropywhich is a function ofµB/kBT . The cool-ing process begins with increasing the magneticfield while keeping the temperature constant.The sample is then thermally isolated, and themagnetic field is decreased isentropically. As aresult, the sample temperature will be

Tf =Bf

BiTi .

In reality, a magnetic moment is effected by themagnetic field produced by the surrounding mo-ments, changing all of the magnetic fields listedabove to

√B2 +B2

0 whereB0 is the field pro-duced by the magnetic moments. At tempera-tures of a few Kelvins, paramagnetic salts canbe used as the coolant, but most of these order attemperatures above several milliKelvins. In or-der to reach even lower temperatures, adiabaticnuclear demagnetization usingnuclearparam-agnets is necessary.See alsoadiabatic nucleardemagnetization.

cooling, nuclear Seeadiabatic nuclear de-magnetization.

cooperativity In general, subsystems work-ing together for a common effect. In the biolog-ical realm, organisms working together for mu-tual survival. In the chemical/physical realm,molecular movement in tandem rather than inrandom motion.

Cooper pairs In a system of fermions withlong-range interactions, it is possible for the in-teractions to become attractive at sufficientlylow temperatures. This causes two fermions toform a composite boson, a Cooper pair. Thetemperature at which this occurs is the super-fluid or superconducting transition temperature,Tc. In low temperature superconductors (e.g.,Pb, Sn, Al), the pairs form as an s-wave pair,with one up- and one down-spin of equal and op-


posite momenta. In the high temperature super-conductors, the pairing is d-wave, while super-fluid 3He is a p-wave superfluid. Some heavy-fermion superconductors show evidence of alsobeing in a p-wave state. In all cases, the Cooperpairs behave collectively and coherently, lead-ing to the superfluid and superconducting prop-erties associated with such materials. In real-ity, a given electron will not remain paired withanother electron for very long, rather the firstelectron will be paired with first one, then an-other, and then yet another electron. An analogyoften made is with a dancer who, while contin-uing to dance, changes partners quite often butnever misses a beat.See alsosuperconductivity;helium-3, superfluid.

CO2 laser Emits infrared radiation of wave-length 10.6µm in the continuous mode. Theoutput power can be very large. The laser isused in medical surgery due to the high absorp-tion of 10.6µm radiation in water to make pre-cise incisions and to vaporize malignant tissue.Industrial applications such as welding and cut-ting are also common.

cords, vocal Triangular shaped folds of tissuelocated in the larynx. Vocal cords consist of twopairs: the tone cords and the false cords. Thereis no gap between them at the front, and thereis a varying gap at the back, which is called theglottis. The voice is produced by air stream fromthe lungs through larynx and mouth, which isaffected by vibrating vocal cords.

core Generally some ferrite or powdered-ironmaterial placed inside a coil or transformer toincrease its inductance.

core, fiber optic The inner portion of an op-tical fiber. Seecladding, fiber optics.

core loss Loss of energy due to induced cir-culating currents and hysteresis effects producedin the core.

cornea A transparent tissue devoid of bloodvessels but full of nerve cells through which lightenters the eye. It is 12 mm in diameter and 0.6mm thick at the center with a refractive index of1.376.

corner cube reflector Three mutually per-pendicular reflectors meeting at the corner of acube. Such a device reflects incoming rays backalong their original direction.

Cornus spiral This is a useful way of graphi-cally evaluating the Fresnel integrals that appearin diffraction theory. In the integral

s∫0

exp(i π w2

2

)dw = C(s) + iS(s) ,

a graph ofC(s) vs. S(s) is called theCornuspiral. A straight line segment drawn from twopoints corresponding tos1 ands2 gives the valueof the integral between those two limits. Thereal and imaginary parts are given by the projec-tions on theC(s) andS(s) axes, respectively.

corona discharge The discharge of electric-ity causing a faint glow adjacent to the surfaceof an electric conductor.

correlation function The time correlationfunction between two radiation fieldsE1(t) andE2(t+ τ) is defined as the time average over aperiodT (> τ),

Γ12(τ) =1T

T∫0

E1(t)E#2 (t+ τ) dt .

Thenormalized correlationfunction,γ12, is ob-tained by dividing the irradiances of the twofields fromΓ12(τ).

correspondence principle A principle enun-ciated by Niels Bohr in 1923, which states thatthe predictions of quantum theory for the behav-ior of any physical system must agree with thoseof the classical physics in the limit the quan-tum numbers specifying the system become verylarge. This principle reconciles the apparentdifferences in the behavior of microscopic (de-scribed by quantum mechanics) vs. macroscopic(described by classical mechanics) phenomena.Another situation where this principle is appliedis to reconcile the predictions of the relativis-tic (Einsteinian) mechanics which should agreewith those of non-relativistic (Newtonian) me-chanics in the limit the speed of light is very


large compared to the speeds of particles in thesystem.

cosine law of emission Suppose the radi-ant intensity (unit: W/sr) from a plane-diffuseradiator is viewed at a fixed distance from it,the intensity at an angleθ from the normal tothe surface,I(θ), decreases withθ according tothe Lambert’s cosine law:I(θ) = I(0) cos θ.The radiance or the intensity per unit solid an-gle per unit of projected area (unit: W/sr-m2),however, is constant withθ. A surface with aconstant radiance independent of the viewingangle is called aLambertian surface.

cosine law of illumination Suppose a surfaceof areaA2 is illuminated by a diffuse radiatingsurface of areaA1, and the line joining one el-ement of the radiator to one of the receiver (ata distancer) makes anglesθ1 andθ2 with thenormals to the radiating and receiving elements,respectively: the total radiant powerΦ (unit:Watts) received byA2 is

Φ =∫A1

∫A2

LdA1dA2 cos θ1 cos θ2r2

.

HereL is the radiance (unit: W/m2-sr) of thesource.

cotransporter A substance that is activelytransported across a plasma membrane and thatbrings another substance across the membranewith itself in the same direction.

Cotton-Mouton effect (1) Magnetic field in-duced double refraction in optically isotropicsubstances. This is a magnetic analog of theKerr electro-optic effect. It is observed in liq-uids and is proportional to the square of the ap-plied magnetic field.

(2) This effect occurs when a dielectric be-comes doubly-refracting when in a magneticfieldH. The ordinary ray becomes retarded rel-ative to the extraordinary ray by an amountδgiven byδ = CmλlH

2 whereλ is the wave-length of the light,l is the length of the path andCm is the Cotton-Mouton constant.

coulomb Unit of electric charges in mks.One coulomb is defined as the amount of charges

transported in one second through a wire carry-ing a current of one ampere.

coulomb field/force The electric field (orelectric force on a test charge) produced by apoint chargeq. Coulomb field is proportional tothe inverse of distance square,E = kq/r2.

Coulomb’s law This is an empirical law dis-covered by Charles Coulomb. It states that theforce between two chargesQ1 andQ2 sepa-rated by a distanced is proportional to the prod-uct of the two charges and inversely propor-tional to the distance square. In mks, the pro-portional constant in vacuum is given byko =8.987551× 109N m2/C2.

counter A basic digital counter counts aninput strobe orclock signal. It is assumed thatthe clock signal makes regular and periodic tran-sitions from logicallow to high and back. Thecounter counts clock transitions and provides anoutput, in some format, representing the count.Counters are used in diverse applications; com-puters, industrial applications such as countingnuts and bolts, and measuring speed are a fewexamples.

The fundamental components of a counterare set of J–K flip–flops (FF). There are twobasic topologies for connecting the flip–flops,and hence ways of encoding the output.

1. A ring counteris a ring of J–K flip–flopsconnected in a manner similar to a shift regis-ter, except that the output of the last FF,Qn, isreturned to the input of the first,J1.

A simple ring counter using n flip–flops (FFs).

Initially, all outputs of the FFs are set to log-ical 0 except the first, which is loaded with a


logical 1. Upon each clock transition, the1 isshifted to the next FF. Hence, the count is repre-sented by determining which FF is set to0. Themaximum count withn FFs in a ring counter isn. For example, consider the output of a ringcounter with 4 flip–flops as a function of thenumber of pulses received:

Clock J–K Outputpulse 1 2 3 4

0 1 0 0 01 0 1 0 02 0 0 1 03 0 0 0 14 1 0 0 0

A twisted ring, Moebious,orJohnsoncounteris similar, except that the complement of the lastFFs output,Qn, is returned to the first FF. Ini-tially, all FFs are set to0. Upon the first clocktransition, a1 will be loaded into the first FF dueto the “twisted” ring. The nextn clock pulsescontinually load a1 into the first FF while shift-ing the1s across the ring until all FFs are set to1. The next clock will load a1 = 0 into the firstFF and the nextn pulses will successively load0s into each FF. Thus, this counter can count to2n with n flip-flops. Again, consider the outputof a counter with 4 flip–flops as a function of thenumber of clock pulses:

Clock J–K Outputpulse 1 2 3 4

0 0 0 0 01 1 0 0 02 1 1 0 03 1 1 1 04 1 1 1 15 0 1 1 16 0 0 1 17 0 0 0 18 0 0 0 09 1 0 0 0

2. The second topology encodes the countmodulo2n, since each FF can have two states.Hence, each FF represents a binary order ofmagnitude, and the counter can count to2n. Anasynchronouscounter uses the output of eachFF,Qi, for the clock input for the next highersignificant bit. This type of counter suffers frompropagation delays, i.e., an input clock transi-tion must “ripple” through each flip–flop. A

synchronouscounter, on the other hand, has theinput clock signal going to each FF as in thering counter/shift register. Proper counting iscontrolled by logic gates between each stage.Thus, each flip-flop changes states simultane-ously and the propagation delay of the counteris minimized.Seecounter, asynchronous.

counter, asynchronous A basic counter us-ing a set of J–K flip–flops. Each J–K output isused to represent a binary order of magnitude.The clocking signal is sent to the first flip–floprepresenting the least significant bit (LSB). Thisoutput is used as a clocking signal for the nexthigher significant bit;see figure.Thus, the firstflip–flop’s output is toggling between1 and0 inresponse to the clock, causing the next flip–flopto toggle. Each flip–flop makes a toggle onlywhen the preceding flip–flop has made a1 to 0transition. Each J and K input are tied to logi-cal1 and hence have no effect on the counting.This type of counter arrangement is relativelysimple, however it can suffer from propagationdelays causing decoding errors as the LSB willcomplete its change before the last flip–flop.

An asynchronous counter using n flip–flops (FFs).

counterions Counterions are ions of oppo-site charge to that of the colloidal particles insuspension.

countertransporter A substance that movesacross a biological membrane in the oppositedirection to active transport.

coupled circuit A coupled circuit is a circuitthat consists of several subunits that are con-nected through a capacitor or an inductor. Typi-


cally a coupled circuit is designed such that onlythe AC components of the signal are transferred.

coupling, intercellular A pairing, or joining,of one or more cells in a biological system.

creep Seefilm, Rollin.

critical angle When light passes from amedium of high index (n1) to low index (n2),it will bend away from the surface normal. Theangle of refraction is given by Snell’s Law. Atthe critical angle of incidenceθc, the angle of re-fraction will be 90. The critical angle is givenby:

θc = sin−1

(n2

n1

).

For angles of incidence greater thanθc, the raywill experience total internal reflection, i.e., itwill be reflected back into the medium.

critical damping Seedamping factor.

critical magnetic field This is the field belowwhich a superconducting material is supercon-ducting and above which the material is normalat a specified temperature and in the absence ofa current.

cross talk (1) Leakage of light from one op-tical fiber to another by frustrated total internalreflection. This can be caused by inadequate ordefective cladding material.Seecladding, fiberoptic.

(2) Crosstalk in a multi-channel communica-tion system refers to the undesirable spill-overof the transmitted signal in one communica-tion channel onto another channel. Crosstalkmay be caused by the nearness of the transmis-sion media, such as electromagnetic mutual cou-pling between different pairs of twisted copperwires in a telephone cable. Crosstalk may arisefrom intermodulation distortion in frequency-division multiplexed systems. Crosstalk mayalso occur as inter-symbol interference fortime-division multiplexed systems due to chan-nel non-linearities such as that in frequency-selective mobile communications channels.

Crosstalk in these various cases may be re-duced by, for example, better insulation amongthe twisted pairs, a wider frequency guard-band

between adjacent frequency-division channels,and pulse-shaping, respectively.Crosstalk re-sistancerefers to the capability of a multiplexedcommunication system to forestall crosstalk.Although various channels are separated fromeach other in frequency, time and/or space,transmission efficiency and systems economicsare at times maximized at the expense ofcrosstalk resistance. In an optical transmissionsystem, crosstalk refers to the leakage of opti-cal power from one optical conductor to anotheroptical conductor.

crown glass Optical crown glass is a lowindex, commercial quality glass. It is designatedas B270 and has a dispersionnF − nC , equalto 0.0089. The Abbe factor (constringence) isequal to 58.8.

cryobiology The study of the effects of lowtemperature on biological systems.

cryogen, biological and medical uses ofCryogens and low temperature technology havefound their way into hospitals and doctors’ of-fices in several areas. Specifically, liquid nitro-gen is now used to remove small growths such asmoles. Both liquid helium and liquid nitrogenare used in magnetic resonance imaging (MRI)machines. A relatively new field, magnetic en-cephalography (MEG, the magnetic analog toEEG) uses superconducting quantum interfer-ence devices (SQUIDS) to measure processesin the brain. These SQUIDS need liquid ni-trogen to work. In most hospitals, the oxygenused for patients throughout the hospital is nowshipped to the hospital as liquid oxygen whereit is boiled off as necessary before use. In bio-logical research laboratories, liquid nitrogen isoften used to preserve cell cultures for futurestudy and use.

cryogenics The science of producing andmeasuring extremely low temperatures, usuallytemperatures below roughly 100 K. Low tem-perature physics utilizes cryogenic techniquesto study the properties of matter at reduced tem-peratures.

cryogens Liquids used in cryogenic appli-cations, also known as liquefied gases. Typ-


ical cryogens include nitrogen, argon, hydro-gen, helium. The cryogens used most frequentlyare liquid nitrogen, with a liquefaction point of77 K, and liquid helium, with a liquefactionpoint of 4.2 K.

cryopumping Cryopumps use physi-adsorption to decrease the pressure in a vessel.The physi-adsorption is accomplished throughthe use of a material with large specific sur-face area, e.g., activated charcoal, contained in aclosed container. This closed container is oftenattached to a dipstick to allow easy immersionin a storage dewar of liquid helium. The con-tainer is connected to the region of interest viaa tube, and the cryopump is then immersed inliquid helium. When the cryopump becomesvery cold, gases adsorb onto the surface of thecharcoal, thereby decreasing the pressure in thesystem. One gram of charcoal can adsorb halfa liter of helium gas (STP) when the charcoal isat 4.2 K, producing a final pressure of 10−4 to10−5 mbar.

cryostat A cryostat is a device that allowsa region to be maintained at low temperaturesfor extended periods. In its simplest form, acryostat is often a dewar containing a cryogenin which an experimental sample is immersed.More complicated cryostats may include multi-ple cryogens separated by vacuum spaces (ac-curately described as “dewars-within-dewars”)to allow operation at very low temperatures forextended periods of time.See alsocryogens,dewar.

cryotron A three-terminal electronic de-vice in which the control element is controlledthrough a magnetic field. The control element isa superconductor, and the magnetic field limitsthe current through the control element.

crystal A particular form of solid, charac-terized by periodicity of building blocks calledcellsin all three directions. Many crystals showfascinating optical properties. Partial crystalscan exist in one or two dimensions.

crystal, biaxial Triclinic, monoclinic and or-thorhombic crystalline systems possess two dif-ferent optic axes and are called biaxial crystals.

These crystals, such as crypsum, feldspar, mica,or topaz have three distinct indices of refraction.

crystallography The study of the architec-ture of atoms in a crystal constitutescrystallog-raphy. Scattering and diffraction studies usingX-rays or neutrons is a technique to determinethe structure and dynamics of crystals.For de-tails of this extensive field, see“Crystal Struc-tures” by R.W.G. Wyckoff, Vol. 1-5, New York,John Wiley & Sons.See alsoBragg’s law.

crystal, optically negative In crystals show-ing double refraction or birefringence, an inci-dent unpolarized ray of light causes the emer-gence of two refracted beams in addition to areflected beam. One of the refracted rays doesnot follow Snell’s law of refraction and hence iscalled anE (for extraordinary) ray in contrastto the other which is called theO (for ordinary)ray. In an optically negative crystal, e.g., cal-cite, the index of refraction of theE ray is lessthan that for theO ray. In biaxial optically neg-ative crystals (e.g., mica, aragonite),β is nearerto γ than toα. (For a biaxial crystal,α, β andγ are the principal refractive indices such thatα < β < γ.)

The axes in a birefringent crystal in which theordinary and extraordinary rays of light propa-gate with the same velocity. In other directions,the light passing through the crystal is dividedinto two polarized rays that pass with differentvelocities. The difference of the speed increasesfrom zero along the optic axis to a maximum forpropagation normal to the optical axis. An uni-axial crystal and a biaxial crystal have one andtwo optical axis, respectively.See alsoopticalaxial plane.

crystal, optically positive If the index of re-fraction for theE ray is greater than that for theO ray, the uniaxial crystal (e.g., quartz) is calledan optically positive crystal. In biaxial opti-cally positive crystals (e.g., topaz, turquoise),β is nearer toα than toγ. Seecrystal, opticallynegative.

crystals, quartz piezoelectric Quartz crys-tals that exhibit the piezoelectric effect. At theend of the last century, quartz crystals were usedfor detailed study of piezoelectric effect. Piezo-


electric properties also have many other naturaland artificially grown crystals and piezoceramikmaterials (polycrystal solid solutions exposedto polarization in electric field). Piezoelectriccrystals and materials are widely used in piezo-electric transducers such as loudspeakers, mi-crophones, etc.Seepiezoelectric effect.

crystal symmetry An ideal crystal containsan infinite regular repetition of identical struc-tural units. A periodic lattice with a group ofatoms (called the basis) situated at each latticepoint constitutes a crystal. The crystal structureremains unchanged under certain symmetry op-erations of translation, rotation and reflection (ora combination of these) due to the property ofcrystal symmetry. For example, a crystal with acubic symmetry remains indistinguishable if itis translated by one or integer multiples of thelattice spacing.Seecrystallography.

Curie law For paramagnetic materials themagnetizationM produced by a magnetizingfieldH is proportional toH and inversely pro-portional to the temperature of the material. Theconstant of proportionality is theCurie constant.

Curie temperature This is the temperatureabove which the arrangement of electron spins ina ferromagnetic material becomes randomizedby thermal agitation. At that point, the sponta-neous magnetization of the material vanishes.

Curie Weiss law This gives the relationshipbetween the paramagnetic susceptibilityχ of amaterial well above the Curie temperatureTc atwhich a ferromagnetic material becomes para-magnetic, i.e.,χ = C/(T −Tc), whereT is theabsolute temperature andC is the Curie constantof the material.

current, acoustic Mean flow in a fluid or gascaused by absorption of intense sound. Propa-gation of a sound wave in a medium always re-sults in acoustic displacements of medium par-ticles. These displacements can be convertedinto a mean flow in the medium if theabsorp-tion of sound in the medium and theintensityofsound are sufficiently high. The acoustic cur-rents can occur near walls and away from them,and always have a form of eddies.

current balance This is a type of balance inwhich the force required to prevent the move-ment of one current-carrying coil in the mag-netic field of a second coil carrying the samecurrent is measured by means of a balancingmass.

current density A physical quantity thatmeasures the amount of charge that passesthrough a unit area per unit time,J = Q/(At).

current generator A type of generator thatgenerates a constant current through a load andis independent of the resistance of the load.

current, membrane A flow of ions and po-lar molecules across the membrane. Movementof ions and polar molecules may be facilitatedby active transport or restricted by proteins em-bedded in the membrane.

current source Seeconstant current source.

curvature of a surface The reciprocal of theradius of a circle that most nearly approximatesthe section of a surface. It is one of the geometricfactors determining the reflective and refractiveproperties of a surface. Only for a spherical sur-face, the curvature (measured in diopters) is thesame in all meridians and is equal to the inverseof the radius of curvature (measured in meters),the sign depending on the sign convention used.

curvature of field An aberration caused byoff-axis rays leading the image plane to becurved rather than flat. This defect is undesir-able in cameras, enlargers and projectors. Ifthe image is obtained on a flat surface, the cen-tral region will be in sharp focus and blurredat the edges. In a two-lens system, correctionis obtained by meeting the Petzval conditionn1f1 + n2f2 = 0, wheren1, n2 are the indicesof refraction andf1 andf2 are the focal lengthsof the lenses.

cutin voltage For diodes this is the approx-imate voltage for which it begins to conduct.Current will always flow through an ideal diodefor any applied voltage. However, given the ex-ponential current dependence on the voltage andpracticalities in dealing with real, physical de-


vices, currents of the order of 1mA or 1% ofmaximum rated current may be considered asa turn-on current. The corresponding voltagefor a silicon diode is≈ 0.6V and≈ 0.2V for agermanium diode.

cut-off region Region in frequency wherean amplifier’s gain has fallen by 3 dB.See alsohalf-power frequency.

c-w wave Also known as continuous wave.Lasers that produce a steady output with time asopposed to a pulse or series of pulses of shortduration.

cybernetics Cybernetics represents the the-oretical study of control and communicationwithin large-scale complex engineering sys-tems, human organizations, or human societyas a whole. Cybernetics also study the con-trol and communication between engineeringdevices and humans, and further studies the sim-ilarity between human-made machines and bio-logical organisms. Man-made engineering sys-tems studied in cybernetics may be electronicor mechanical, and human systems investigatedin cybernetics may relate to corporate organiza-tion and management, education, public healthcare, urban development, socio-economics, na-tional policies, the environment, or human ecol-ogy. Cybernetics also investigates the behav-ior of a complex system involving as its pivotalcomponent a human being monitoring and re-sponding to the dynamic behavior of the rest ofthe complex system. Cybernetics models, sim-ulates, optimizes, tests, and evaluates variouselectronic or mechanical assemblages or humanorganizational systems using techniques in tra-ditional numerical analysis, automatic controltheory, artificial and computational intelligence,pattern recognition, adaptive and learning sys-tems, cognitive sciences, game theory, data fu-sion, neural networks and fuzzy logic.

cycle, acoustic One complete oscillation (vi-bration). Suppose that a one-dimensional os-cillation is mathematically described asξ =ξ0 sin(2πt/T − 2πx/λ). Here,ξ is an oscil-lating quantity,ξ0 is its amplitude,T andλ area period and a wavelength of oscillations,t istime, andx is a spatial coordinate. Then, at agiven pointx = x0, one time cycle of oscilla-tions is a dependence ofξ on t from any timemomentt0 to the time momentt0 + T . Simi-larly, for a fixed time momentt = t0, one spatialcycle of oscillations is a dependence ofξ on xfrom any pointx0 to the pointx0 + λ.

cyclotron A particle accelerator in whichpositively charged particles are accelerated inD shaped magnets (dees); the energy is suppliedby a high frequency voltage applied across thedees. When the radius of the paths of the parti-cles reaches that of the dees, they are electricallydeflected out of the dees to form a high energybeam for use in nuclear experiments.

Czerny-Turner mount An arrangement ofdiffraction grating and other optical elements ina spectrometer. In this system, the light from theentrance slit is collimated by a concave mirror,dispersed by the grating and focused on to theexit slit by another concave mirror.

Czerny-Turner mount.


Ddamped oscillations Oscillations in whichthe amplitude decreases over the course of timet. The decrease is caused by a loss of energyin the oscillations due to friction and/or othermechanisms. The simplest model of damped os-cillations is one-dimensional damped harmonicoscillations, which are described by the follow-ing second order differential equation:

Mdx2

dt2+R

dx

dt+ Sx = 0 .

Here,x is a coordinate of a physical quantity un-der oscillation,M is the mass of the quantity,Ris thedamping coefficient,andS is the stiffnesscoefficient. ForR < 2

√SM , this equation has

the following solution

x = Ae−Rt2M sin

(√S/M − (R/2M)2t+ φ

),

whereA andφ are arbitrary constants. This for-mula describes damped harmonic oscillationsin which the amplitude exponentially decreasesover the course of timet.

damping coefficient A coefficientR of ve-locity term in the equation fordampedharmonicoscillations. The damping coefficient deter-mines a rate at which the amplitude of oscil-lations is decreased over the course of time.

damping factor A dimensionless index usedin second order systems. This index describesthe system’s tendency to oscillate. Examplesof mechanical second order systems are drivenvibrating strings and pendulum with a meansof mechanical energy loss, i.e., friction. Thedamping factor would influence the response ofthe system to the driving frequency, in contrastto the natural, undamped frequency.

This parameter is used in the analysis ofamplifier and filter circuits with a double-poletransfer function. It thus governs the overallfrequency response of the circuit. Consideringa general filter with an input (driven) voltage,

the output voltage (response) as a function offrequency for a low pass circuit is given by

Vout

Vin(dB) = −20 log

√(1− ω2)2 + 4δ2ω2 ,

where ω is the ratio of the input frequency(ωdriven) to the natural, undamped frequency(ωn) andδ is the damping factor. As illustratedin the graph,δ controls the overall shape of thesystem’s frequency response.

For δ = 1/√

2, the response is maximallyflat. For values ofδ <

√2, the response of the

system is peaked near the natural frequency. Thesmallerδ, the closer the peak response will beto the natural frequency. In filter design, prac-tical values are betweenδ = 0 and2. A filter–amplifier with zero damping will thus tend tooscillate at its natural frequency.

Low pass frequency response of a second–order filter

for various values of the damping factor, δ.

The damping factor also determines the sys-tem’s transient response to an instantaneous stepinput. In solving the transfer function of a sec-ond order system and thus determining the out-put due to a step input, there arise three specialcases for the value ofδ.

1. Underdampedδ < 1: The output over-shoots the step response and tends to oscillatewith an exponentially decaying sinusoidal wave,i.e., ringing. The output response is proportional


to

Vout

Vin∝ 1−

[δ√

1− δ2sin(√

1− δ2ωnt)

+cos(√

1− δ2ωnt)]e−δωnt .

2. Overdampedδ > 1: The output slowlyapproaches the final output value. The responsefunction is given by

Vout

Vin∝ 1

2√δ2 − 1

[1d1e−d1ωnt − 1

d2e−d2ωnt

]with d1 = δ−

√δ2 − 1 andd2 = δ+

√δ2 − 1.

3. Critically damped δ = 1: The outputalso approaches the final output asymptotically.This, however, is the fastest response withoutoscillation.

Usually, a damping factor of≈ 1√2

is accept-able as a best compromise between ringing andslow response.

Temporal response of a second–order system to an in-

stantaneous step input for various values of the damp-

ing factor, δ.

Daniell cell A type of cell that was originallyinvented by J.F. Daniell in 1836. It consists ofa zinc anode and a copper cathode immersed insulfate electrolyte containing zinc ions and cop-per ions separated by porous wall. The chemicalreactions during discharging include the release

of electrons at the Zn electrode and the recom-bination of electrons and copper ions at the Cuelectrode.

Darcy’s law In percolation theory, this lawyields a model of gravitational flow of a liquidthrough a permeable membrane. LetJ be therate of flow of the liquid through the membranein cubic meters per second,G be the amountof hydraulic pressure lost per flow distance (hy-draulic gradient), andA be the cross sectionalarea of membrane through which the liquidpasses. Then Darcy’s law states:J = kGAwherek is the proportionality constant.

Darlington connection A useful connectionscheme for a direct coupled transistor ampli-fier. Shown in the figure are two transistorsconnected in the common–emitter (CE) formof the Darlington compound connection. Themain advantages of this connection scheme areincreased input impedance and improved powerhandling capacity as compared to a single tran-sistor. The overall current gain is approximatelythe product of the CE current gain,hfe (seeh-parameters), of each individual transistor. Tran-sistorT1 is assumed operated as a common col-lector (CC) stage, thus it has a relatively highinput resistance. The base current ofT2 is sup-plied viaT1 emitter current. SoT2 carries mostof the current and is usually a higher power tran-sistor.

Two transistors connected in the common–emitter Dar-

lington connection.


DC bias A constant voltage or current that issuperimposed with an AC signal. Vacuum tubesgenerally require a bias voltage added to the sig-nal applied to the control grid. Transistors, onthe other hand, generally require a current biasapplied to the base in addition to the signal to beamplified. This is done, in both cases, to ensurethat the device operates in the intended area ofits characteristic curve.

DC feedback The constant, or average, di-rect current component of the return signal in afeedback control system.See alsofeedback.

DC source Provides voltage to power elec-tronic circuits, e.g.,VCC orVEE voltages for op-erational amplifiers. The voltage source main-tains a prescribed DC voltage level, independentof current loading. Thecomplianceis the cur-rent range for which the source can maintain thedesired voltage level.

DC voltage regulator A device that con-ditions a poorly specified input voltage sourceand provides a stable, well-specified DC volt-age. The regulator’s output voltage should beindependent of such things as load current, tem-perature changes, or any temporal behavior ofthe input voltage source. Sometimes, protec-tion circuitry is incorporated in the regulator toprevent overload conditions.

deafness Partly or completely impaired hear-ing. There are three main types of deafness:cortial, nerve, and conductive deafness. Cortialdeafness is the inability of a human brain to ap-propriately “understand” nerve signals from theinnerear even if these signals are unimpaired.Cortial deafness is typical mainly of elderly peo-ple. Nerve deafness is due to impairment ofthe cochlea located in the inner ear. Conductivedeafness is caused by defects of sound transmis-sion from the outer ear to the cochlea. Deafnessis studied byaudiometry.Hearing aids are com-monly used devices to compensate deafness.

Debeye-Huckel constant Peter Debeye andE. Huckel described the behavior of strong elec-trolytes in dilute solutions yielding the distribu-tion of and the interaction forces between ionsand counterions [Debeye, P. and Huckel, E.Z.,

Phys. vol. 24 (1923)]. For low potentials, thedecay of the potential away from a membranesurface follows an exponential law in which thedecay length (Debeye-Huckel constant) is pro-portional to the charge density of the counterionsand inversely proportional to the square root ofthe temperature.

De Broglie wavelength The wavelength,λof a particle of momentump given by the for-mulaλ = h

p whereh is the Planck’s constant(6.626 × 10−34 J.s). For sub-atomic particles,this wave aspect is significant, resulting in theobservation of wave-like phenomena such as in-terference and diffraction. This wavelength alsodescribes the uncertainty in the position of a par-ticle with the uncertainty in momentum of theorder ofp.

debugging The process whereby faults insoftware are corrected. If the software is faulty,the problem, which may be in just a small mod-ule, is corrected and the software generally re-leased as a newer version. If there is a signifi-cant amount of reworking, it is then released asa higher version.

Debye-Scherrer rings Diffraction of X-raysby a powder sample (usually contained in a fineglass tube and rotated about a vertical axis) pro-duces a set of reflections for different atomicplanes satisfyingBragg’s law. As shown in thedrawing, a monochromatic beam of X-rays isscattered by a sample in the middle of the cir-cle. The film is a circular strip, and diffractedbeams are cones coaxial with the incident beamand intersect the film in arcs. The exposed filmwill contain rings, namedDebye-Scherrer rings,centered around the incident beam direction. Bymeasuring the diameter (s) of a given ring andthe radius (R) of the film cylinder, one can de-duce the Bragg angle, which isθ = s

4R , andhence the atomic spacing of the correspondingplanes that cause diffraction. This technique isused for identification of samples by comparingthe ring pattern to that of known substances.

decay of sound A decrease of acoustic en-ergy densityE in a room over the course oftime t after the shut-down all sound sources. Inmost cases, sound decays exponentiallyE =


Exposed film strip; Debye-Scherrer ring.

E0e−t/τ . Here,E0 is acoustic energy density

at timet = 0, andτ is the characteristic decaytime.

DECCA A radio navigation system operat-ing in the range of 70 to 130 kHz that is usedby the British. Positions in air or at sea can bedetermined by comparing the phase differencesreceived from two or more fixed synchronizedradio stations. The operating range is about 400km.

decibel One tenth of a bel. 1dB = 0.1B,where dB and B are abbreviations fordecibelandbel. If I2 andI1 are quantities having di-mensions of energy, intensity, power, etc., theirratio in decibels is given byN = 10 lg(I2/I1).In acoustics it is customary to measure and re-port sound pressure levels in decibels.

declination, magnetic The angle betweenthe magnetic meridian and the geographicalmeridian is known as themagnetic declination.

decoding In communication, a message insource symbols at the transmitter can be recov-ered at its destination from the string of values orsymbols from its coded representation. This isdone by using an algorithm for decompressingdata at the destination. It is of great importancethat the decoded symbols be error free.

decoding, decision Transmission can cause asignal to degenerate and interfere with the taskof the receiver to establish the transmitted se-quence. The accuracy with which the decisionthreshold must be placed depends on the severityof the distortion suffered during transmission. A

single decision threshold is therefore requiredfor binary sequence.

decoding, feedback A type of decoding inwhich each decoding decision on transmitted in-formation is fed back to affect future decisions.This method can cause the undesirable propertyof error propagation caused by an incorrect de-cision in a previous step.

decoding, hard decision A type of decisionfrom the demodulator when regenerating the in-formation sequence. The Hamming distance be-tween the received symbols and the estimatedtransmitted symbols in the trellis are used asa measure of confidence, known as the metric.This decoding method is optimum since it min-imizes the probability that the entire sequenceis in error. The demodulated signal at the de-modulator output is sampled and hard limited toregenerate the binary signal for channel decod-ing.

decoding, likelihood ratio Noise degener-ates the decision-making process at the output-stage of transmitted signals. It is therefore nec-essary that the decision-making is statisticallyquantified by indicating the probability of mak-ing an erroneous decision. The error probabilityis generally treated as that due to Gaussian noise.Thelikelihood ratiois the ratio of two Gaussianpoint distribution functions of receiving a signalx under the hypothesis thaty andz are trans-mitted. Small changes in signal to noise greatlyaffect the error probability. The path with thesmallest distance, from all the paths in the trel-lis, is selected, in Viterbi algorithm. This resultsin a minimum bit error rate. Equalizers are em-ployed in circuits to reduce such problems.Seedecoding, Viterbi.

decoding, metric The test statistic that isused in maximum likelihood decoding for eachcomplete path in a tree, and determines thepath for which certain optimized values will belargest. This is useful for minimizing errors as-sociated with decoding.Seedecoding, trellisdiagram.

decoding, soft decision In this approach, thesignal variations at the output of the demodu-


lator are sampled and quantized. The demod-ulator passes a sequence of quantized levels tothe decoder instead of a sequence of data bit.Sometimes incorrect decoding that occurs withhard decision decoding can be rectified by us-ing soft decision decoding.Seedecoding, harddecision.

decoding, state diagram A convolutionalcode tree withk inputs can be represented bythis type of diagram. In general, each state isassociated with a previousk− 1 input bits lead-ing to it and the transition between states by theoutput sequence produced by the input bit caus-ing the transition.Seedecoding, trellis diagram.

decoding, trellis diagram This method isused for more effective utilization of availablebandwidth and power, where convolutional cod-ing and modulation are treated as a single en-tity. There is dependency between successivesignal points such that only certain patterns orsequences of signal points are permitted. Thisproduces the trellis structure. Maximum likeli-hood of decoding trellis codes consists of find-ing that path through the trellis with minimumsquared Euclidean distance to the received se-quence. The coding of points is done to maxi-mize the chance of detecting errors.Seedecod-ing, Viterbi.

decoding, Viterbi An algorithm procedurethat involves considering paths through a trellisdiagram, in comparing the received sequencesof codes with all the possible sequences thatcan be obtained with the encoder. This pro-cedure involves considering the retained pathsin the trellis diagram so that a continuous pathis formed through the trellis with a minimumaggregate Hamming distance. The decoding al-gorithm makes use of the repetition property ofthe convolutional code tree to reduce the numberof comparisons.

de-coupling Removing the inter-relationshipbetween two entities. For example, in a multi-stage amplifier, it is necessary to de-couple thepower supply of the input stage from the remain-der of the amplifier. The reason for this is thatthe supply voltage changes with current becauseof the effective internal impedance of the power

supply. Any small change in the power supplyvoltage alters the bias on the first stage, and isamplified in the same way as an input signal.

dedicated line Generally refers to phonelines in which the path is set up from source todestination such that it is assigned exclusivelyto a particular connection or call.

defibrillator An instrument that provides anelectric shock in such a way as to restore a nor-mal heartbeat by arresting fibrillation of the ven-tricular muscle.

deflector coil The coil used in a cathode raytube to deflect the direction of electron beam iscalled thedeflector coil.

degeneracy acoustic Existence of differentnormal modes in a vibrating system that havethe same frequency. For example in a vibrat-ing square membrane, there can be two differentnormal modes corresponding to the allowed fre-quencies. Various linear combinations of thesenormal modes give an infinite number of pos-sible vibrations in a square membrane for thegiven frequency.

degree of coherence Seecoherence, degreeof.

de Haas van Alphen effect If the magneticsusceptibility of metals is measured at low tem-peratures in a magnetic field,χ is found to os-cillate as a function of magnetic field. Carefulanalysis shows that the oscillations are actuallyperiodic in1/H, not inH. It can be shown thatthe length of the period in1/H is inversely pro-portional to an extremal cross-sectional area ofthe Fermi surface normal to the magnetic field.For an ideal Fermi sphere, the extremal area issimply a circle of radiuskF , not providing muchinformation. In real metals, however, the shapeof the Fermi surface can be fairly convoluted.It is possible, by varying the direction of themagnetic field, to reconstruct the Fermi surfaceusing this effect, thede Haas van Alphen effect.This can be a complicated task when the Fermisurface has several extremal cross-sections in agiven direction, so it is often easier to develop atheoretical Fermi surface, and then match a the-


ory to the data. Similar oscillations are foundin the electrical conductivity (where they areknown asShubnikov-de Haas oscillations), ther-mal conductivity, magnetoresistance, sound at-tenuation, and all other physical properties ofmetals that depend on the electronic density ofstates.

dehardening The thawing of a biologicalsystem that had been subjected to prolongedcold.

delay Generally refers to the transient timeinvolved in switching networks or digital gatesin response to a stimulus. When considering theswitching properties of a device (e.g., a transis-tor), the delay time is the time required for theoutput to rise to 10% of the full asymptotic out-put in response to a step input. Other parametersused to describe the response are the rise, stor-age, and fall times. These are illustrated below.

Definition of delay, rise, storage, and fall times for a

device responding to a step input.

delay distortion A type of transmission im-pairment caused by different Fourier compo-nents travelling at different speeds. For digitaldata, fast components from one bit may catchup and overtake slow components from the bitahead, thereby increasing the probability of in-correct reception.

delay equalizer This is used as a solution tothe problem of a receiver getting several signalsfrom a transmitter, each of which has traveled adifferent path between transmitter and receiver.

In this method, delayed and attenuated imagesof the direct signal are subtracted from the actualreceived signal.

delay line A communications or electroniccircuit that has a built-in delay. Acoustic delaylines were used to create the earliest computermemories by using tubes of liquid mercury thatwould slow down the digital pulses long enough,e.g., a fraction of a second, to serve as storage.An optical fiber of a precise length can also beused to introduce a delay in a light wave pulsewhich is equal to the time required for the pulseto propagate from beginning to end.

delay, signal Associated with multipath dis-persion, where multiple signals originating fromthe same transmitter follow different paths to thereceiver. This causes signals relating to a previ-ous bit/symbol to interfere with the signals relat-ing to the next bit/symbol.Seedelay equalizer.

Dellinger effect A sudden fade-out of radiosignals that can occur in the band from about 1MHz to about 30 MHz. This is caused by partialor complete absorption in the ionosphere oftenas a result of abnormal solar radiation affectingthe transmission path.

delta connection In a three-phase, three-wiremotor, an arrangement such that the phases be-tween any two wires are±120 apart. There areseveral advantages including:

1. compactness of the device,2. greater initial torque,3. ease of starting, and4. minimization of the Joule loss on the lead

wires.A schematic diagram of a three-phase circuit

is shown in the figure below. The namedeltaconnectioncomes from the geometry of the cir-cuit.

delta network One of two common connec-tion schemes in the generation and loading ofthree-phase electrical power, the other beingY–network.These are a system of three sinusoidalvoltage sources, each with the same magnitudeand frequency, but 120 out of phase with eachother. The delta connection scheme is shownbelow for both source and load. If the currents


Delta network.

in each “side” of the delta are equal, then thesystem is said to be balanced. In this case, theequivalent Y–network can be easily computed.

demagnetization The process of renderingthe orientation of magnetic domains randomlyto reduce the magnetization to zero.

demagnetization, adiabatic One techniquefor demagnetizing in which the material isheated to its Curie point and then cooled in theabsence of an external magnetic field.

demodulation The process of extractingmeaningful information from a composite wave-form by performing the inverse process of mod-ulation. Knowledge of the modulation scheme(e.g., amplitude, frequency, or pulsewidth mod-ulation) is required to decode the waveform.Seemodulation.

de Morgan’s laws A statement of the re-lationship between the elementary operationsconjunction and disjunction in boolean algebra.De Morgan’s laws are a direct implication ofthe duality principle and are useful in simpli-fying complicated boolean expressions. Con-sider an arbitrary number of boolean variablesA,B,C, . . . each of whose value is either 1 or 0(true or false) and can represent an input vari-able, constant, or functional result. Then, thetwo forms of de Morgan’s law are

1. the inverse of a product of variables is equalto the sum of inverses of the individual variables,

A⊗B ⊗ C ⊗ . . . = A⊕ B ⊕ C ⊕ . . . .

2. and the inverse of a sum of variables isequal to the product of inverses of the individual

variables.

A⊕B ⊕ C ⊕ . . . = A⊗ B ⊗ C ⊗ . . .

In the above equations, the bar(. . . ) repre-sents the inverse of that variable or expression(NOT),⊗ represents the conjunction (AND) op-eration, and⊕ is the disjunction (OR) opera-tion: the three elementary boolean operationsand their implementations in digital electronics.

These rules serve as a mathematical basisfor constructing arbitrary logic functions basedon a subset of the elementary gates. For ex-ample, in TTL (transistor-transistor logic) cir-cuitry, it is usually more economical to constructNOR≡NOT OR circuits. Thus, theAND gatecan be formed by applying de Morgan’s law:

A⊗B = A⊕ B ;

that is to say,

A AND B = (NOT A) NOR ( NOT B) .

This functional equivalence of the electronicgates is illustrated below.

Equivalent digital electronic circuits as shown by

de Morgan’s laws.

depletion layer The electrostatic dipole layerformed at a p-n semiconductor junction. It isformed by electrons in the n-type region nearthe junction diffusing to the p-type side, leavingbehind positively charged donor ions. Addition-ally, holes will flow from the p-region to theadjacent n-type side leaving behind negativelycharged acceptor ions. Therefore, there aretwo adjacent layers of fixed equal-but-oppositecharges at the p-n junction.See alsodiode junc-tion.

The effective width of the depletion layer canbe calculated by solving Poisson’s equation:

d2V

dx2= −ρ ,


Pictorial representation of the depletion layer formed

at a p-n semiconductor junction.

whereρ is the charge density andV , the elec-trostatic potential. The junction is assumed tobe positioned atx = 0 and the depletion layerextends into the p- and n-type sides by−ln and+lp respectively (see accompanying figure). In-tegrating Poisson’s equation in the n-type regionyields

dV

dx= −ρDx+ constant,

whereρD is the donor ion density. Given theboundary conditiondV

dx ≡ 0 atx = −ln, then

dV

dx x=0= −ρDln .

By a similar argument on the p-side of the junc-tion,

dV

dx x=0= ρAlp ,

thus implying−ρDln = ρAlp, or, owing to theopposite charges ofρA andρD,

NAld = NDlp ,

whereNA andND are the number densities ofthe acceptor and donor ions, respectively. Thedepletion layer(ld + ln) therefore depends onthe relative degree of doping (see alsodoping)of each side of the junction.

depolarization The effect that leads to theloss of the polarization state of a beam of lightas it interacts with a medium. When light is re-flected from mirrors or transmitted through in-terfaces of dielectrics at angles other than nor-mal incidence, the ratio ofTE (s-type) toTM(p-type) polarized light changes according toFresnel’s equations. Polarization-dependent ab-sorption and scattering can also lead to depolar-ization.

Calculation of the width of the depletion layer (see text).

depth gauges Seeliquid refrigerant level;surface detection.

depth of field The range of distances of an ob-ject from an optical system to produce an imageconsidered to be in focus (seedepth of focus).The depth of field is greater for smaller aper-tures (larger f-stop number) and longer objectdistances.Seecamera, depth of field of.

depth of focus The greatest distance throughwhich an image screen (and hence image of anobject) can be moved with a tolerable blur (ornoticeable lack of sharpness of an image). Thisis similar todepth of field,which is the greatestdistance through which an object can be movedwith tolerable blur.Depth of fieldanddepth offocusdepend on the aperture of the system.

depth sounding Finding water depth by us-ing anecho sounder.

de Sauty bridge A type of AC variation ofWheatstone bridge. It is used to measure un-known capacitance. The balance condition forthe de Sauty bridge is given by the followingequation;

CX = CS

(R2

R1

).

desorption Desorption, the reverse of ad-sorption, is when an adsorbed atom or moleculeleaves the surface of the substrate and moves intothe gas phase. Desorption and re-adsorption ofgases at low temperatures can cause problemsfor the experimentalist due to heat transport.The desorbed molecules can increase their ther-mal energy by coming into contact with warmersurfaces, then deposit that energy on a cold sur-face, providing another source of heat into the


de Sauty bridge.

experiment. Desorbed gases can also be used tocontrol the temperature of a sample located inthe path of gas flow by controlling the rate ofdesorption.

detectivity It is the reciprocal of the mini-mum detectable power, called thenoise equiv-alent power(NEP), of a detector. The NEP(unit: W/Hz1/2) of a detector is the rms valueof sinusoidally modulated monochromatic radi-ation in a 1 Hz bandwidth, which gives rise toa signal voltage equal to the noise voltage ofthe detector. The detectivity,D(λ), is limitedby the inherent noise mechanisms such as radi-ation noise (that result from statistical fluctua-tions of photons) and Johnson noise (caused bythermal fluctuation of charge carriers). SpectralD star,D∗(λ, f), is obtained by normalizing theeffects due to the detector area and bandwidth(unit: cm.Hz1/2/W).

detector (1) In communication, a device thatrecovers information from a transmitted signal.Also referred to as ademodulator.

(2) A unit or device used to measure the pres-ence of a given entity, such as the emission ofenergy, flux of particles, or static electric or mag-netic fields. Usually, the output of the detectoris an analog voltage proportional to the strengthor amount of that which is detected.

detector, crystal A crystal used as a trans-ducer to convert mechanical energy into electri-

cal energy. The operating principle is the piezo-electric effect; the unbalancing of the positiveand negative charges in the crystal due to me-chanical stress.

detector, phase sensitive Phase sensitive(synchronous)detectionis a useful techniquefor measuring small signals that are obscured bylarger and/or noisy background signals. Phasesensitive detection is the basic operating princi-ple of lock–in amplifiers.

This detection scheme requires an excitation,or reference, modulation signal with frequencyω. In an experimental measurement, this signalis used to modulate a parameter of the exper-iment and hence indirectly modulate the mea-surement signal, as exemplified by the figure.

Representative measurement illustrating phase sensi-

tive detection.

The principle of phase sensitive detection isbased on mixing (multiplying) the detected sig-nal with a sine wave in phase with the modu-lation reference, as illustrated in the followingblock diagram:

Block diagram illustrating phase sensitive detection.


It is assumed that the detected signal isA =Vsig cos(ωt + φsig) whereφsig is the phase dif-ference measured relative to the modulation sig-nal. A reference sine wave is generated fromthe modulation signal, usually with a phase-lockloop (PLL), and is given byB = Vref cos(ωreft+φref). The product of the two signals is deter-mined by the trigonometric identity:

cos(A)× cos(B)

≡ 12

[cos(A−B) + cos(A+B)] ;

thus, the mixed output signal is

Vmixed =12VsigVref

[cos(ωreft+ φref − ωt− φsig)+ cos(ωreft+ φref + ωt+ φsig)] ,

and there will be two beat frequencies in theoutput, one at the difference and one at the sumfrequency. Sinceωref = ω, then

Vmixed =12VsigVref [cos(φref − φsig)

+ cos(2ωt+ φref + φsig)] .

A low pass filter is used to block the doublefrequency component, therefore leaving a DCsignal proportional to the desired signal. Noisesignals far from the reference frequency are at-tenuated by the low pass filter. Signals withfrequencies close toωref will yield a very lowfrequency AC component in the output. The at-tenuation depends on the bandwidth of the lowpass filter, and the bandwidth of detection forthe whole device is determined by the filter. Se-lecting a longer time constant for the filter willimprove signal-to-noise ratios; however it willalso reduce response times.

With the final output ideally given by

VX ∝ VsigVref cos (φref − φsig) ,

the phase of the reference-generated signal isadjusted such thatφref = φsig for a maximalVX . Alternatively, the detected signal can beindependently mixed with another referenceVref

cos(ωreft+ φref + π2 ), yielding

VY ∝ VsigVref sin(φref − φsig) ,

after proper filtering. Then the final, phase-independent, output can be determined fromVout =

√V 2

X + V 2Y . This alternate scheme is

not shown in the figure.

detector, square law (1) In communication,noncoherent AM demodulation can be accom-plished by rectifying the input signal (AM mod-ulated carrier). If the rectifier has a characteris-tic

Vout = constant × V 2in ,

then the detector is said to follow a square law.However, the detector also generates second har-monic frequencies and thus leads to some dis-tortion of the demodulated carrier.

(2) Inverse square law:The signal detectedas a measuring device is moved away from thesource it is viewing, e.g., an optical power meterdetecting the light emitted from an incandescentlight source. Assuming the source emission isisotropic in space, the square law indicates thatthe detected signal will decrease as a functionof 1/r2 wherer is the distance from the source.This is because the emitted power, or flux, isbeing distributed over larger and larger (imagi-nary) spheres centered on the source. So a de-tector that views the source with a fixed area ofdetection will be intercepting less and less radi-ation as the detector is moved away because thesolid angle of detection is decreasing.

deuterium arc lamp Deuterium gas (alsocalled heavy hydrogen) under high pressure ina high voltage discharge tube produces intensecontinuous ultraviolet (UV) radiation from 180nm to 400 nm. The emission in the 400 to 700nm range contains broadened line spectra. Thelamp is used as a source of radiation inUV spec-troscopy.

deviation ratio Ratio of the maximum allow-able deviation in the frequency to the maximumallowable modulating frequency for FM trans-mission.

dewar A container that holds a cryogeniccoolant such as liquefied nitrogen or helium.Also calledacryostat.

dextrorotatory When linearly polarizedlight propagates through some optically active


substances, the polarization direction rotateswith distance. Viewing the beam head-on, ifthe rotation is clockwise, the substance is calleddextrorotatory (as in a type of sugar called dex-trose). In contrast, levorotatory materials pro-duce a counter-clockwise rotation.

D-field The electric displacement field givenby: D = εoE + P whereεo is the permittivityof free space,E is an external electric field andP is the polarization produced.

dial A device used to generate the pulse sig-nals needed for the automation of telephone ex-changes. The rotary dial used contacts withinthe dial to make and break an electrical cir-cuit from a battery in exchange-through loopmade by the line to the customers’ premises andthrough the phone itself.

dialysis The separation of salts from solutionby placement of the solution on one side of apermeable membrane with water on the oppositeside. The ions diffuse across the membrane dueto osmotic pressure; however, larger moleculesare held back in solution.

diamagnetic materials Materials that ex-hibit diamagnetism and that consequently havea negative susceptibility.

diamagnetism A weak magnetism in whicha material exhibits a magnetization that is oppo-site in direction to the applied field.

diaphragm A flexible membrane or a thinplate that is used in transducers to radiate or re-ceive sound. In order to radiate a sound wave byloudspeaker or other source, a diaphragm is setinto vibrations. In receiving transducers such asa microphone, a diaphragm is set into motionby an incident sound; this motion is then trans-formed into an electrical signal. The theory of avibrating diaphragm is that of a vibrating mem-brane or a thin plate.

dichroic mirror A mirror that can reflect aspecific color of light. Such mirrors are usedin color television cameras. The principle ofoperation depends on the property that the color

of some dyes is concentration-dependent.Seedichromatism.

dichroism A class of anisotropic media thatpolarize light by selective absorption of only oneof the two rectangular components of vibrationof the electric field vector. The wavelength ofthe absorption edge of a crystal depends uponthe linear polarization of the light. A commondichroic crystal is the mineral tourmaline; someorganic compounds such as Polaroid also exhibitthis effect.

dichroism, circular Unequal absorption ofleft- and right-handed circularly polarized light.First observed in solutions by Cotton in 1895;the anomalous rotatory dispersion observed iscalled theCotton effect.

dichroism, circular, fluorescence detected(FDCD) Anomalous dispersion of circularlypolarized light near an absorption edge in achemical substance and leading to fluorescencelight with preferred circular polarization.

dichroism, linear Dichroism is polarizartionof light (electromagnetic radiation) by selectiveabsorption of the radiation along one preferredaxis of two referred to as theO andE axes.Dichroism results from asymmetry in the molec-ular structure of the substance.

dichromate cell A primary cell in whichpoles of carbon amalgamated zinc are im-mersed in a solution of potassium dichromate(K2Cr2O7) in dilute sulfuric acid. The emf of adichromate cell is 2.03 volts.

dichromatism The presence of two absorp-tion bands in an optical material with differentabsorption coefficients. This effect is seen in,for example, the material used for green sun-glasses that look red when doubled over so thatobservation is through twice the normal thick-ness.

dielectric A material that does not conductelectric charge is called adielectric. It is alsoknown as aninsulator.There are two types of di-electric: polar and non-polar. A polar dielectricconsists of polar molecules that have permanent


dipole moment. When an external electric fieldis applied to a polar dielectric, the moleculescan be aligned, while in the non-polar dielectric,the applied field will induce an electric dipolemoment in the atom or molecule and align thedipole moment.

dielectric breakdown When the electricfield applied to a dielectric material exceedsthe dielectric strength of the material, the elec-tric charges will force themselves through thedielectric material. This is called adielectricbreakdown.

dielectric constant The ratio of permittivityof the material to that of the free space,κ =ε/εo, is known as thedielectric constantof thematerial.

dielectric heating Heating of a dielectric ma-terial through the use of radiation of high fre-quency electromagnetic wave.

dielectric hysteresis The dependence of thepolarization of ferroelectric materials on theirprevious history is calleddielectric hysteresis.The dielectric hysteresis in dielectric materialsis analogous to the magnetic hysteresis in ferro-magnetic materials. It is also known asferro-electric hysteresis.

dielectric strength The maximum electricfield a dielectric material can withstand withoutbreakdown is calleddielectric strengthof thematerial.

difference frequency One of the signal com-ponents obtained by mixing two signals withdifferent frequencies. Ignoring phase and am-plitude differences, the two signals can be de-scribed as

V1 = cos (ω1t)

V2 = cos (ω2t)

The mixed signal will yield two components:

V1 × V2 ∝ cos (ω1 − ω2) + cos (ω1 + ω2) ,

a component at the difference frequency and oneat the sum frequency.

differential conductance The inverse of thedifferential resistance. Seedifferential resis-tance.

differential input The voltage difference be-tween the two input terminals of an amplifier,particularly an emitter-coupled amplifier stageas shown below. Referring to the diagram, thedifferential input voltage isV∆ in = Vin 1 −Vin 2.

Most operational amplifiers use an emitter-coupled amplifier as their first amplificationstage.

A basic emitter–coupled transistor amplifier.

differential output The difference betweenthe two output voltages of an emitter–coupledpair. Referring to the above figure, the differen-tial output isV∆ out = Vout 1 − Vout 2. See alsodifferential input.

differential resistance The effective resis-tance between the two input terminals of anamplifier, particularly an emitter-coupled am-plifier stage. Contrast this with the common–mode input resistance, which is the resistancefrom either input to analog ground. An idealoperational-amplifier will have these resistancesinfinite. See alsodifferential input.

differential voltage gain The change in dif-ferential output voltage per unit change of dif-


ferential input, expressed as:

g∆ =V∆ out

V∆ in.

Seedifferential input, differential output.

differentiator A basic circuit, commonlybased on an operational amplifier, that differ-entiates an input voltage with respect to time.Shown below is a simple differentiator circuit.The input capacitor does not allow any directcurrent to flow, only the displacement currentwhich depends on the time rate of change of thevoltage across the capacitor. Since the voltageat the− terminal of the op-amp is an effectivenull, it can be shown that the output voltage ofthe presented ideal differentiator is given by

Vout = −RC dVin

dt.

Thus, a differentiator circuit is useful for mea-suring the rate of change of an input voltage.

A simple differentiator.

diffraction The propagation of light waves inany manner that departs from rectilinear propa-gation predicted by the laws of geometrical op-tics. The term originates from the observationthat light bends around opaque obstacles, result-ing in shadows that have slightly blurred bound-aries. Patterns are produced near the edges ofthe shadow that depend on the size and shape ofthe obstacle. This breaking up of the light as itpasses the object isdiffractionand the observedpatterns arediffraction patterns.

Since the dimensions of obstacles encoun-tered by light are not large compared with thewavelength, the effects are subtle but ubiqui-tous in our common experience. The luminousborder outlining a mountain profile in the first

seconds prior to the sun rising behind it and thestreaks of light seen with half shut eyes viewinga strong light source are but a few of the manycommon examples of the diffraction phenom-ena. It was first commented on by Grimaldi inhis book published in 1665.

Diffraction is now known to be a direct conse-quence of the wave nature of light. Much of thephenomena can be quantitatively described bya mathematical form of Huygen’s principle for-mulated by Kirchoff that is an approximation tothe wave equation, making it unnecessary to rig-orously solve the wave equation to understanddiffraction. There are two convenient catego-rizations of diffraction phenomena, dependingon whether a parallel beam of light passes thediffracting object. When either the light sourceor the observing detector, or both, are a finite dis-tance from the diffracting obstacle or aperture,the diffraction is classified asFresnel diffraction.When either the source or detector, or both, are atinfinite distance, effectively making the beam oflight passing the obstacle parallel, the diffractionis termedFraunhofer diffraction,for historicalreasons. Fresnel diffraction is easiest to observeof the two as no lenses are needed and it was thefirst to be investigated. However the mathemat-ical theory is much more difficult than the planewaves of Fraunhofer diffraction.

diffraction, crystal Destructive and con-structive interference of waves scattered by theperiodic placement of electrons, nuclei or forcefields in the lattice of a crystal, resulting in apattern of discrete spectra.

diffraction, Fraunhofer The diffractionphenomena observed when both the source andobservation point are at very large distancesfrom the diffracting object. In this limit, themathematics of diffraction is much simpler thanFresnel diffraction. A point source at the fo-cal point of a converging lens or collimator isfrequently substituted for an infinitely far lightsource. The diffracted light may be collected bylenses and fringes observed in the focal planeof the lens, rather than from an infinite distance.For diffraction from a slit, it is readily found thatthe diffraction pattern has an absolute maximumat the center of the line appearing on the focalplane of a lens, and a diffraction pattern that is


symmetrical about this center. The width of theprincipal maximum is twice the width of the sec-ondary maxima, and both the principal and sec-ondary maxima are inversely proportional to thewidth of the slit. For a point source, the Fraun-hofer diffraction pattern is a line perpendicularto the slit, whereas with Fresnel diffraction itwould be a band. More elaborate mathemat-ics is required for Fraunhofer diffraction froma circular aperture than from a single slit. Inthis case, the diffraction pattern consists of abright circular disk surrounded by a series ofdark and bright fringes that rapidly decrease inintensity. The detailed results for the circularaperture are of practical use for the properties ofoptical instruments. Typically a lens is limitedby the circular rim, so a converging lens does notproduce an exact point image of far-away pointsources, despite careful correction for aberra-tion. Concave spherical mirrors such as thoseused for telescopes also exhibit this spreading.These diffraction disk images limited by the re-solving power of the optical instrument, and forthis reason objective lenses and mirrors of tele-scopes are made with large diameters.

diffraction, Fresnel Diffraction phenomenaobserved when either the source and observ-ing screen or both are at a finite distance fromthe diffracting object. In the case of a circu-lar aperture, one or more Fresnel zones are un-covered, and the amplitude of the optical dis-turbance is estimated from the area of the zone,and neighboring zones have opposite signs. Thelight intensity goes through a series rings ofmaxima and minima due to the appearance anddisappearance of successive positive and nega-tive Fresnel zones. Near the axis of an aper-ture with dimensions comparable to the distancefrom the observation point, the illumination isnearly identical to that produced by the unob-structed wave. If a circular obstacle is usedrather than an aperture, then given the contribu-tions of various Fresnel zones, the pattern con-sists also of concentric bright and dark rings, butat the center the intensity is always a maxima.

diffraction, Kirchoff’s formula A mathe-matical form of Huygen’s principle formulatedby Kirchoff that is an approximation to the waveequation, making it unnecessary to rigorously

solve the wave equation to understand diffrac-tion phenomena. Using Green’s theorem, thelight wave at any point in space is expressed asintegral over a closed surface surrounding thepoint. The differential contributions from sur-face elements provide the Huygen’s secondarywavelets. In principle, if the part of the closedsurface coincides with the diffracting screen,then the solution of any diffraction may be ob-tained by evaluating this integral with suitableboundary conditions.

diffraction of waves Propagation of acous-tic and electromagnetic waves that follows lawsof geometrical acoustics and optics. Many phe-nomena in wave propagation are the result ofdiffraction, for example: penetration of a waveinto a region of geometrical shadow; penetrationof a wave through a small opening in a screen;and propagation of waves along a surface (sur-face and greeping waves). Usually diffractioneffects become important whenλ > d, whereλ is a wavelength, andd is a characteristic geo-metrical scale of a problem such as the diameterof anaperture,the size of an inhomogeneity ina medium, etc. Diffraction pattern also dependssignificantly on the distancex of sound propa-gation. Ifx ∼ d2/λ, rays from the opposite sideof the aperture, inhomogeneity, etc. have differ-ent phase increments at the point of observation.This case is calledFresnel diffraction(diffrac-tion in converging rays). Ifx d2/λ, thesephase increments are almost the same. This isknown asFraunhofer diffraction(diffraction inparallel rays).

diffractometer An apparatus used in con-junction with optical diffraction methods tosolve problems of X-ray structure analysis. Asource of light is imaged by a lens onto a pinholeand the emerging light from the pinhole is madeparallel by a lens after the pinhole. The interfer-ing beams are made to focus with an additionallens identical to the first lens after the pinhole.A plane mirror is then used to reduce the lengthof the instrument, and either a diffracting screenor a microscope is used to view the resultingdiffraction pattern.

diffuse spectra There are three main se-quences of lines in the spectra of neutral al-


kali atoms: theprincipal, sharpanddiffusese-ries. Theprincipal series are the strong lines,the sharpseries are the very narrow lines, andthediffuseseries spectra quite broad. This ter-minology applies as well to series arising fromthe same types of electron transitions in otheratoms. The physical characteristics of the lines,however, may be very different from the sim-ple observational character of the alkali atomicspectra.

diffusion, cell membrane (1) Lateral dif-fusion: Two-dimensional effective transport ofmolecules within the cell interior. Mechanismsconsist of Brownian motion and percolation, inaddition to active transport.

(2) Translational diffusion:One-dimension-al transport of molecules across a membrane.Mechanisms include facilitated transport and ac-tive transport.

diffusion coefficient, translational A mea-sure of the rate of flow across a permeable mem-brane due to diffusion, having units of squaremeters per second and relating the ratio of fluxto concentration gradient. That is diffusion co-efficient,D = −J/(dC/dx), whereJ is theflux anddC/dx is the concentration gradient.

diffusion, cytoplasm The specific diffusionof potassium, calcium, and sodium atoms acrossthe cytoplasm giving rise to an action potential.The kinetics of this diffusion.

digital arithmetic Digital arithmetic is per-forming mathematical operations on numbersusing digital electronic circuits. The operationsare performed in a binary number system be-cause they are easiest to implement with logiccircuits. Addition and subtraction are the twobasic operations on which all others are based.

Since only the two statestrue and falseareavailable to represent a digit in digital electron-ics, decimal numbers are represented in theirbase 2 (binary) equivalent. Instead of the char-acters0, 1, 2 . . . 9 to represent digits, there areonly 0 and 1. For example, “13” in base 10 rep-resents1 × 101 + 3 × 100. Using thenaturalrepresentation scheme, this is written in binaryas 1101 implying1×23+1×22+0×21+1×20.

However, to represent signed numbers, i.e.,negative numbers, a convention must be used.The three most common schemes to representsigned numbers aresigned magnitude, 2’s com-plement,and1’s complement.Positive numbersin all three schemes are identical to the naturalrepresentation. Insigned magnitude,one of thenumber digits is reserved to indicate the sign. Itis usually the first digit with 1 indicating a neg-ative number. So, 1011 represents−3 in thisscheme. In1’s complement,a positive num-ber is represented with 1 in the first digit andthe remaining digits inverted. Therefore, 1011implies negative(011) which is−4. Negativenumbers are represented in2’s complementbyusing 1 in the first digit position and1−X forthe numerical part.

Example representation ofdecimal numbers in different,3–digit binary representations

Scheme 000 001 010 011

natural 0 1 2 3sign–mag 0 1 2 31’s comp. 0 1 2 32’s comp. 0 1 2 3

Scheme 100 101 110 111

natural 4 5 6 7sign–mag −0 −1 −2 −31’s comp. −3 −2 −1 −02’s comp. −4 −3 −2 −1

Digital arithmetic operations are based on theaddition and/or subtraction of single digits, start-ing with the least significant digit position andpropagating a carry or borrow digit to the nexthigher position. First consider addition of twosingle-digit binary numbers, an addend and au-gend. The addition is defined for all possiblevalues as: Thus, aside from the sum, a carry is


generated as well when adding the single digits.In adding multiple digit numbers, however,

consideration must be given to a carry from thenext least significant digit. Letxi be a digit ofthe addend andyi a digit of the augend. We alsowant to consider a carry from thei − 1 sum,ci−1. Then, the sumsi and carryci from addingthese two digits are defined in the accompanyingtable. A subtraction truth table can be definedas well.

Digital addition truthtable

i yi ci−1 ci si

0 0 0 0 00 1 0 0 11 0 0 0 11 1 0 1 00 0 1 0 10 1 1 1 01 0 1 1 01 1 1 1 1

Digital subractiontruth table

i yi ci−1 bi di

0 0 0 0 00 1 0 1 11 0 0 0 01 1 0 0 10 0 1 1 10 1 1 1 01 0 1 1 11 1 1 0 0

As an illustration, consider adding the binarynumbers 0110 and 0111 (which represent 6 and7 respectively in the natural scheme). Using thetabulated rules, which is consistent with7+6 =

13. The rules for subtraction vary somewhatdepending on which number-coding scheme isbeing used.

The implementation of the addition and sub-traction tables with electronic circuits is donewith a full–adder. A full–adder comprises twohalf–adders (seehalf–adder), and will acceptxi,yi, andci−1 inputs. It will generatesi andci as per the truth table. Multiple digit additioncan be carried out serially or in parallel. In serialaddition, the digits to be added are delimited bya clock pulse. The addition is carried out by onlyone full–adder with the carry being held in a Dflip–flop for the next digit. In parallel addition,there are separate full adders for each digit.

digital circuit Seedigital electronics.

digital combining In digital transmission,some processing tasks are involved from the datasource via a communication channel to a dis-tant data terminal. The two main categories aresource coding and channel coding. Greatly im-proved efficiency can be obtained by combiningsome tasks.Seemultiplex.

digital communication Digital communica-tion refers to a mode of communication whereinthe transmitted information signal is definedonly at a discrete set of time instances, whenthe signal may take on any one value from adiscrete set of values.

A common block-diagram model of a digi-tal communication system involves the follow-ing blocks connected in series: a source, sourceencoder, discrete channel encoder, digital mod-ulator, communication channel, digital demod-ulator, discrete channel demodulator, source de-coder, and a user. The source outputs a stream ofinformation-bearing signal, which may be ana-log (for example, speech, music, a photographicimage) or digital (for example, digitized data orcomputer files). The source encoder takes thesource output and reconstructs it into a streamof binary bits with a lower data rate while impos-ing minimum distortion of the information con-tained therein. The source encoder achieves thisdata compression task by removing the redun-dancy in the information-bearing signal outputby the source. The channel encoder accepts andtransmutes the binary bit stream from the source


encoder into another stream of discrete-time anddiscrete-valued symbols with a higher data rate.This increased data rate aims to incorporate cer-tain intelligent redundancy in the data stream toendow it with the capabilities of transmissionerror self-detection and transmission error self-correction against possible distortions caused bythe modulator-channel-demodulator unit. Themodulator inputs and transforms the digital datastream from the channel encoder into an analogwaveform for the multi-user multiplexed ana-log channel. Note that all physical communica-tions medium, including optical fibers carryinglaser light pulses, must necessarily be analog.This transmitter unit (consisting of the sourceencoder, channel encoder and modulator, con-nected in series) sends onto the channel the mod-ulator’s analog output, which is received by thereceiver. Each component in the receiver — de-modulator, channel decoder, and source decoder— performs the reverse function of its corre-sponding counterpart in the transmitter.

Digital communication contrasts withdiscrete-time analog communication, whereinthe discrete-time signal may take on any onevalue from an uncountably infinite set ofvalues. Digital communication also differsfrom fully analog communication wherein theinformation signal is defined over a continuoustime duration and may take on any one valuefrom an uncountably infinite set of values.An analog communication system wouldneed to perform any source compression,channel coding and modulation all in analog— a difficult task, which often means thesource output would be directly modulatedwithout source compression or channel errorself-detection/self-correction coding. Themajor advantages of digital communicationsare easy error detection and correction, easysignal manipulation, and increased dynamicrange. These advantages arise partly becausethe source/channel encoder/decoder transmutesthe wasteful redundancy in the source outputinto an intelligent redundancy that facilitatestransmission error self-detection and self-correction by the received signal itself. Themain disadvantages of digital communicationsare the requirement for wider bandwidth thananalog communication and the need for signalsynchronization.

digital electronics Electronics applied to theprocessing of binary variables or numbers. Theprocessing circuits which employ componentssuch as diodes, transistors, resistors, etc. to con-struct gates, which perform logical operations.Theselogic circuitsare generally operated in anon-linear, switching mode to accomplish theirintended design. They are the building blocksof larger and more complicated functions likearithmetic operations and memories, also en-compassed in digital electronics.See alsogate.

digital signature Used for message authen-tication as a trailer at the end of a message. Theencrypted trailer is analogous to a signature atthe end of a letter since it verifies the person whoactually sent the message.

digital switch A mechanical or electronic de-vice that can be used to direct the flow of electri-cal or optical signals from one side to the other.Switches with more than two ports, such as aLAN switch or PBX, can be used to route traffic.The semiconductor switch, known as a transis-tor, performs the same function as the familiaron/off light wall switch. The switch is elec-tronically closed by pulsing the semiconductormaterial, which makes it conduct.

digital television Digital TV or DTV debutedin major cities in the United States in 1988. Inorder to receive DTV, a new digital TV set or aset-top box for an existing analog TV is neededsince transmission in the radio frequency is donedigitally. Digital TV sets will support analogTV transmission, which is expected to be broad-cast until at least 2006. Digital TV offers 18formats from SDTV (Standard Definition TV),without snow or ghost. More SDTV channelscan be transmitted within the same bandwidth.Therefore, it will be up to the broadcasters, cableproviders and satellite companies to determinethe amount of content versus quality. Digital TVcan provide 14 progressive scan and 4 interlacedformats.

digital voltmeter Device that can samplean analog signal and quantize the voltage level.It converts the analog signal to a usable digi-tal number with an analog–to–digital converter.The digital number is then presented to a user


indicating the measured voltage in decimal for-mat. Seeconverter, analog/digital.

diode An electronic circuit element. A diodeis the simplest integrated semiconductor struc-ture. It consists of a junction between an n-type and p-type semiconductor. Ideally, cur-rent is only allowed to flow in one direction.Shown in the figure are the diode’s electronicschematic symbol and its semiconductor con-struction. Also shown is an illustration of a typ-ical discrete diode; the n-type side is markedwith a band.Seediode junction.

Diode, a) semiconductor junction, b) schematic sym-

bol, and c) discrete diode with marking scheme. See

text.

When the voltage of the p-type side is posi-tive with respect to the n-type side, the diode isforward biased. Then, current flow is defined aspositive from p– to n–, the magnitude of whichdepends on the magnitude of the applied volt-age. WhenV is such that the p-type side is morenegative than the n-type side, only a very smallamount of current flows through the diode. Thecurrent flow through a diode as a function ofappliedV (for either direction) is given by

I = Is

(exp

(eV

kT

)− 1)

with Is, the reverse saturation current,e =1.602 × 10−19, C the electronic charge,k =1.38 × 10−23J/K Boltzmann constant, andTin Kelvin. A typical diodeI−V curve is plottedbelow.

Quintessential diode I-V curve.

From a practical standpoint, a current flow of1% of rated current can be considered as a nomi-nal “turn–on” current for most applications. Fortypical silicon diodes, this corresponds to an ap-plied forward bias voltage ofVσ ≈ 750mV ;thecutin voltage. In reverse bias, only a smallamount of current is allowed to flow,Is indepen-dent of reverse bias. However, if the reverse biasis large enough, then reverse breakdown will oc-cur (seediode, avalanche) and current will flowin the reverse direction. Provided that powerdissipation in the diode does not exceed designlimits, reverse bias breakdown will not harm thediode.

If a diode is being used in a high speed appli-cation where the reverse bias voltage can changerapidly, then consideration must be given to thecapacitance of the reverse biased diode junction.If this barrier or transition capacitanceis largeenough, then the current intended to be stoppedin reverse bias will still flow due to the displace-ment current of the capacitance. The reversebias capacitance varies as1√

Vand this behavior

can be exploited in frequency locking or modu-lating circuits or in parametric amplifiers.

There are different special types of diodesengineered to have special properties. Exam-ples include theZener,the tunnel,and thelightemitting diode.

diode, avalanche A diode with sufficientpower dissipation capabilities to be intentionallyoperated in a reverse bias breakdown condition.Such a diode is useful in a voltage regulation


circuit. A simple voltage regulation circuit isshown below illustrating this. Provided thatRx,ILoad, andVSupply are such that the designVLoad

is in the breakdown region (VB) of the diode’s I–V curve, the diode will accommodate moderatechanges inVSupplyor ILoad thus regulatingVLoad.See alsodiode.

A basic voltage regulation circuit based on an

avalanche diode.

Characteristic I–V curve for an avalanche or Zener

diode.

A similar device to the avalanche diode is theZener diode. It has a qualitatively identical I–Vcurve as the avalanche diode and behaves simi-larly in circuit. However, the physics behind theZener diode are quite different.

In a reverse bias situation, an extremelylarge electric field can be imposed in the re-gion of the p–n junction. Considering a genuineavalanche situation, the charge carriers (elec-trons and holes), which constitute the relativelysmall reverse conduction current, can be accel-erated to large kinetic energies. If the energy

is large enough and they spend sufficient timein the junction, these carriers can ionize atomsupon impact with the crystal lattice thusexcit-ing free electron-hole pairs. These extra carriersare also subject to the large electric field and cancontinue the process until a very large (reverse)current is obtained hence the termavalanchemultiplication. This effect strongly depends onthe applied electric field, therefore accountingfor the rapid increase of reverse current at thebreakdown voltage. The degree of doping de-termines the physical distance of the depletionlayer and how much time a charge carrier willspend under the influence of the electric field.This, in turn, determines whatVB will be.

On the other hand, by tailoring the doping ofeach semiconductor, it is possible to make thejunction between p– and n– become physicallyvery narrow. Then at some particular value ofreverse bias, it is possible for the electrons toquantum-mechanically tunnel from the valenceband of the p–side to the conduction band of then–side. This is essentially the Zener effect andis shown in the figure below.

In silicon diodes, breakdown voltages in therange of 10 to 100V is due to avalanche mul-tiplication. The Zener effect is responsible fordiodes withVB ≈ 1 to 2V. Diodes withVB inthe range of 6 to 8V are due to both mechanismssimultaneously.

Energy band levels in a diode junction illustrating the

mechanisms behind reverse bias avalanche and Zener

operation.


diode, Gunn A diode with I-V characteristicssimilar to that of a forward biased tunnel diode.The electrical characteristics of a Gunn diode,however, arise from the peculiar band structureof bulk GaAs (gallium arsenide) rather than anycharacteristics due to a p-n junction.

The band structure of GaAs essentially per-mits the existence of two different types of elec-trons. One, which is normally responsible forconduction in the semicondutor, has an effec-tive mass of≈ 0.08me, whereme is the normalfree electron mass. The other allowed electronhas an effective mass of≈ 1.2me.

Under normal electric fields, the semicon-ductor behaves normally, the light electron beingresponsible for current flow. However, at higherapplied electric fields(≈ 104V/cm)some of thelight electrons can be excited into the other en-ergy band, thus becoming aheavyelectron. Thiscauses the conductivity to decrease since the ef-fective mobility of the electrons has decreasedwith increasing effective electron mass. This,in turn, decreases the current through the de-vice although the applied voltage is increasing.Hence, the device exhibits negative differentialmobility.

Under these conditions, moving domains ofhigh electric field strengths are created withinthe material. The thickness of the active mate-rial can be chosen so that the frequency of these“electron-waves” is of the order 10 GHz. Gunndiodes find uses in high frequency oscillator cir-cuits (e.g., police radar) as well as high speedlogic switching circuits.

diode junction A junction formed by themutual contact of an n– and p–type semicon-ductor. This junction has unique and useful elec-trical properties (seediode) that result from thephysics of the junction.

In the bulk of an n–type material, an abun-dance of conduction electrons can be found, neu-tralizing the space charge of the donor ions. Sim-ilarly, there will be an abundance of holes avail-able in the bulk of the p–type material. Whenthe n– and p–type materials are joined, the elec-trons and holes cannot remain separated unlessthere is an electric field at the interface.

When the two materials are in contact, chargetransfer will occur until the Fermi levels areequalized. Initially, the excess electrons on the

n–side will diffuse into the empty electron stateson the p–side. Simultaneously, excess holes onthe p–side will diffuse and fill vacant hole stateson the n–side. This initial flow of charge willleave behind the negatively charged donor ionsin the n–side and positively charged acceptorson the p–side of the junction. This will form anelectrostatic dipole layer at the junction. (Seedepletion layer.) Therefore, the resulting elec-tric field (and associated potential energy differ-ence) will generally oppose further diffusion.

In the p– and n–type materials, the Fermi en-ergy (EF ) lies approximately at the acceptor anddonor levels respectively. When the junctionis formed, charge transfer equalizes the Fermilevels. Thus, the positions of the valence andconduction band edges must vary relative toEF

within the transition region of the junction.

A p–n junction (diode junction) in thermal equilibrium

and zero applied voltage. Shading suggests electron

population in filling of energy bands. For clarity, poten-

tial energy, current flow, and band filling shown are for

electrons only.

A very simple argument explaining the rec-tification nature of the diode junction is nowgiven. In what follows, only electron currentsand energies will be considered. There is alsohole transport in the junction as well. The de-scription for hole current parallels that for theelectron case since the energy of a hole is mea-sured opposite that of an electron and becauseits charge is opposite of the electron.

Even though the depletion layer produces apotential energy differenceδE, which preventselectrons from flowing to the p–side, there willbe a small number of electrons with non-zerothermodynamic probability to have an energy


greater thanδE. These electrons can flow to thep–side where they recombine with holes. Thisis the recombination electron current flow,Jr,and its magnitude will depend on the doping inthe material. There will also be conduction elec-trons thermally generated on the p–side as well,but they can easily drift “down” the potential hillat the junction; this is the generation currentJg.If there is zero voltage across the diode (as in thefigure), then these two currents must balance.

Jr(V = 0) ≡ −Jg(V = 0) .

Now, if a voltage is applied such that the n-side is more positive than the p–side then thepotential barrier will be raised. This conditionis reversebias. Again, the electron current flowthrough the junction is determined by the ther-modynamic probability of an electron havingsufficient energy to overcome the barrier. Therecombination current will therefore be reducedby the Boltzmann factor

Jr (Vreverse) = Jr(V = 0)× exp(

−e|V |kT

).

The generation currentJg is nominally unaf-fected by the reverse bias as this current is goingdown the potential hill anyway.

Reverse biased diode junction.

If the applied voltage is nowforward biased,the potential barrier is reduced and more currentcan flow. The increase in flow is again deter-mined by the thermodynamic probability factor

Jr(Vforward) = Jr(V = 0)× exp(

+e|V |kT

).

And again, the generation current is mostly un-changed.

Forward biased diode junction.

There will also be a contribution to thetotalconduction current by the hole current as well.Hole current through the p–n junction will be-have similarly and the currents due to electronsand holes will add. The total current through thediode as observed in a circuit is then given by

I = IS

(exp

(eV

kT

)− 1)

whereIS is the sum of the two generation cur-rents and is essentially the reverse bias current.The currentI is defined using common circuitconventions as positive flowing from p– to n–side with positiveV indicating the p–side poten-tial with respect to the n–side. Thus the diodejunction current flow depends on the relativebias of the applied voltage.See alsodiode.

diode-transistor logic (DTL) A realizationof logical gates using discrete or integrated elec-tronic components; particularlogic family; de-tailed electronic circuit implementation of logicgates exhibiting a common theme or conventionin the operation of the circuit.

A simple example circuit employing DTL isshown below. This discrete component circuitoperates as a NAND gate assuming the positivelogic convention. It has two inputs,A andB, aswell as the outputC. The basic electronic oper-ation of the DTL gate is now described. If eitherinputA orB is a logical0 (i.e., eitherVA orVB

is near 0 volts) then the current flowing through


Rx is shunted by the diodesDA orDB respec-tively. In this situation, the base–emitter currentof the transistor,IBE , is small and the transistoris “turned-off”. Thus the output voltageVC isnear 5 volts, logical1.

On the other hand, if bothA andB are logi-cal1 (i.e.,VA andVB are near 5 volts) then thecurrent flowing throughRx is no longer shuntedand is divided between resistorRz and the tran-sistor. Then,IBE is such that the transistor issaturated and the outputVC is near zero volts.The resistorRz is used to remove charge fromthe transistor during the transition time from sat-uration to off.

When driving other gates, the transistor mustbe able to sink the current provided through theinput diodes of the other gates. This consid-eration leads to the maximum number of gateswhich can be driven — the fan-out — by thecircuit. The DTL logic family has a greaterfan-out capability over RTL (resistor-transistor-logic) but is somewhat slower.

A simple DTL circuit implementation of a NAND gate.

diopter The curvature of a wavefront at agiven distance from the source is given in termsof the diopter and is expressed in reciprocal me-ters. Powers of lenses and other optical systemsare usually expressed in terms of diopters.

dip A perfectly freely suspended magnetwould in general align itself with the direction ofgreatest field strength. The angle between thisdirection and the horizontal is called thedip.

dip circle This is a mechanical device con-sisting of a thin steel magnet suspended so thatit can rotate in the vertical plane and also berotated about the vertical axis to determine az-imuth. The instrument is used to measure dip.

diplex operation The use of a single circuit,carrier or antenna for the simultaneous transmis-sion or reception of two signals.

dipole acoustic Two identical monopoles lo-cated at a distanced with the amplitudes equalin magnitude and opposite in sign. A dipole iscalled the point dipole ifkd 1, wherek is awavenumber of a sound field radiated by bothmonopoles. An acoustic pressure fieldp of adipole is the superposition of those of the twomonopoles. For a point dipole,p contains a fac-tor (1+i/kr) cos θ that is not present in the fieldof a monopole. Here,r is the distance to the ob-servation point, andθ is the angle between theradius vector to this point and the dipole axis(the line connecting the two monopoles). Dueto this factor, a directivity pattern of a dipole (thedependence ofp onθ) and its near field (acousticpressure field forkr 1) is different from thatof a monopole.

dipole, in dielectric In dielectric material,each atom or molecule can be either a perma-nent dipole or an induced dipole. These dipolescan be aligned by an external electric field andcontribute the properties of the dielectric mate-rial.

dipole magnetic A permanent magnet, cur-rent loop or particle with angular momentumthat experiences a torque when placed in a mag-netic field. It acts as if it consists of two magneticpoles separated by a small distance.

dipole magnetic moment A vector whosecross product with the magnetic induction of amagnetic field is equal to the torque exerted onthe system by the field.

dipstick A dipstick is a colloquial term usedto describe a small experimental probe that isdesigned to be inserted directly into a storagedewar of a liquid cryogen, most often liquid he-lium. Such a probe consists of whatever elec-


trical connections and wiring are necessary at-tached to a tube or rod with the experimentalsample clamped at one end. The other end con-tains the electrical connections that interface theexperiment with any peripheral equipment.

direction finder A device used for the de-termination of one’s terrestrial location. It isusually a radio receiver and becomes an inte-gral part of a larger system of radio transmittersof known location. Early direction finders em-ployed a rotating loop antenna; the direction toan established transmitter could be determinedfrom the known detection pattern of a dipoleantenna. Modern location usually involves anarray of transmitters; location is determined us-ing a form of hyperbolic differential distanceranging (see figure).

The transmitters may be one of two basictypes:

1.Pulsed with a common carrier.The differ-ential distance is determined from differences inpulse arrival times, or

2. Continuous wave using different, but re-lated carrier frequencies.Here, the differenceis determined from phase differences.

Hyperbolic ranging to determine position. The hyber-

bolic ranges are such that AB + BO − AO is a

constant.

directivity of sound Angular distribution ofradiated or received sound power by an acous-tic antenna, array, loudspeaker or microphone.Directivity of an antenna, etc. characterizes its

ability to radiate (receive) sound from one di-rection better than from others. This property ismathematically characterized by the directivityfunction which is defined as the ratio of the ra-diated (received) power in a particular directionto the maximum possible power in the directionknown as the acoustic axis.

discharge The release and eventual re-combination of opposite electric charges of abattery or a capacitor when a load is connectedto its terminals. Energy is released during thedischarge process.

discharge tube Enclosure used in the pro-duction of a glow discharge. The inside is evac-uated and partially filled with the intended gasto be used in the discharge.See alsoglow dis-charge.

discrete channel A discrete channel may re-fer to a communications channel that allows thetransmission and reception of information rep-resented as a discrete sequence of alphabets. Adiscrete channel may alternately refer to a com-munication channel whose effect on the trans-mitted signal may be represented as a discrete-time filter. A discrete-time channel refers to acommunication channel whose response is de-fined only at a set of discrete time instances.

disjunction (logic) A fundamental booleanlogic operation over two or more logical vari-ables. Sometimes referred to asOR, it is thelogical sum

A⊕B ≡ A OR B ,

where the variablesA andB are elements ofthe set1, 0 (or, equivalently, the settrue,false). In other words, the disjunction of thesetwo variables is true ifeitherA orB is true. Thepostulated rules of the logical sum are

0⊕A = A

and1⊕A = 1 .

Also, for every variableA there exists the suminverse,NOT A ≡ A, such that

A⊕ A = 0 .


The logical sum is commutative as well as asso-ciative:

A⊕B = B ⊕A

A⊕ (B ⊕ C) = (A⊕B)⊕ C .

Given these rules, all possible values of theconjunctionY = A ⊕ B can be tabulated in atruth table. This is illustrated in the accompa-nying table.

Disjunction (logical sum) forall possible values of thevariables A and B

A B Y = A OR B

0 0 00 1 11 0 11 1 1

In implementing this operation in digitalelectronics, voltage signals are used to representthe logical variablesA andB following a pre-defined logic convention. A circuit, frequentlyreferred to as agate,is used to determine the dis-junction and the result is provided on an output,Y . The symbol representing such an electroniccircuit performing anOR operation is shownbelow. See alsoconjunction (logic).

Symbol representing disjunction (OR) in digital elec-

tronics.

disk of least confusion Seecircle of leastconfusion.

dispersion The variation of refractive in-dex,n, with frequency of electromagnetic fieldstraversing a material medium. The dielectricconstant is a function of the frequency of thefields and phase velocity is not the same foreach frequency component. In a nondispersivemedium, the index of refraction is independentof the frequency, and the phase and group ve-

locities are bothc/n. In a dispersive mediumthe velocity of energy flow differs greatly fromthe phase velocity, or may even lack a precisemeaning. Dispersion is explained by taking intoaccount the actual motion of the charges in theoptical medium traversed by the light. This mo-tion is modeled using the damped forced oscil-lator of the charge bound to a fixed atom site.The solution gives a polarizability which is in-versely proportional to the mass, so it is the elec-trons that determine the index of refraction inthe transparent regions of the optical spectrum.The ionic polarizabilities only dominate in de-termination of the refractive index in resonantregions.

dispersion, anomalous In the vicinity of anabsorption band, the index of refraction cannotusually be measured because the substance willnot transmit radiation of this wavelength. Onthe long wavelength side of the band, the indexis quite large, but decreases very rapidly with in-creasing wavelength. On the short wavelengthside the opposite behavior of the index is ob-served — it is very small but increases rapidlyas wavelength is decreased. The index of refrac-tion thus has a large discontinuity in the vicin-ity of an absorption band, which causes anoma-lous dispersion. The longer wavelengths havea higher value of the refractive index and aremore refracted than the shorter wavelengths inthis region. This situation is anomalous, unlikethe rest of the dispersion curve where index ofrefraction increases as wavelength decreases.

dispersion, chromatic The decompositionof a beam of white light into separate beams ofcolor that spread out to produce spectra.

dispersion, normal Away from an absorp-tion band, the index of refraction exhibits normaldispersion. The index of refraction increaseswith decreasing wavelength, and the rate of itsincrease becomes larger at shorter wavelengths.For a variety of materials, the refractive indexversus wavelength curve is steeper if the mate-rial has a larger refractive index, but the curvefor one substance cannot be merely shifted in or-dinate scale to obtain the curve for another. Thislatter property of normal dispersion means thatthe spectra from prisms of different substances


never agree exactly in the relative spacing ofspectral lines. Since there is no simple rela-tion between the normal dispersion curves ofdifferent substances, their dispersion is termedirrational. In the visible region, all transparentsubstances that are not colored exhibit normaldispersion.

dispersion, partial Difference in the indexof refraction at two specified wavelengths. Usedby optical designers to compare various pairs ofglasses to determine which will give the leastsecondary spectrum in an achromat. The spec-ified wavelengths are usually at the so-calledFraunhofer lines that are designated by the let-tersA,B,C, . . ., starting at the extreme red.Cdenotes the redC line of hydrogen at 656.3um, D denotes the average wavelength of thetwo yellow D lines of sodium, agreed on as589.3 um,F denotes the blueF line of hydro-gen at 486.1um. Glass catalogs denote the var-ious partial dispersions at these wavelengths bynD − nC, nC − nA, nF − nD, the subscriptsdenoting the spectral lines at which the indiceswere determined.

dispersion, sound Dependence of the phasevelocity of a harmonic wave on the frequencyof this wave. Sound dispersion can occur inducts, waveguides, anddispersive media.In thelatter case, sound dispersion is caused by inclu-sions in a medium (i.e., bubbles in water), byeffects of thermoconductivity and viscosity onsound propagation, or by acousticrelaxation.Sound dispersion due to acoustic relaxation iswell studied; in this case the dependence of thesound speedc on the frequencyω is given byc2(ω) = c20 + (c2∞ − c20)

ω2

ω2r+ω2 . Here,c0 and

c∞ are the values ofc at ω = 0 andω = ∞,respectively, andωr is the relaxation frequency.

dispersive medium A medium in which thephase velocity of a harmonic sound wave de-pends on a frequency of this wave. Propagationof a sound wave through a dispersive mediumresults insound dispersion.

dispersive power A measure of the way therefractive index varies with wavelength in a sam-ple of glass. LetF andC denote the blueF lineand the redC line of hydrogen. The disper-

sive power is define by the equation1/nu =nF − nC/nD − 1 where the subscripts denotethe spectral line at which the index of refrac-tion is determined. The reciprocal of dispersivepower, denoted by the Greek letterν, is between30 and 60 for most optical glasses.

displacement, acoustic Deviation of a parti-cle in a medium from its equilibrium position be-cause of the passage of a sound wave. This wavealso causes deviations (fluctuations) inpressure,fluid velocityanddensityin a medium.

displacement current Is the partial timederivative of the displacement, i.e.,∂D

∂t . It wasfirst introduced by James Clark Maxwell to com-plete Amperes Law.

dissipation, acoustic Transformation of en-ergy of acoustic oscillations into other forms ofenergy, such as heat.

dissonance Nonharmoniuos sounding of twoor more tones played together. The term disso-nance is opposite toconsonance.Dissonanceoccurs when unpleasant beats occur betweenpartials of tones that form an interval.

distortion When the output of an amplifieris not simply a scaled exact replica of the inputsignal; i.e., the transfer function of the amplifieris not perfectly linear for all inputs.

Different types of distortion can be iden-tified based on how the amplifier’s imperfec-tions affect the output signal.Linear distor-tion describesamplitude distortion: differentfrequency components are amplified by differ-ent effective gains, andphase distortion:differ-ent frequency components have unequal phaseshifts. Non-linear distortionproduces new fre-quencies in the output of an amplifier that werenot present in the original signal.Seedistortion,harmonic.

distortion (optical) In the third order the-ory of aberrations, distortion is the fifth of thefive Seidel sum representing deviations from thepath prescribed by the Gaussian ray tracing for-mulas. A system is free of distortion when ithas uniform lateral magnification over its entirefield. Pinhole cameras show no distortion as


all straight lines connecting each pair of conju-gate points pass through the opening. Lensesshow barrel distortion when the magnificationdecreases toward the edge of the field, andpin-cushion distortionwhen there is greater magni-fication towards the edge.

distortion acoustic Any undesired change inan amplitude, frequency or phase of a signal inits transmission or reproduction in devices suchas microphones, earphones, loudspeakers, etc.The main types of acoustic distortion areampli-tude distortion, frequency distortion,andnon-linear distortion. Amplitude distortionoccurswhen the ratio between the amplitudes of theoutput and input in a device is not constant fordifferent values of the input amplitude for a fixedfrequency. Frequency distortionoccurs whenthe ratio between the amplitudes of the outputand input is not constant for different frequen-cies of the harmonic input signal.Non-lineardistortion occurs when there is no linear rela-tionship between the input and the output. Thisdistortion can result in production of harmonicsin the output even if they are not presented inthe input.

distortion, cross over In push-pull ampli-fiers that employ two Class B transistor ampli-fiers, one transistor is used to amplify the pos-itive going portion of an input signal while theother is used to handle the negative going por-tion. In principle, the amplifier should oper-ate linearly for all portions of an input waveform. However, due to the initial curvatureof the emitter-base diode characteristics of thetransistor (i.e., increased input impedance forlow signal levels), small signals are not accu-rately reproduced. Therefore, a sinusoidal base-voltage excitation will not yield a sinusoidal out-put current.

For example, shown in the figure is a typicaldynamic characteristic transfer function for thetwo transistors of a push–pull amplifier. It is as-sumed that the bases of the transistors are givena pure sinusoidal input signal. The peaks of thesine wave are reproduced in the transistor out-put currents, but the signal near zero is distorted.This shows up as distortion in the output waveform as the output swings through zero volts:cross-over distortion.

Dynamic transfer showing cross-over distortion of

push–pull transistor amplifier.

distortion, harmonic A convenient meansby which the deviation from a sought wave-form from an amplifier or signal generator canbe measured. For example, it is a measure ofhow accurately an amplifier (or generator) canreproduce (generate) a sinusoidal wave. The de-viation from a pure sinusoidal wave is expressedby thetotal harmonic distortionas

THD = 100%×√A2

2 +A23 +A2

4 + · · ·

whereAk is the ratio of the amplitude of thekthharmonic to the amplitude of the fundamentalfrequency in the Fourier series representation ofthe given waveform.

diverging wave A wave in which amplitudeand energy are decreased with the distance ofpropagation. Point source, dipole, quadrupoleand most other sources emit diverging waves.Two important particular kinds of divergingwaves are aspherical diverging wave,the am-plitude of which decreases with the distancerof propagation as1/r, andcylindrically diverg-ing wave,the amplitude of which decreases as1/√r.

diversity system This refers to the techniqueof providing more than one path for the estab-lishment of a channel or circuit, thus providinga more reliable service.


divider (1) Voltage or current divider. Asystem of resistors, or loads, intended to reducea given voltage or current to a desired fractionof the original value. A simple voltage divideris shown below. The output voltage is

Vout =R2

R2 +R1Vin ,

assuming no current is drawn on the output.

A voltage divider based on two resistors.

(2) Frequency divider.Seefrequency divider.

D-lines of sodium The emission spectra ofsodium vapor consists of two bright yellow spec-tral lines called the sodiumD1 andD2 lines.Their wavelengths are 5889.95 Å and 5895.92Å, respectively. They result from transitions be-tween the spin-orbit split3p excited states andthe3s ground state.

DNA structure DNA (deoxyribonucleicacid) is a polymeric molecule in which themonomer is composed of a ribose sugar, a phos-phate group and one of four bases (the purinesadenineandguanine,and the pyrimidinescyto-sineandthymine). The deoxyribose and phos-phate groups form the backbone of the polymer.Double-helical DNA is formed by hydrogen-bonding of the bases on two strands of DNA.The nature of the hydrogen bonding is such thatadenine always binds with thymine and gua-nine always binds with cytosine. The stabil-ity of the double helices are affected both bythe hydrogen-bonding within base pairs and bystacking interactions (van der Waals in nature)of the planar bases. Four different right-handedconformations (denoted A, B, C and D) havebeen observed and one left-handed conforma-tion (called Z) has been reported. DNA in so-lution adopts the B conformation, making it

clearly important for living cells. The A con-formation might have biological relevance sinceRNA is always found in the A conformation. Z-DNA has been associated with gene regulation.The C and D conformations appear to exist onlyin the laboratory. The right-handed conforma-tions differ by their helical pitch as well as theposition and orientation of the base pairs withinthe double helix. The genetic code itself is con-tained in the sequence of the bases along one ofthe two strands (known as thesense strand).

dominant wavelength One of the three ob-jective parameters (dominant wavelength, col-orimetric purity and luminosity) used in colori-metric representation of the psychological sen-sation of light. In the monochromatic method ofcolorimetry, half the photometric field is illumi-nated by the color to be matched, and the otherhalf by a mixture of controllable amounts ofmonochromatic light of adjustable wavelengthsand white light of definite spectral quality. Thedominant wavelength is the wavelength of themonochromatic light. The luminosity is the sumof the monochromatic and white light luminosi-ties. The colorimetric purity is the ratio of theluminosity of the monochromatic radiation tothe total luminosity.

Donnan equilibrium Gibbs-Donnan equi-librium exists when, on opposite sides of asemipermeable membrane, the product of dif-fusable anion and counterion concentrations areequal, and when the sum of the concentrationsfor diffusable and nondiffusable anions and thesum of the diffusable and nondiffusable coun-terions are equal. This creates the membranepotential.

donor A donor is an impurity added toa semiconducting material, either during thecrystal growing process, or later by diffusion.The donor atoms are substituted for the origi-nal semiconductor atoms within the crystalinestructure. However, in forming the covalentbonds with neighboring atoms, the donor atomhas one (or more) extra electron(s) that can actas extra charge carriers in the bulk of the crystal.

For example, pure silicon (Si) has four elec-trons in its outermost unfilled shell (valenceelectrons). In forming the crystal, Si forms four


covalent bonds with four different neighbors;each bond is two electrons with opposite spins.The geometrical arrangement of the four bondsis tetrahedral. Thus, the crystaline structure isdiamond-like.

If a pentavalent atom (like P, As, Sb, or Bi)is substituted for one of the original Si atoms,four of the five valence electrons will partici-pate normally in the diamond-like structure andthere will be an extra electron. (See the firstaccompanying figure.) Normally this electronwould be tightly bound to the donor ion. How-ever, because the electron moves in the bulk ofthe semiconductor, the binding energy of thisatom to the donor ion is greatly reduced. Thisis because the electron now moves in a materialwith a dielectric constant,ε, much higher thanvacuum and thus feels a reduced Coulumb at-traction, and also because the effective mass ofthe electron,m∗

e, is reduced within the solid.

Two-dimensional projection of a tetrahedral crystaline

structure. Also shown is a donor atom with its (extra)

loosely bound electron.

An estimation of the binding energy of thiselectron to the donor can be calculated by ap-proximating the donor ion and electron pair asa hydrogenic system. Then, from elementaryquantum mechanics, the binding energy is givenby

ED =−e4m∗

e

2ε2~2,

or≈ 0.02 eV , using numbers for a typical semi-conductor. Here,e is the elementary charge ofthe electron and~ is Planck’s constant dividedby 2π.

In the solid state, the allowed energies of theelectrons formbandsand for the pure semicon-ductor, most of the electrons are in the valenceband and cannot contribute to the bulk conduc-tivity. However, the electrons associated withthe donors are only≈ 0.02 eV (see above) fromthe conduction band. Since thermal energies atroom temperature are≈ 1

40 eV, this electron caneasily be ionized by thermal agitation and partic-ipate as a charge carrier. (See the second acom-panying figure.) Thus, donor electrons also havethe effect of raising the Fermi level (the energyof a state for which the probability for occupa-tion is 1

2 ) closer to the conduction band.Seealsoacceptor; doping.

Qualitative illustration of allowed electron energies in

a donor doped (n-type) semiconductor.

doping Doping is the deliberate addition ofimpurities to a semiconductor. If the impurityatom is approximately the same size as the in-digenous semiconductor atoms and does not dis-rupt the normal crystalline structure, then addingimpurities has the effect of replacing some of theoriginal atoms with the impurity atoms. This isdone to change the available number of chargecarriers in the bulk semiconductor, thus enhanc-ing the conductivity. When this is done, theconductivity changes fromintrinsic to extrinsicconductivity and can have vastly different tem-


perature dependencies.See alsointrinsic con-ductivity.

If an impurity contributes extra electrons tothe semiconductor, it is called a donor and thecrystal as a whole becomes ann–typesemi-conductor. If, on the other hand, the impuritycontributes a hole, then it is called an acceptorand the semiconductor becomesp–type.This issummarized in the accompanying table.

Summary of basicsemiconductor dopingschemes

impurity type contributedclass carrier

donor n-type electronacceptor p-type hole

Electrical devices with desirable electricalcharacteristics can be constructed by judiciouslyforming junctions of different types and concen-trations of impurities. For example a suitablejunction between an n–type and a p–type semi-conductor forms ap–n junction— a basic diode.See alsodonor; acceptor; diode.

Doppler effect (1) The change of the appar-ent frequency of the source of electromagneticor acoustic radiation due to the relative motionof the source and observer. If the source emitslight of frequencyν, wavelengthλ, and its mo-tion is towards the observer with a velocityu, theobserver receivesu/λ waves in addition to thenumberν = c/λ that would reach the observerin the absence of relative motion. If the motionis away from the observer, thenu/λ fewer wavesare counted.

(2) Difference in frequencies of a sound orelectromagnetic wave radiated by a source andreceived by an observer which are in relativemotion. In acoustics, the Doppler effect in ahomogeneous non-moving medium is given bythe formulaω′ = ω

1−n·u/c . Here,ω andω′ arethe angular frequencies of the radiated and re-ceived wave,u is the source velocity,c is thesound speed,n is the unit vector in the direc-tion of wave propagation, and it is assumed, forsimplicity, that the receiver is at rest. The differ-

ence∆ω = ω′ − ω is called theDoppler shift.The Doppler shift∆ω is positive (negative) ifthe source is moving toward (away from) the re-ceiver. In electrodynamics, the Doppler effect in

a vacuum is given byω′ = ω√

1−u2/c2

1−n·u/c , wherethe notations are the same, exceptc, that is, lightvelocity. The difference between the formulaefor the Doppler effect in acoustics and electro-dynamics is the factor

√1− u2/c2, which is

due to the Lorentz transformation in the theoryof relativity.

Doppler shift The change in frequency seenby an observer of a source of sinusoidal waveswhen the observer and source are in relativemotion. For electromagnetic waves, all inertialframes are physically equivalent, so the Dopplershift depends only on the relative velocity. Forsound waves, the medium is the preferred ref-erence frame, and the shift can depend on thevelocity of the medium as well.

dosimetry A procedure for measuring ab-sorbed radiation dose.

dosimetry, thermoluminiscent Measure-ment of radiation dose absorbed by lithium flu-oride by quantification of the light output of theheated material.

double Kevin bridge Double Kevin bridgeis a type of resistance bridge designed to min-imize the effects of lead or contact resistance.It is used to measure low values of resistanceprecisely. A diagram of double Kevin bridgeis shown below. Here, the unknown resistance,x, and the standard resistance,S, are connectedin series with a battery, variable resistor, a Gal-vanometer, and low resistance wire,l. The resis-tancesr1, r2, r3, andr4 are contact resistances.When the balance is achieved, the value ofX isgiven by

X = SA

B+Bl(a+ b+ l)

(A

B− a

b

).

Balance is achieved by adjusting variable resis-tance on the other two arms until they are equal.

double refraction The presence of two re-fracted beams from an unpolarized incident


Double Kevin bridge.

beam on an anisotropic material in place ofthe usual single refracted beam observed forisotropic materials. By measuring the anglesof refraction, one of the double refracted beamsis found to obey Snell’s law, and is termed theordinary ray, while the other does not and istermed theextraordinary ray. Double refrac-tion in crystals of calcite and quartz allow theproduction of polarized light over a wider rangeof wavelengths than is possible using dichroicmaterials such as Polaroid. The frequency ofthe light appears to the observer to be increasedby the ratio of the propagation speed to the prop-agation speed reduced by the relative speed. Ifthe motion is away from the observer, the fre-quency appears to the observer to be decreasedby the ratio of the propagation speed to the sumof the propagation speed and the relative speed.

doublet A lens combination of opposite signsused for the elimination of spherical aberration.The amount of spherical aberration introducedby one lens of such a combination must be oppo-site to that introduced by the other. Neutraliza-tion is possible in a doublet because the sphericalaberration varies as the cube of focal length andtherefore changes sign with the focal length.

downlink The transmission of data from aspace vehicle, e.g., a missile or satellite, to theground. It is usually modulated onto subcarriersand then onto RF carriers.

drain One of the connections to the chan-nel of an FET (field effect transistor).See alsotransistor, field effect.

drift (1) Slow changes in operating param-eters or conditions that affect the output of, forexample, an electronic amplifier. This wouldbe indistinguishable from a very low frequencyinput signal.

(2) Electron motion in a conductor or semi-conductor under the influence of an applied elec-tric field. The drift velocity is given by

v = µE

whereµ is the (material-specific) mobility coef-ficient andE is the applied electric field.

drift velocity Mean velocity of the currentcarrying particles upon the application of anelectric field. It is given byv = J/(ne), whereJ = current density,n = density of charge car-riers ande = their charge. Typical values formetals are10(−5) m/s.

drum An early high-speed, direct access stor-age device that used a magnetic-coated cylinderwith tracks around its circumference. Each trackhad its own read/write head.

drum factor The ratio of the length of thedrum usable for scanning to its diameter, for areceiver or transmitter drum.

drum receiver Facsimile apparatus in whichthe recording medium is attached to a rotatingdrum. This is scanned helically by a recordinghead.

drum speed The speed with which the mag-netic coated cylinder, used as a direct access de-vice, rotates.Seedrum.

drum transmitter Facsimile apparatus inwhich the document to be transmitted is attachedto a rotating drum. This is scanned helically bya reading head.

dry cell A cell in which the electrolyte issoaked by absorbing material to prevent thespilling or leakage of the electrolyte is called


adry cell. A common example of a dry cell is acarbon-zinc (Laclanche) cell.

duality principle The duality principle statesthat the logical value of an boolean expressionremains unchanged if

1. every conjunction is replaced with a dis-junction,

2. every disjunction is replaced with a con-junction, and

3. every variable, term, and functional resultis inverted.

An example of the duality principle isde Morgan’s laws. To illustrate the principle,the expression

Y = A⊗B

is logically equivalent to

Y = A⊕ B ,

where the bar(. . .) represents the inverse of thatvariable,⊗ represents conjunction, and⊕ is thedisjunction operation.

One application of the duality principle is de-termining the logic function of a circuit if onechanges the logic convention. If a circuit is de-fined in, say, positive logic, then the correspond-ing function implemented by the same circuitusing negative logic is found by replacing eachconjunction with a disjunction and vice versa.Note, however, that by “inverting” the logic con-vention, the truth value of each input signal (theinput boolean variable) and output (functionalresult) is necessarily inverted. This can be illus-trated by using the equations above as an exam-ple.

Consider a circuit with two inputsA andBwhich, when operated with a positive logic con-vention, acts as anAND gate. Shown in the firstsection of the acompanying table is the output ofthe gate,Y , for all possible states of the inputsusing 1 and 0 to represent logicaltrue andfalse.The second section is obtained by re-writing thisinformation in terms of voltage levelsH andL(seeconvention; logic gates) with the view thatthe gate is now just an arbitrary circuit. Now, ifthe circuit is operated using a negative logic con-vention (by redefining the relative potential of 0volts and being mindful of proper power supplyconnections), the truth table shown in the third

section is obtained. Thus, by inspection of thetable, the circuit now operates as anOR gate innegative logic.

Operation of a circuit used in positive ornegative logic

Positive Arbitrary Negativelogic circuit logic

A B Y A B Y A B Y

0 0 0 L L L 1 1 10 1 1 L H L 1 0 11 0 1 H L L 0 1 11 1 1 H H H 0 0 0

duct (waveguide), acoustic (1) A pipe ortube along the interior of which sound is trans-mitted. These ducts can be of various shapeand form (i.e., rectangular, cylindrical) and arewidely used in practice.

(2) A medium where sound can propagateonly between two layers or surfaces. Examplesof these waveguides are an atmosphere betweenthe ground and the height of temperature inver-sion, where sound waves can be trapped, and anocean where sound is trapped between its sur-face and bottom.

duplexing Term used to describe two sys-tems that are functionally identical. They bothmay perform the same functions, or one may bestandby in the event the other fails.

duty cycle The capacity of a machine to workunder normal conditions. For example, for aprinter, it would indicate the number of pagesthat can be printed per month without a problem.

dynamic characteristics The dynamic char-acteristic curve determines the output of a cir-cuit or circuit element for a given input voltage.Usually, characteristic curves are a simple, one-to-one functional relationship between the inputand output. With more complex characteristiccurves, the output may be multi–valued and de-pend on the previous value of the input, e.g.,hysteresis.

As an example, consider the diode–resistorcircuit shown below. A voltage from a sourceis applied,Vs, across the series diode and re-


sistor circuit, and the voltage developed acrossthe resistor will be considered the output volt-age. From elementary circuit analysis, the volt-age dropped across the diode is

Vdiode = Vs − IdiodeR .

Example circuit for determining a dynamic character-

istic curve.

To actually determine the voltage droppedacross the diode, itsstatic characteristic is re-quired, relating the voltage across and currentthrough the diode. The solution can be deter-mined graphically by plotting, on the samegraph, theload line,

Idiode =Vs

R− Vdiode

R

whereVdiode is treated as the independent vari-able and plotted on the x–axis andIdiode is plot-ted on the y-axis (see the second figure). Thus,the intersection of the static characteristic andload line determines the actual voltage and cur-rent for the given (and instantaneous) value ofappliedVs.

To determineVR, the output voltage, thedy-namiccharacteristicIdiode as a function ofVs isto be determined. This is done by drawing a linefrom the solved value ofIdiode and intersectingwith a line extended from the appliedVs (this isshown in the figure by the dashed lines). Differ-ent values of applied voltage,V ′S for example,will yield a family of load lines and thus a familyof currents corresponding to the applied voltage.In this manner, the dynamic characteristic of thecircuit can be determined from the static curveof the diode. This curve is also shown.

Given the dynamic characteristic of the cir-cuit, the resulting output for a given input can bedetermined. For time varying signals, the inputis plotted as a function of time below the hori-zontal axis of the dynamic characteristic curve.

This is illustrated in the third figure, using thedynamic characteristic above as an example and

Graphical construction of dynamic curve from static

characteristic and load line for the example circuit

above.

assuming a sinusoidal input. The output is de-termined by reflecting the input waveform aboutthe dynamic characteristic curve, also shown.

A dynamic characteristic curve used to determine the

output voltage as a function of time for a given input

waveform.

dynamometer A device, consisting of a setof parallel rollers, which allows thewheelsof avehicle to be driven while the vehicle remainsstationary. Typically used for engine diagnos-tics.dynamotor A device that contains amotorand one or more generator(s). The motor andthe generator(s) have a common magnetic fieldand separate armature windings. One of the ar-maturewindingsreceivesdirect current and op-erates as a motor. The motor rotates and the


other armature windings operate as generators.It is used for transformation of DC voltage.

Eear A component of a complex acousticalsystem of hearing. Three main parts of the hu-man ear are the outer ear, the middle air, andthe inner ear. The outer ear consists of the pinnaand the auditory canal which is closed at the endby the eardrum. The middle ear has three smallbones: the hammer, the anvil and the stirrup,which are connected to each other. The hum-mer and the stirrup are attached to the eardrumand to the oval window, respectively. The ovalwindow is a membrane that divides the middleear from the inner ear, which consists in part ofthe cochlea andthe basilar membrane.A soundfrom the outer ear sets the eardrum into vibra-tions. These vibrations are further transmittedthrough the bones of the middle ear to the ovalwindow and the basilar membrane, which hasnerve endings connected to the brain. This pro-cess results in the hearing of sound. Human earscan localize a source of sound and distinguish itsloudness, timbre and pitch.See alsoaudibility,limits of; audio frequency; binaural.

ear, artificial An artificial device to permithearing for those whose ears do not function.

earthquake Sudden motions in the earth’scrust. The most common mechanism of earth-quakes is displacement along a fault that gen-erates seismic waves. Earthquakes can lead todevastating distractions on the earth’s surfaceand tsunami in the ocean. Seismic waves gener-ated by an earthquake can propagate through thecrust, the mantle and the core of the earth, en-abling retrieval of information about the internalstructure of the earth.

echelette Diffraction grating with large in-tervals and flat grooves inclined at an angle toreflect radiation in the direction of the order in-tended to be brightest. Term used by R.W. Woodfor infrared grating of this type. Echelette grat-ings are now generally calledblazed grating.

echelon Diffraction grating for high reso-lution studies of a small portion of the spec-trum, such as the hyperfine structure of linesor the Zeeman effect. Twenty to forty accu-rately plane-parallel plates are stacked togetherand staggered to form a series of steps with aconstant offset of about 1 mm. The thicknessof each plate is usually about 1 cm, so the grat-ing space is very large and the concentration oc-curs in an extremely high ordering. The lightis concentrated in a direction perpendicular tothe fronts of the steps. At most, two orders ofa given wavelength appear under the diffractionmaxima. But the order is so large that the resolv-ing power (order times the number of plates) is100,000 to 1,000,000 even though the numberof plates is relatively small. Echelon was pio-neered by A.A. Michelson and was the first useof the principle of concentrating a light in par-ticular order.

echo A wave reflected or scattered by anobject, surface or inhomogeneity in a medium.Echoes are a very common phenomenon in thepropagation of sound. For example, a listener ina room hears not only direct sound from a sourcebut also multiple echoes from walls (seeacous-tics of rooms). Bats use echoes to navigate andto find prey. Echoes are also used in acousticalinstruments, such assonarandecho sounders,to measure the distances to an object.

echocardiography The use of ultrasound tostudy heart structure and motion. In this non-invasive technique, a transducer is held againstthe chest to send a beam of very high frequencysound waves to the heart. A certain fractionof these ultrasonic waves are reflected by theinterface between two different types of tissue.The same transducer receives the reflected sig-nal that is then displayed to reveal the structureof the heart and the motion of the various partsof the heart throughout the cardiac cycle.

echoes, flutter A succession of echoes froma single impulse source that do not overlap andoccur rapidly one after the other. Flutter echoescan appear in auditoriums and rooms with highlyreflective walls. Flutter echoes significantly de-crease the acoustic quality of auditoriums androoms and, therefore, should be eliminated.


echo, harmonic An echoproduced by scat-tering or reflection of an overtone rather thanthe fundamental frequency of a complex sound.Harmonic echoes can occur because scatteringamplitude is proportional to a power of fre-quency and the frequency of overtone is alwaysgreater than the fundamental frequency. There-fore, the amplitude of the harmonic echo can begreater than that produced by scattering of thefundamental frequency.

echo, musical A specific kind of flutterecho. Musical echoes occur when scatteringobjects are located at approximately uniformlyincreased distances from a source, resulting inperiodic multiple echoes.

echo sounder A navigational acoustic sys-tem that is used to measure water depth. Anecho sounder consists of a transducer that emitsan acoustic pulse, which propagates from theocean surface to the bottom, reflects and comesback to the transducer. A measured characteris-tic is the time interval∆t between pulse emis-sion and reception. The ocean depthh is calcu-lated by using the formulah = c∆t/2, wherecis the sound speed in water.

eddy currents Currents induced in a con-ductor due to the presence of an applied, chang-ing magnetic field through the conductor. Thesecurrents always circulate in such a way that theproduced magnetic field opposes the change inthe applied field, in accordance with Lenz’s law.

effusive beam A beam of molecules formedby leakage of gas through a fine orifice. Gra-ham’s law applies when the mean free path issmall compared to the dimensions of the orifice,the flow being analogous to a fluid jet forced outby pressure. At low pressures, the mean freepath is large compared to the dimensions of theorifice, the mechanism of escape of gas is dif-ferent, and it is termedmolecular effusion.Inthis case the volume diffusing per second intoa vacuum is directly proportional to the area oforifice and inversely proportional to the squareroot of molecular weight.

Einstein function The Einstein model forthe heat capacity of phonons assumes all of the

phonons are at the same energy. This assump-tion produces a heat capacity CV

CV = 3NkB

(ωkBT

)eω/kBT

(eω/kBT − 1)2

whereω is the energy of an oscillator. At hightemperatures, the heat capacity becomes3NkB ,the classical Dulong and Petit value. At low tem-peratures, the model predicts the heat capacitywill decrease as exp(-ω/kBT ) in contrast to ex-periments which show the heat capacity follow-ing the Debye model (T 3 at low temperatures).The equal-energy approximation for the oscil-lator energies is valid for the optical branchesof the phonon spectrum, and this is where theEinstein model is most often used.

electric attraction According to Coulomb’slaw, electric charges with opposite polarity willattract each other. This is known aselectric at-traction.

electric conductivity The ability of a mate-rial to conduct electric current. It is expressed interms of the current per unit of applied voltage.

electric conductor An electric conductor isa material that conducts electric current. Typi-cal electric conductors are metals with free elec-trons as carriers.

electric dipole When two charges, oppositein sign (q and−q), are placed a very short dis-tanced apart, they form an electric dipole withelectric dipole momentp = qd. An ideal elec-tric dipole exists whereq goes to infinity anddgoes to zero while maintaining the productqdconstant.

electric dipole moment A measure of thestrength of an electric dipole. In the simple pointcharge model, the electric dipole moment is de-fined asp = qd, whereq is the charge andd isthe distance between the two point charges. Ina more general form,p =

∫xρ(x) dx.

electric displacement The vectorD, whichis related to the sum of the electric fieldE andthe electric polarizationP , byD = ε0E + P .By Gauss’s law it can be shown that the flux of


D through a closed surface is equal to the freecharge within the surface.

electric field The force per unit charge ona test charge in a given electric field. For thecase of a static charged point particleQ, its elec-tric field is found via Coulomb’s lawE(r) =F (r, q)/q = Q/(4πr2)r.

electric field, energy in The energy stored inan electric field is given byE = ε0/2

∫E.E dv.

Thus the energy per unit volume in the field isε0/2E2.

electric field gradient The gradient of theelectric field vector. The artificial electric fieldgradient is used for particle acceleration. Theelectric field gradient around an atom is affectedby the anisotropy of the electric charge distribu-tion of the atom. It can be observed by using thenuclear magnetic resonance (NMR) technique.

electric field, induced The electric field pro-duced by a time variance.

electric flux By definition, this isE.da,whereE is the electric field, andda is a vec-tor normal to an infinitesimal surface of areada.Gauss’s law states that the electric flux througha closed surface is related to the enclosed chargeQ.

electric flux density Also known aselec-tric displacement,and normally denoted by asymbolD. The density of theoretical lines offorce that extend in all directions from an elec-tric charge or a charged body. It is measured incoulombs-per-meter squared. The permittivityε of the medium is given byD/E. The electricflux densityD can defined as:

D = ε0E + P ,

= εE ,

whereε0, E, andP are the permittivity of freespace, electric field, and polarization, respec-tively. The permittivityε of the medium is givenby D/E. The divergence of the electric fluxdensity is from surface charge densityρ :

divD = ρ .

The rotation of the electric flux density is zero;rotD = 0. The energy stored in a capacitorUis calculated fromE andD:

U =12

∫E ·DdV .

There is a relationship between magnetic fieldstrengthH, magnetic flux densityB, magnetismM , andD:

rotH = rot

(B

µ0−M

)=∂D

∂t,

whereµ0 is the permeability.

electric intensity Normally electric intensityis denoted by a symbolE. Currently, it is calledtheelectric field strengthor electric field inten-sity. The electric intensity is the strength of anelectric field at a given point in the field and isequal to the force exerted by the field on unitcharge at the point.

It is measured in volts per meter. Electricfield strengthE is the total electric field strengthdue to the set of point chargeqi:

E(r) =1

4πε0

∑i

qi(r − ri)|r − ri|3

,

whereε0 is the permittivity of free space. Ascalar function electrostatic potentialΦ(r) is de-fined as:

Φ(r) =1

4πε0

∑i

qi|r − ri|

.

E is derived fromPhi as:

E = −∇Φ .

electric moment The torque exerted on anelectric dipole in an electric field. A distribu-tion of charges can be regarded as a dipole andits electric moment can be calculated as a vec-tor equal to the product of the magnitude of thecharges and the distance between the charges ofthe virtual dipole.

electric polarization P , the electric dipolemoment per unit volume, given byNp, wherepis the dipole moment of the molecule/atom, andN is their number per unit volume. One can


show thatP is related to the volume densityρb

and surface densityσb of bound charges via therelationshipsρp = −∇.P andσb = P.n, wheren is the surface normal.

electric screening A screen of conductivematerial used for the reduction of electric fieldsentering a particular region.Seeelectrostaticshielding.

electric shock A physiological stimulationcaused by electric current passing through tis-sue. It sometimes causes involuntary contrac-tions of the muscles. A severe shock can dam-age heart and even cause death.

electroacoustics The branch of acoustics andtechnique that deals with basic principles anddesign of electroacoustical transducers that areused to convert sound energy into electrical en-ergy or vice versa. One of the main tasks ofelectroacoustics is to find a relationship betweenan acoustic (electrical) input and an electrical(acoustic) output of a transducer. To solve thistask, equivalent electrical circuits of acousticsystemsare often used. The other important taskof electroacoustics is to achieve maximum ef-ficiency of an electroacoustical transducer andminimal acoustic distortion.

electrocardiography The study of electricalpotential produced by the heart beat at variouslocations on the surface of the body. The rhyth-mic contractions of the heart are controlled byan electrical signal generated in a specializedregion of the right atrium called thesinoatrialnode,the heart’s natural pacemaker. This elec-trical signal spreads throughout the heart, caus-ing its contraction.

electrocorticography The study of the elec-trical activity of the brain via electrodes placeddirectly on the exposed cerebral cortex.

electrode A conductor for emitting, deflect-ing, or collecting electric charge carriers in acell, an electron tube, a semiconductor device,and so on. A positive electrode is usually calledan anodeand a negative electrode is called acathode.

electrode dissipation The power dissipatedin the form of heat by an electrode. It is causedby bombardment by electrons and/or ions.

electrode efficiency The ratio of the actualyield of metal deposited in an electrolytic cellto the yield that could be deposited theoreticallyas a result of electric current passed through thecell.

electrode, implanted A conductor that hasbeen placed inside tissue in order to detect elec-trical activity or to supply exciting pulses. Im-plantation may be accomplished surgically.

electrodes, beveled A conductor used to es-tablish contact with a part of a circuit.

electrodes, intercerebral A conductor at-tached to the cerebrum in order to monitor elec-trical activity of that part of the brain.

electrodes, pH sensitive A conducting mate-rial that is sensitive to the concentration of pos-itive hydrogen ions in solution.

electrode, surface A conductor attached tothe outside of a body in order to monitor theelectrical activity inside.

electrodynamometer Instrument for mea-suring small currents.

electroencephalography The study of theelectrical activity of the brain as measured byelectrodes attached to the scalp. Potential dif-ferences up to 100s of microvolts are recordedwith this technique. Spectral analysis of elec-troencephalograms (EEG) show wave-like phe-nomena in several different frequency ranges,particularly when the subject is asleep. The low-est frequency components (between 1 and 3 Hz)are termeddelta wavesand are associated withdeep sleep.theta waveshave frequencies be-tween 4 and 7 Hz. Most brain waves are foundin the frequency range between 8 and 13 Hz.These waves are calledalpha wavesand are as-sociated with light sleep. The highest frequen-cies wave, calledbeta waves,are in the 13 to 30Hz range.


Several diseases, such as cerebral tumors andepilepsy, can be diagnosed by causing unusualfeatures in the EEG. Electroencephalographyhas also been used very extensively in researchon sleep and its various stages.

electrogenic pump The mechanism respon-sible for transferring electrical charge across amembrane, resulting in a potential differenceacross that membrane.

electrokinesis The movement of chargedparticles through a continuous medium underthe influence of an electrical field. Such physi-cal phenomena are frequently associated witha charged surface in an electrolytic solution.The four principal electrokinetic phenomena areelectrophoresis, electroosmosis, streaming po-tential andsedimentation potential.

electroluminescence Process by which elec-trical energy is converted directly into light with-out thermal losses, e.g., electron recombinationof apn junction.

electrolysis The production of a chemical re-action by an electric current passing through anelectrolyte. The chemical action is a oxidation-reduction reaction depending on an electrontransfer at the electrodes. Positive ions migrateto the cathode. Positive ions are reduced (gainelectrons) to form a neutral species at cathode.Negative ions migrate to the anode to be oxi-dized at the anode. Negative ions are oxidized(lose electrons) to form a neutral species at cath-ode.

electrolyte A liquid that contains positive ornegative ions as electric carriers and conductselectricity. It contains substances that act on oneor both of the electrodes and cause a chemicalaction to generate an electric current flow. Elec-trolytes are molten ionic compounds or solutionscontaining ions, i.e., solutions of ionic salts orcompounds that ionize in a solution. Usually, aliquid metal is not regarded as an electrolyte.

electrolytic dissociation The separation of aneutral ionic compound into positive and nega-tive ions, usually caused by dissolution. In caseof reversible dissociation, the equilibrium con-

stant of the reaction is calleddissociation con-stant.

electrolytic polarization The phenomenonof the existence of the electrolytic polarizationvoltage, which is required for a steady currentto flow through an electrolytic cell. The irre-versible chemical reaction around the electrodecauses the electrolytic polarization voltage. Thedelay of diffusion and transport of the substancesto the reaction around the electrode also causeelectrolytic polarization. Depolarizing agentsare used to reduce the electrolytic polarization,also known aselectrochemical polarization.

electrolytic tank A device used to make amodel for solving electrostatic problems analo-gously by measurements made on a model elec-trolyte in the tank.

electromagnetic pump This type of pumpoperates on the principle that a force is exertedupon a conductor that carries current in a mag-netic field. Liquid metals have high conductivityand can be conveniently used in such pumps.

electromagnetic units Electromagnetic units(EMU) are a system of electrical units. For eachsystem,

ε0µ0 =1c2,

whereε0 is the electric constant (the permittivityof free space),ε0 is the magnetic constant (thepermeability of free space), andc is the speed oflight in free space. It is used in the centimeter,gram, second (CSG) system. Usually, electro-magnetic units have the prefixab- attached tothe names of conventional units. In the EMUsystem, the unit of electric current is defined bymaking the coefficient constant of the force be-tween a pair of parallel electric currents equal totwo:

F = 2ii′

r2,

whereF is the force between a pair of parallelcurrentsi, i′ with distancer. The ab-ampereis the EMU of a current 1 ab-coulomb per sec-ond. It also can be said that, in the EMU system,the unit of magnetic poles is defined to makethe coefficient constant of the force between the


magnetic poles equal to one:

F =gg′

r2,

whereF is the force between a pair of magneticpolesg, g′ with distancer. The dimension ofthe ab-coulomb is different from thecoulombofSI units. The quantity of electric charge mea-sured in electrostatic unitsqEMU is related to onemeasured in SI unitsq:

qEMU =√µ0

4π· q SI ,

=1c

1√4πε0

· q SI .

The unit of the electric charge is (s dyn1/2)and its dimension is (length1/2 mass1/2).

electromagnets Electromagnets are tempo-rary magnets that make use of electric currentsto generate the magnetic field.

electrometer A voltmeter with a very highresistance (1014 ohm), suitable for measuringvoltages on small capacitors, etc.

electrometer, Hoffmann A sensitive elec-trometer consisting of a half-vane (only oneblade vane) in a pair of segmented metal boxes(binants).

It is also known as abinant electrometer.

electrometer, Lindemann A highly sensi-tive electrometer. It has a light needle supportedby a torsion fiber surrounded by metal platequadrants on all sides. The opposite metal plateof the quadrants are connected. The voltage be-tween the plates causes a force on the needle. Amicroscope measures the deflection of the nee-dle tip.

electrometer, quadrant An electrometerconsisting of a set of quadrants and a light vanesuspended by a quartz fiber between the quad-rants. The quadrants are oppositely connectedin pairs. A quadrant electrometer is used to mea-sure voltages and charges. The voltage betweenthe pairs of quadrants causes the deflection of thevane. The angle of deflection is proportional tothe voltage. The Dolezalek quadrant electrom-eter is well known.

electromotive force Old term for the inducedelectric potential given by Faraday’s inductionlaw.

electromotive force, motional An emf thatarises in a conductor in relative motion to an ex-ternal magnetic field due to Faraday’s induction.

electromotive force, self induced An emfthat arises in inductances when their self fieldchanges.

electromyography The study of the elec-trical signals associated with muscular activity,usually the skeletal muscles. The voltage sig-nals are detected either with electrodes attachedto the surface of the skin or with needle elec-trodes. This technique is useful in studying neu-romuscular function, possible damage to nerves,and in kinesiology.

electron charge The charge of one electronis equal to1.60218 × 10−19 coulombs. Thisquantity is a fundamental constant of nature anda basic unit of charge.

electronic mail (email) The ability to com-pose, send and receive mail via the computerusing some type of email program. That is, ter-minals can transmit documents such as letters,reports, and telexes to other computers or ter-minals. Such services can be accessed using apublic network through a host computer and canbe retrieved at other terminals.

electronic musical instruments Musical in-struments that generate electromagnetic vibra-tions of desired form and spectrum that are thenconverted into sound by means of electro-acoustical transducers. Examples of electronicmusical instruments are an electronic piano, gui-tar, organ, and carillon. Electronic musical in-struments are used to imitate sound of “stan-dard” musical instruments, to simplify their con-struction and minimize their size, and to developnew musical instruments. Sound produced byelectronic musical instruments is calledsyn-thetic sound.

electron multiplier A device primarily usedfor the detection of single, elementary atomic


particles such as electrons, photons, and ions.An electron multiplier consists of a sequenceof electrodes calleddynodesand produces anoutput pulse of charge for each incident particle.

If the particle to be detected is incident onthe first dynode and has sufficient energy, it willcause some electrons to be ejected from the dyn-ode surface upon impact. These secondary elec-trons are accelerated toward the next dynode inthe sequence (by proper arrangement of rela-tive electrostatic potentials) and they eject moreelectrons upon impact. These tertiary electronsare accelerated to a fourth dynode, and so on.Assuming that, nominally, three electrons areejected from a dynode surface per incident elec-tron, a multiplier withndynodes will have a gainof approximately3n. For example, an electronmultiplier with 10 dynodes will have a showerof 106 electrons collected on the last dynode fora givensingleparticle incident on the first dyn-ode. Thus, there is now a sufficient amount ofcharge collected on the last dynode that can beprocessed with conventional electronics.

The quintessential discrete dynode electron multiplier.

Illustrated is electron multiplication of a single event

(the incident particle) creating a shower of charge col-

lected on the last dynode.

It is possible to have an electron multiplierwith one continuous dynode instead of the dis-crete dynode chain described above. Here, the

dynode is a film of moderately high resistancematerial with suitable secondary electron emis-sion characteristics and is coated on the inside ofa glass tube. A particle impinging on the begin-ning of the tube will eject secondary electronsand cause a shower of electrons to be acceler-ated and multiplied (in a manner similar to thediscrete dynode multiplier) for collection at theend of the tube. A continuous dynode electronmultiplier is usually physically smaller than adiscrete multiplier with comparable gain.

A continuous dynode electron multiplier illustrating

electron multiplication of a single event (the incident

particle). Operation of the device is similar to the dis-

crete dynode electron multipler.

electron nuclear double resonance (ENDOR)technique A technique in which the magneticresonance of a nucleus is detected by observingthe resonance absorption of an associated un-paired electron. This effect is due to hyperfinecoupling of the nuclear and electron magneticmoments. The statistical distribution of nuclearspin alignments within the sample causes eachunpaired electron to experience a different localmagnetic field. Because of the differences in thetime scales for spin flipping, the electrons expe-rience inhomogeneous broadening. To performan ENDOR experiment, the sample chamber isbathed in both microwave and radio-frequencyfields. The applied static magnetic field andthe microwave radiation are tuned to an elec-tron resonance. The intensity of the microwavefield is sufficient to saturate the spin states forthose matching electrons, a subset of all of theunpaired electrons. The frequency of the ra-


dio waves is slowly scanned and the microwaveabsorption of the cavity is monitored. Whenthe frequency of the radio waves is correct toflip more nuclei into the orientation correspond-ing to the subset of unpaired electrons, the ob-served absorption of microwave radiation willincrease. In this technique, the absorption ofthe microwave field is observed while the fre-quency of the radio field is scanned.

electron optics Mathematical analogy be-tween the passage of an electron beam throughmagnetic and electric fields and the passage of abeam of light through refracting media. A lim-ited magnetic or electric field is considered toform a lens, which can be combined in waysanalogous to optics to form various focusing in-struments, such as an electron microscope.

electron-phonon interaction parameterSeesuperconductivity.

electron transport chain, photosynthesisThe chain by which an electron, excited by theabsorption of light, moves through the pigment-protein complex of the light-harvesting struc-tures to create a potential difference. All sub-sequent chemical reactions which convertCO2

and water to carbohydrates are driven by thispotential difference. In the purple bacteriumRhodobactersphaeroides, optical excitation ofa pair of chlorophyll molecules in the reactioncenter leads to the movement of an electron to apheophytin molecule within a few picoseconds.This electron hops to a quinone molecule withinabout 200 psec. The electron hops to a secondquinone molecule in about 100µsec. After thisprocess is repeated, a complex composed of twoexcited quinone molecules participates in chem-ical reactions which create the potential differ-ence across the membrane in which the complexsits.

electron-volt Energy equivalent to the ki-netic energy gained by a particle with one elec-tronic charge that is accelerated across one volt.It equals1.6× 10(−19)J .

electrooptic effect The changes in the prop-agation of light through matter due to the ap-plication of electric fields. Electrooptic effects

include theKerr effect, electric double refrac-tion, the Stark effect,and theinverse Stark ef-fect. In the Kerr effect,double refraction ap-pears with the application of an electric field.Glass, some liquids (nitrobenzene), and gasesexhibit this effect. The magnitude of this ef-fect is proportional to the electric field strengthsquared.Electric double refraction effectrefersto the appearance of double refraction at fre-quencies close to the absorption lines with theapplication of an electric field. In theStark ef-fect,spectral lines are split with the applicationof a strong electric field. The Stark effect withthe lines appearing in absorption is called theinverse Stark effect.

electrophoresis The movement of chargedparticles (usually macromolecules or colloidalparticles) due to the presence of an electricalfield. In most biochemical applications, a gelis used as the medium. Application of an elec-trical field to a collection of dissimilar chargedparticles in a gel will cause the particles to mi-grate in a direction determined by the sign oftheir charge and the polarity of the applied elec-trical field. The particles will move at differentspeeds depending on their net charge, size andshape. Since most species are in low-chargedstates (usually+1e and occasionally+2e), thegeometric differences of the species are the dom-inant factor for determining the speed of theparticle. The weight of the macromolecule it-self is the usual cause for differences in sizeand shape, making electrophoresis an excellentmethod for separating macromolecules on thebasis of weight.

electroretinography The study of the elec-trical potential of the retina in response to stim-ulation via light.

electroscope Instrument for detecting staticelectric charges. It consists of two thin sheetsof conducting material that hang freely from aconducting pivot. When a statically charged ob-ject is brought near the pivot, the plates separatedue to the mutual repulsion of the like chargesthat are on both plates.


electrostatic field The electric field createdby stationary charges. The force on test chargesplaced in such a field is given by Coulomb’s law.

electrostatic focusing Focusing of the elec-tron beam by use of an electrostatic lens thatvaries the electric field.Seeelectrostatic lens.

electrostatic hazards General term ap-plied to potentially dangerous situations thatare caused by the build-up and eventual dis-charge of static electric charges. Such dis-charges can cause fires/explosions in environ-ments with high concentrations of flamable orpowered materials.

electrostatic induction The generation of anelectrical charge on a conductor by a electricfield. A positive charge causes a negative chargeon the uncharged object that is nearest to theoriginal positive charge.

electrostatic lens An device that generatesan electrostatic field to cause electron beams toconverge or diverge. It is used in a cathode raytube (CRT) and a electron microscope.See alsomicroscope, electron. In a CRT, the electrostaticlens consists of a focus anode, accelerating an-ode, and a control grid. The electrodes of anelectrostatic lens have cylindrical form and con-centric with the electron beam. The focus anodeand accelerating anode are maintained at a pos-itive potential with respect to the cathode. Thepotential of the accelerating anode is higher thanthat of the focus anode. The control grid con-trols the energy of the beam and consequentlythe intensity of the beam spot. The focal lengthof this lens depends on the potential of the focusanode. The potential of the acceleration anodealso affects the focal length.

electrostatics The branch of electricity studythat studies electrical charges at rest, such ascharge objects and stationary electric fields, andthe electric fields associated with them.

electrostatic screening An electrostaticshield that consists of a number of parallel con-ducting wires or rods connected at one end inorder to obstruct electric flux, while permittingthe passage of magnetic flux.

electrostatic shielding A grounded conduc-tive screen or enclosure placed around a de-vice or between two devices to obstruct electricfields.

electrostatic units Electrostatic units (ESU)are a system of electrical units in the centimeter,gram, second (CGS) system. For each system,

ε0µ0 =1c2,

whereε0 is the electric constant (the permittivityof free space),ε0 is the magnetic constant (thepermeability of free space), andc is the speedof light in free space.

Electrostatic units have the prefixstat- at-tached to the name of conventional units. In anESU system, the unit of electric charge is definedto make the coefficient constant of Coulomb’slaw equal to two:

F =qq′

r2,

whereF is the coulomb force between a pair ofchargesq, q′ with distance ofr.

The ESU of electric charge is thestat-coulomb. ESU is based on this stat-coulomb,a unit of electric charge that exerts a force of1 dyne on another unit charge at a distance of1 cm in a free space. The dimension of thestat-coulomb is different from thecoulombofSI units. The quantity of electric charge mea-sured in electrostatic unitsqESU is related to onemeasured in SI unitsq:

qESU =1√4πε0

· q SI ,

= c

√µ0

4π· q SI .

This unit system has been replaced for most pur-poses by SI units. Some of the relationships be-tween a quantity in an ESU unit system and onein SI unit system are

EESU =√

4πε0 · E SI ,

HESU =H SI√ε0/4π

,

whereE andH are the electric field strengthand the magnetic field strength, respectively,


Dimension of ESU unitsunit dimension

q cm dyn1/2 L3/2 M1/2 T−1

E cm−1 dyn1/2 L−1/2 M1/2 T−1

H s−1 dyn1/2 L1/2 M1/2 T−2

L: length, M: mass, T: time.

electrostatic voltmeter A voltmeter thatmeasures the voltage applied between a fixedmetal plate and a rotating metal plate placedclose to each other. The applied voltage causesan electrostatic force that deflects the rotatingplate against the torque of a spring. The arc ofthe plate rotated is proportional to the appliedvoltage.

electrosurgery Surgery performed using anactive electrode as a cutting device or to promotecoagulation. An alternating current in the MHzrange is frequently used.

emitter One of three connections on a bipo-lar transistor. It is responsible for injecting themajority charge carriers into the transistor, i.e.,electrons forn-p-ntype transistors and holes forp-n-ptype transistors.See alsotransistor, junc-tion; bipolar code.

emmetropia A visual condition in which aninfinitely distant object is imaged sharply on theretina without inducing any accommodation ofthe eye lens.

encoder (channel) A communication chan-nel by which digital signals are transmitted.Channel coding is concerned with the character-istics of the transmission channel; the processeddata must be compatible with the requirementsof the channel. For example, a television cam-era produces a signal in two clear parts —lumi-nanceandchrominance.These two signals arecombined at the transmitter by this process andthe signals are decoded at the receiver.

encryption The process by which data isscrambled, or made unreadable, to prevent unau-thorized access. It is the process of changingoriginal data to ciphertext so that they cannot beunderstood until the ciphertext is decrypted intocleartext at the distant end of link.

end-of-pulsing signal See end-of-trans-mission character.

end-of-transmission character Also knownasend-of-pulsing signal.A signal sent forwardto indicate that the address signals have all beentransmitted.

endoscopy The inspection of an internal cav-ity through a device made for that purpose. En-doscopy is important both in diagnosis of dis-ease or injury as well as in surgery. The in-creased vision made possible via endoscopy per-mits surgeons to make much smaller incisions.This results in much less trauma to the tissuesof the patient and speeds recovery time.

energy conservation, acoustic, law of Therelationship between the acoustic energy densityE and the acoustic energy fluxI: ∂E

∂t +∇·I = 0.Here,t is time, and∇ = (∂/∂x, ∂/∂y, ∂/∂z).According to this relationship, acoustic energyis conserved in a process of sound propagation.The law of acoustic energy conservation holdsin a non-moving medium, whereE and I aredetermined byE = p2/(%c2) and I = Ecn.Here,p is the acoustic pressure,% is the am-bient density,c is the sound speed, andn is aunit vector in the direction of wave propagation.However, the law of acoustic energy conserva-tion can be violated in a moving medium due toan exchange of energy between the mean flowand a sound wave. The total energy in a systemof the medium and the sound wave remains, ofcourse, the same.

energy, exchange Seeinteraction, exchange.

energy, in magnetic field This is given by thevolume integral of the energy density1/2µoH

2

whereµo is the permeability of free space andH is the magnetic field strength.

energy, zero point The energy present ina quantum mechanical system at absolute zerotemperature. In the quantum harmonic oscilla-tor, for example, the zero point energy is1/2 ωwhereω is the angular frequency of the oscil-lator. In a molecule, the zero point energy canmake up a substantial fraction of the binding en-


ergy. In a crystal, the zero point energy is foundin the lattice vibrations.

entrance window The stop image on the ob-ject side that subtends the smallest angle at thecenter of the entrance pupil. Alternately, the im-age of the field stop formed by the part of theoptical system that precedes it.

envelope A group of waves having slightlydifferent frequencies travelling together charac-terized by the group velocity. In a public datanetwork, a group of binary digits consisting of abyte together with additional digits required forthe operation of the network, such as start andstop pulses.Seeenvelop delay.

envelope delay Also known asfrequency de-lay. Different frequencies arrive at the remoteend at different times; frequencies around 1200Hz are received first with the lower and higherfrequencies arriving later. Frequencies in range2900 Hz may come in more than 2 ms later; thatis, bands of frequencies travel together in en-velopes. It creates a degradation of the signalsimilar to what attenuation causes.

EOR An abbreviation for anexclusiveORlogic function, whereOR represents the dis-junction. In other words, the output of this func-tion is logicaltrue if and only if one of the inputsaretrue. Compare this to theOR whose outputis true if either (i.e., all) input variables aretrue.

TheEOR is a basic (but not elementary) logicgate that frequently occurs as an independentunit. If A andB are input boolean variables,the output of the functionEOR can be definedusing the elementaryAND and OR functions(seeconjunction; disjunction) by the booleanexpression

Y = A EOR B ≡ (A⊗B)⊕ (A⊗ B) ,

whereA ≡ NOT A is the logical inverse ofA. Given this equation and the rules concern-ing AND andOR, all possible values ofY =A EOR B can be tabulated in a truth table. Us-ing 1 and0 to representtrue and false respec-tively, a truth table is presented below.

A circuit performing theEOR function indigital electronics is represented by a symbolsimilar to that for theOR — with an extra curved

Output values of EOR for allpossible values of inputs Aand B

A B Y = A EOR B

0 0 00 1 11 0 11 1 0

line on the input side of the gate. This is illus-trated in the following figure.

Symbol representing exclusiveOR (EOR) in digital

electronics.

episcope An optical system used to project areal, enlarged image of an opaque object (gen-erally a flat picture). The object is illuminatedas strongly as possible. Light reflected from theobject is reflected from a mirror and then fo-cused by a projection lens. The lens has to havethe largest possible aperture to collect the scat-tered rays, resulting in rather poor image quality.(Also calledepidiascope.)

equalization The process of compensatingfor frequency-dependent gain in the amplifica-tion, transmission, and/or reproduction of data,particularly voice data.

Transmission lines do not transmit all fre-quencies at the same velocity or suffer the sameattenuation. So, to compensate for losses anddistortion, equalization circuits can be insertedthat compensate for the frequency-dependenttransmission. This is accomplished by tailor de-signing the frequency response of the equaliza-tion amplifier.

Also, in sound recording and playback, dif-ferent analog recording techniques will natu-rally have different frequency responses inher-ent to each process. Upon playback, equaliza-tion circuits can compensate the recorded signal.Recommended frequency responses to be usedon playback with the various techniques (mag-


netic tape, magnet phonograph, crystal phono-graph, etc.) have been provided by societiessuch as the Record Industry Association ofAmerica (RIAA).

equipotential surface A surface in spaceover which the electric potential is constant. Fora point charge, the equipotential surfaces areconcentric spheres centered on the charge. Theelectric fieldE is always normal to the equipo-tential surface.

equivalent circuit A circuit that can replacea given circuit while maintaining the same func-tionality with respect to measurable voltages andcurrents. Usually such replacement is done inthe context of an approximation or simplifica-tion for the purpose of analyzing a complex cir-cuit. Circuits with two terminals can be replacedwith their Thevenin or Norton equivalents. Cir-cuits with three terminals, particularly those inelectricity generation and distribution, can beanalyzed using Y or∆ equivalent circuits. Anexample of the application can be found in theanalysis of transistors.Seeh-parameters.

equivalent electrical circuit of an acousticsystem An electrical circuit that is de-scribed by the same differential equations asthose for an acoustic system under consider-ation. Many acoustic systems and electricalcircuits are described by analogous differentialequations. Therefore, an analysis of an acous-tic system can usually be reduced to that of acertain electrical circuit that is called theequiv-alent electrical circuit. Then, a considerationof this equivalent electrical circuit is done bywell-developed methods of circuit analysis, andthe results obtained are used in analysis of theacoustic system. Equivalent electrical circuitsare often used in theory and design of acousticdevices and electroacoustical transducers.

ergodicity Refers to behavior of random pro-cesses. Deals with correlations that may existbetween parts of the same signal or betweenparts of one signal or another. If a process isergodic, the autocorrelation function obtainedfrom a member function, of sufficiently long du-ration, is the same as that obtained from the pro-cess as a whole, i.e., the autocorrelation function

may be expressed equivalently as an ensembleaverage or as a time average.

erlang Unit of measure of telephone trafficengineering, which gives a measure of the totaltraffic load on link, after Danish engineer A.K.Erlang. For example, if the average number ofsimultaneous calls in progress in a given periodover a particular group of trunks isN , then thetraffic intensity isN erlang. If there is one per-manently engaged circuit, the traffic is 1 erlang.

etalon Fabry-Perot interferometer that con-sists of two semi-silvered optically flat platesthat are fixed accurately parallel to each other,with an air separation gap ranging from mil-limeters to centimeters. Etalons produce sharpfringes and high resolving power, and are usedto accurately compare wavelengths and in thestudy of hyperfine line structure.

ether Medium filling all space that was be-lieved necessary for propagation of electromag-netic waves. The medium had mechanical prop-erties that were adjusted to provide a consistenttheory for electricity, magnetism, action at a dis-tance, and the transmission of light and heat.To account for the transmission of light, etherwas assumed to pervade all space and matter,and have greater density in matter than in freespace, and be so elastic as to transmit transversewaves with the speed of light. Michelson andMorely attempted by optical means to measurethe motion of the earth through the ether, andfailed to detect any ether drift. Einstein’s theoryof relativity has shown that these experimentalresults and all theoretical ideas connected withthe concept of an ether can be systematized in aself-consistent manner without reference to theproperties of the ether medium. From this pointof view, the ether is no longer required for an ex-planation of the empirical facts and has becomean unnecessary appendage of physical theory.

Ethernet A method of connecting devices ina local environment developed by Xerox corpo-ration. It allows for transmission of data, usingnetwork topology at up to 10 million bps for upto half a mile. Workstations can exist on thesame cable but are only able to communicateone at a time. To overcome these problems,


switched Ethernet and fast Ethernet were in-vented and combinations of them are also used.

evaporation (low temperature) At lowtemperatures, evaporation, like so many otherphysical processes, takes on a quantum nature.Quantum evaporation occurs when an excita-tion propagates to a free surface of the materialin question, annihilates, and emits one atom ormolecule into free space. This can only hap-pen when the energy of the excitation is greaterthan the binding energy of the material. In mostsolids, the atoms or molecules are bound tootightly to be liberated from the surface by a sin-gle excitation. Quantum evaporation is possiblein superfluid4He, however. Both phonons androtons can carry enough energy to cause quan-tum evaporation. Experiments studying quan-tum evaporation can provide direct measure-ments of phonons and rotons at high energies(E > 10 K) while at low temperatures (T < 0.2 K).

excimer A molecular complex of two, usu-ally identical, atoms or molecules that is stableonly when one of them is in an excited state. Lit-erally, a contraction of “excited dimer”. An ex-cimer laser is a rare-gas halide or rare-gas metalvapor laser emitting in the ultraviolet range (126to 558 nm). It operates on electronic transitionsof excimer molecules whose ground state is es-sentially repulsive. Lasing gases include the di-atomic molecules ArCl, ArF, KrCl, KrF, XeCland XeF. Excitation may be by electric dischargeor electron beam.

exciplex Strictly used, the term “excimer”refers to excited species made by the combina-tion of two identical atoms or molecules. Ex-cited complexes that do not fall into this cat-egory are more accurately calledexciplexesorheteroexcimers.

excitation of vibrations Setting an acousti-cal, electrical or mechanical system into vibra-tions. Excitation of vibrations always results insupplying energy to the system. Vibrations canbe excited by direct action on a system, e.g., bya driving force that starts acting on a system (seeforced oscillations). An example of such excita-tion is a pendulum pushed at some time moment.Vibrations can also be excited by changing pa-

rameters of a system, e.g., by changing a lengthof an oscillating pendulum.

excitons A bound electron-hole pair foundin nonmetallic solids. The two main types ofexcitons areFrenkelandWannier.TheFrenkelexcitonusually exists in molecular solids and ishighly localized. A Frenkel exciton may be con-fined to a single molecule, but can move throughthe solid by a hopping mechanism. TheWannierexcitonusually exists in semiconductors and isvery delocalized with the electron in the con-duction band and a hole in the valence band.The coulombic interaction binding the electronand hole together is diminished by the dielectricscreening of the material.

exercise testing The process of performingmedical examinations on a patient during phys-ical exercise, such as walking or running on atreadmill. This allows for observation of bod-ily functions under a wide variety of physicalstresses and helps with the diagnosis of diseaseconditions.

exit window The image of the entrance win-dow, as formed by the complete optical system.Alternately, the image of the field stop formedby the part of the optical system that follows it.

extinction coefficient The imaginary part ofthe complex index of refraction. The extinc-tion coefficient simplifies expressions of lightintensity in the vicinity of the absorption bandin dispersion theory, and refers to the extinc-tion of electromagnetic waves during propaga-tion through an absorbing medium. The termattenuation indexis also used. The termex-tinction ratiorefers to the ratio of the power of aplane-polarized beam that is transmitted througha polarizer with its polarizing axis parallel to theelectric field vector of the beam, to the transmit-ted power when the polarizer’s axis is perpen-dicular.

eye It is the sense organ of vision.

eye diagram Deterministic degradationssuch as errors in equalizing, offsets in decisiontiming, and gain errors in digital systems are as-sessed using this type of diagram. In the absence


of noise, the width of the eye opening gives thetime interval over which the received signal canbe sampled without error. The rate of the closureof the eye gives the sensitivity of the system todecision timing errors. The height of the eye ata specified decision time determines the marginover the noise.

Eye diagram.

eye, far-sighted Also known ashyperopia.In this condition, sharp focus in the relaxed (non-accommodating) eye occurs behind the retina,resulting in defective vision for near objects.This is a form ofametropia.

eye, near-sighted Also known asmyopia.In this condition, sharp focus, in the relaxed(non-accommodating) eye occurs in front of theretina, resulting in defective vision for far ob-jects. This is a form ofametropia.

eyepiece An eyepiece magnifies the imagefrom a microscope objective and presents thisimage to the observer’s eye.

eyepiece, compensating Modifications ofthe Huygen’s eyepiece in that both the lensesused are doublet lenses, thus avoiding the chro-matic differences in the magnification.

eyepiece, erecting Usually, a system consist-ing of two convex lenses to render the final image

erect. The use of lenses introduces aberrationsand may make the optical system considerablylonger. These disadvantages may be avoided byusing a pair of erecting prisms.

eyepiece, Gaussian A modification of theRamsden eyepiece obtained by adding a thinparallel-sided plate of glass at an angle of45

to the optical axis. When light enters an aper-ture on the side of the tube and, after partialreflection by the plate, travels through the axisof the eyepiece, it illuminates the crosswires atthe foci of the objective and the eyepiece. Whenused in a telescope, such an eyepiece enables thetelescope to be focused and to be placed perpen-dicular to a plane surface by requiring that theimages of the crosswires formed from a planemirror placed at the objective end be focusedand coincide with the crosswires themselves.

eyepiece, Huygen Also called thenegativetype eyepiece.It consists of two simple plano-convex lenses separated by half the sum of thefocal lengths, and the focal plane falls close tothe field lens. This eyepiece is usually used withlow-powered standard objectives.

eyepiece, Kellner A modification of theRamsden eyepiece, in which the eye lens is re-placed by an achromatic doublet.

eyepiece, Ramsden’s Also called thepos-itive type eyepiece.It consists of two plano-convex lens facing each other and having thesame focal lengths. The focal plane lies outsideof the optics on the objective side and therefore areticule can be placed in the tube of the eyepieceat the focal plane.

Eyring theory A theory, based on statisticalmechanics, that determines the rate of a givenreaction in terms of the allowed energy levelsand the temperature.


Ffacsimile A method of transmitting imagesor printed matter by electronic means. The im-age is optically scanned line by line at the trans-mitter, and the light and dark areas are con-verted into digital information. This is trans-mitted over the network using telephone linesor fax modems and reconstructed at the receiv-ing station and duplicated in some form of filmor paper. More sophisticated machines are ableto skip over blank lines thereby reducing trans-mission times.

fading (signal) Adverse transmission condi-tions can cause signals to be received simulta-neously over more than one transmission path.In microwave transmission it can be caused byreflections from large objects such as buildingsor aircraft.

farad The unit of capacitance, equal to onecoulomb per volt.

Faraday cage A cage made of an electricallyconducting material that is used to protect inter-nal devices from outside electric fields. It workson the principle that there can be no electric fieldinside a conductor.

Faraday disk dynamo A device consistingof a copper disk in which a radial EMF is inducedwhen the disk is rotated between the poles of amagnet.

Faraday effect Rotation of the plane of po-larization of a beam of light passing through cer-tain materials in the direction of applied mag-netic lines of force. First discovered by Faradayin 1845 in heavy flint glass, which exhibits theeffect markedly; it was one of the earliest indica-tions of a connection between light and electro-magnetism. The effect was also discovered inwater and in quartz, and has since been observedfor many solids, liquids, and gases. The effectis thousands of times stronger for thin transpar-

ent films of ferromagnetic materials. TheFara-day effectis not restricted to optical frequencies,and has been observed with microwave and ra-dio frequencies. The rotation of polarization inthe Faraday effect is independent of the sensein which the beam follows the field lines, whichdistinguishes it from the natural optical activityof the media — if the beam is reflected backalong its path, the rotation is doubled only forthe Faraday effect. The angle of rotation is pro-portional to the strength of the magnetic fieldand to the path length in the material. The pro-portionality constant is calledVerdet’s constant,and is the ratio of the angle turned to the dis-tanced traversed evaluated for a unit magneticfield. The phenomenon can be understood interms of a difference in the index of refractionfor left- and right-hand circularly polarized light— the velocity of circularly polarized light de-pends on the direction of rotation. This variationof velocity comes about because the absorptionfrequencies and thus the dispersion of light de-pend on the polarization. The theory of the Fara-day effect is closely related to the theory of theZeeman effect.A well-known present day ap-plication of the Faraday effect is in protectivedevices used to prevent the destruction of high-power laser systems by back reflections from thetarget.

Faraday’s law of induction The law of elec-tromagnetic induction states that the current in-duced in a conductor when it is subjected tocchanging magnetic flux is proportional to thetime rate of change of the magnetic flux.

far-field pattern A region sufficiently farfrom an aperture or source (such as a light-emitting diode, injection laser diode, or the endof an optical waveguide) where the diffractionpattern is essentially the same as that at infin-ity. For all points within the far-field region, thediffraction pattern does not change significantlywith distance.

fault current The current that flows in a cir-cuit under abnormal conditions. It typically oc-curs in a circuit in which there has been a lossof insulation between conductors or between aconductor and the ground.


Fechner fraction If the threshold in bright-ness (also known asluminous fluxor luminoussterance) for the eye to just distinguish an ob-ject differs by an amount dB from a large back-ground of brightnessB, the contrast sensitivityis dB/B, a fraction of the total brightness, andis termed theFechner fraction.It is the small-est difference of brightness that can be detectedby the eye as a fraction of the total brightnesswhen the two objects are side by side, as in pho-tometry. Fechner’s Law(1860) states that thesensation of brightness varies as the logarithmof the stimulus and can be deduced from the as-sumption that the Fechner fraction is a constant.For moderate degrees of brightness, it is onlyroughly constant. The term has been appliedfor sensations other than brightness.

feedback The principle of sending part of anoutput signal from a device to be re-evaluatedwith the original input signal.

A basic feedback system is shown in the fig-ure. It consists of a summing network indicatedby Σ, an amplifier with an open loop gain givenbya, and a feedback network with transfer func-tion b. The signals processed by such a systemcan be pneumatic or mechanical, but electricalvoltage signals will be used in the following.There are two broad distinctions of feedback: Inanegative feedbacksystem, the feedback signalis subtracted from the input signal by the sum-ming network and generates an error signal,

Vε = Vin − Vfeedback.

On the other hand, if the summing network pro-vided

Vε = Vin + Vfeedback,

then the system would be apositive feedbacksystem. Negative feedback is normally used forgeneral signal processing amplifiers, while pos-itive feedback is used in non-linear systems suchas comparators.

The amplifier in the system provides an am-plified signal,

Vout = aVε .

This signal is sampled by the feedback networkand produces the feedback signal

Vfeedback= bVout .

Thus the amount of signal sent back is deter-mined byb, and henceb is sometimes referredto as the feedback factor. The closed loop gain,G, of the entire system is found by eliminatingVfeedbackandVε from the above,

G =Vout

Vin=

1b

11 + 1/ab

.

If a is sufficiently large, as is usually the situ-ation with operational amplifiers, then the gainreduces to

G =1b

Basic feedback system.

feedback, acoustic Feedback in an acousticsystem. Acoustic feedback can occur when aportion of the output sound in a system comesback to the input of the system.

feedback factor Seefeedback.

feedback network The circuit responsiblefor sampling the output of a feedback systemand producing a feedback signal. The transferfunction of the network, or sometimes the feed-back factor, determines how much of the outputsignal is re–sampled by the system. It can alsobe reactive and thus make the entire system fre-quency dependent.See alsofeedback.

feedback voltage Seefeedback.

Fermat’s principle Also known as theprin-ciple of shortest optical path.It states that theoptical length: ∫ B

A

nds ,

of a ray between two pointsA toB in a mediumof indexn is shorter than the optical length of


any other curve that joins these points. It is alsoknown as theprinciple of least time.

ferrimagnetism A specific type of orderingin a system of magnetic moments. A material isferrimagnetic when (a) all moments on a givensub-lattice point in a single direction and (b) theresultant moments of the sub-lattices lie parallelor anti-parallel to each other.

ferrite Ferrite, or ferrimagnetic oxide, is a ce-ramic material that is usually dark grey or blackand very hard and brittle. The material has goodmagnetic properties and high bulk resistivities.It is therefore suitable for low loss transformerand inductor cores. The crystaline structure ofa simple ferrite is cubic, mineral spinel. Themagnetic properties arise from the metallic ion,usually a diavalent transition metal such as Mn,Fe, or Co, occupying a particular position rela-tive to the oxygen atoms in the ceramic. Someferrites have near-rectangular hysterisis curves,thus miniature toroidal ferrites have been usedin early memory elements.

ferromagnetism Ferromagnetic materialsexhibit a high magnetic permeability, e.g., fer-rite and powdered iron.

fiber An optical fiber consists of a glasscore that is completely surrounded by a glasscladding used in transmission of light pulses fortelecommunications purposes.Seefiber opticcable.

fiber absorption Transmission of lightthrough fiber undergoes attenuation, which de-pends on the wavelength being used. The at-tenuation properties are of the order of about5% loss per kilometer for bands centered on 1.3and 1.55 microns.Absorptionis one of two ba-sic loss mechanisms in optical fibers. The othermechanism isdispersion.Small traces of metal-lic impurities, e.g., Fe, Cu, in the silica can in-crease the loss in the fiber considerably. A com-mon impurity is water as hydroxyl ions.

fiber, inhomogeneous An optical fiber inwhich the refractive index changes throughoutthe fiber, as a function of the spatial coordinates.

fiber, low loss Optical fiber transmission sys-tems have low transmission loss, which permitslonger repeater sections than with a coaxial ca-ble system, thereby reducing costs. Fibers areso free from impurities that very little energy islost and there is usually an attenuation of lessthan 1 dB per kilometer; i.e., there is low energyloss per unit length of fiber. The energy loss isinversely proportional to the square root of thefrequency being transmitted.

fiber, multimode Optic fibers with thickercores, typically 50 or 62.5 microns, that allowmany different modes of transmission since lighttravels in multiple paths such that it is reflectedfrom the cladding back into the core as it travelsdown the core. Examples arestepped index,andgraded index multimode.

fiber optic cable Transmission systems em-ploying the use of a pulsed light-wave as a car-rier. This is sent in digital form using a bi-nary code. The optical cable consists of a glasscore that is completely surrounded by a glasscladding. The core transmits the lightwavesand the cladding minimizes surface losses andguides the lightwaves. It has low transmis-sion loss, wide bandwidth, small cable size andweight, and immunity to electromagnetic inter-ference. Its great advantage is that it can carrythousands of different frequencies without dataloss.

fiber optics The branch of optical technol-ogy concerned with the transmission of radiantpower through fibers made of transparent ma-terials such as plastic, glass, or fused silica. Afiber is a thin filament of these materials that is


drawn or extruded so as to have a central coreand a cladding of a material of a lower refractiveindex so as to promote total internal reflectionand travel along the length of the fiber with-out escaping reflection. It may be used as asingle fiber to transmit pulsed optical signals(communications fiber) or in bundles of fibersto transmit light or images. Cables of opti-cal fibers used for this purpose are smaller andlighter than conventional cables using copperwires or coaxial conductors, yet they can carrymuch more information, making them usefulfor transmitting large amounts of data betweencomputers and for carrying data-intensive videoor large numbers of simultaneous phone con-versations. Optical fibers are immune to elec-tromagnetic interference and to crosstalk fromadjoining fibers. To keep a signal from deterio-rating over long distances, optical fibers requirefewer repeaters than does copper wire over agiven distance. Most fibers for long distancepurposes are made of quartz because of the lowlosses down to 0.1dB/km, while some short-distance fibers have less expensive and easier-to-handle plastics as the core material. In everycase, the light guidance is provided by total re-flection inside the fiber core, which has a slightlylarger index of refraction than the rest of thefiber (the cladding). An additional outer plas-tic coating protects the fiber from mechanicalor chemical damage. Standard telecommunica-tion fibers have core diameters of 9 microns andan outer cladding diameter of 0.125mm (9/125monomode fiber) or 50 microns (50/125 multi-mode fiber). For long distance telecommunica-tions only monomode fibers are used, becauseof the absence of modal dispersion.

fiber optics communications Fiber opticscommunications represent a form of opticalcommunication through optic fibers made of sil-ica glass. Because free unguided laser beams arehighly vulnerable to obstruction by fog, rain orsnow, laser beams need to be transmitted withinprotective pipes for earth-bound telecommuni-cations. The efficacy of information transmis-sion via optical fibers is a result of the orders-of-magnitude more superior transparency of silicaglass to visible and infrareds, as compared toany other previously known solid medium forelectromagnetic waves of any other frequency

spectrum. Transmission is typically in base-band, with the information signal representedas a sequence of on-and-off light pulses. Trans-mitted signals may lose half their power only af-ter having traveled along optical fibers for over10 km or up to 50 km, depending on the op-tical fiber and optical source used. A signalrepeater takes attenuated incoming on-off op-tical pulse trains, converts them into electronicpulses, then amplifies and re-times the electronicpulse trains and uses them to excite an opti-cal transmitter to regenerate the received opti-cal pulse trains to travel another length of theoptical cable network. Semi-conductor photo-diodes represent the most common optical com-munication receivers. Because the optical signalgenerally travels unidirectionally along the op-tical fiber, network nodes are typically arrangedin a ring configuration (rather than, say, a star-shaped topology). Signal distortion, however,occurs in long-distance transmission as the lightwave reflects off the boundary between the twoglass layers in the coaxial cylindrical step-indexfiber optical fiber, wherein the core layer is madeof glass with a slightly higher refraction indexthan the coaxial outer layer. Because opticalfibers would accept only light entering the fiberat a low angle from the fiber axis, laser sources(with their narrow spatial directivity) are typ-ically used. Fiber optics communications us-ing laser beams is characterized by lasers’ verywide frequency spectrum, thereby permittingvery high data transmission rates. Optical fiberscan also be more easily insulated from outsideinterference, and are smaller and lighter thanmetallic cables. Fiber optics link the centraltelephone switching offices of major Americanand European cities.

fiber optics data link An optical link capa-ble of handling data in digital form. This dealswith layer 2 of the open systems interconnec-tion (OSI) networking model, which concernsdata packets and reliable data transfer. The datalink layer detects and may correct errors in thephysical layer (layer 1). The data link layer isresponsible for several specific functions suchas providing a well-defined service interface tothe network layer as well as regulating the flowof frames.


fiber scattering Also known asdispersion.Light pulses sent down a fiber spread out inlength as they propagate, the amount dependenton the wavelength being used. Rayleigh scat-tering of the light occurs within the moleculesof glass material itself. This loss is independentof the light intensity but varies inversely withthe fourth power of wavelength. It is thereforeadvantageous to operate at longer wavelengths.Favorable operating windows for fiber optics arein the region of 800 nm, 1300 nm, and 1500 nm.Dispersion effects can be canceled out by mak-ing pulses in a special shape known as solitons.These can be sent for thousands of kilometerswithout distortion.

fiber, single mode Optic fibers with verysmall diameter core, typically 8 - 9 microns,that allow only a single mode of transmissionof light pulses. An example is thestepped indexmonomode.

Single mode fiber.

fiber, W-type A doubly cladded optical fiberwith two layers of concentric cladding, in whichthe core usually has the larger refractive indexand the inner cladding has the lower refractiveindex. There are several advantages of this typeover conventional fibers, such as reduced bend-ing losses.

Fick’s first law Entropy drives diffusion of asubstance in the presence of a concentration gra-dient.Fick’s first lawconnects the flux (J) of thesubstance to the concentration gradient (∂c/∂xin one dimension) via the diffusion coefficient

D:

J = −D ∂c

∂x.

Fick’s second law Connects the time rate ofchange of the concentration of a solute (∂c/∂t)in terms of the diffusion coefficientD and thespatial rate of change of the concentration gra-dient (∂2c/∂x2):

∂c

∂t= D

∂2c

∂x2.

fidelity The degree to which an electronicsignal can be reproduced at the output comparedto the input signal.

figure of merit This indicates the perfor-mance of a device for a particular application.

1. For a magnetic amplifier: the ratio ofpower amplification to control time constant

2. For atransistor: the ratio of gain to thebandwidth

3. For agalvanometer:the ratio of the de-flection to the current, also calledsensitivity

4. The ratio of reactance to resistance.

film, Rollin A Rollin film is the film of su-perfluid helium found on any surface in contactwith bulk superfluid helium. This film flows,or creeps, along all surfaces that are below thelambda temperature, 2.1768 K. Due to the highthermal conductivity of the film, a superfluidfilm can increase the heat flow into a low temper-ature experiment. In such cases, heaters or knifeedges are used to prevent such films. The heaterboils off the film using less heat than would becarried by the film, and the knife edge is a sharpedge over which the superfluid film cannot flow.As the superfluid flows over the knife edge, itbecomes thinner and thereby must flow fasterto keep the mass flow the same. A sufficientlysharp knife edge will force the fluid to exceed thecritical velocity, thereby turning the fluid nor-mal. See alsohelium-4, superfluid.

filter circuits Circuitry that selectively elim-inates or passes.

filters (optical) A homogeneous opticalmedium that is used for attenuating particular


wavelengths or frequencies of light (includinginfrared and ultraviolet) while passing otherswith relatively no change. Filters are used tocontrol or alter the relative energy distributionof a beam of light. In photography, filters maybe placed over the light source or over some partof the optical path to the camera, frequently overthe lens. Typically, transparent substances (col-ored glasses or films) are used. One type of col-ored filter is thegelatin filter,which can be dyedwith a wide range of materials. Another typeof colored filter is made from polyester and hassome of the same characteristics. A filter basedon a completely different principle is theinter-ference filter,which uses destructive interfer-ence of waves transmitted directly through thefilter and those reflected2n times from the frontand back faces of the filter to yield a narrowerband of wavelength — 10 to 100 angstroms. Alinear polarizing filtertransmits light waves thatvibrate in a single direction only. It eliminatesvarious degrees of reflected light from glass, wa-ter, plastic, paper and similar surfaces. It canalso eliminate light reflections from vapors orfloating dust to emphasize a blue sky.Polariz-ing filters transmit light waves that vibrate in asingle direction only; the effect can be seen inthe viewfinder as the filter is rotated. Acircularpolarizing filter converts linear polarized lightwaves to circular polarized light waves. Re-quired whenever polarizing is desired, as withuse of autofocus cameras and cameras that havea semi-silvered reflex mirror. Aneutral den-sity (ND) filter reduces the amount of transmit-ted light without affecting color balance. Ex-posure change is rated by filter factors, such asND2 or ND4. ND2 reduces light to 1/2 andND4 to 1/4. Acolor compensating (CC) filterenables fine adjustments of color tone or colordensity in color photography. Acolor (temper-ature) conversion filteralters color temperatureof light to make it suitable for the film in use,thereby enabling the photographer to use day-light film indoors or tungsten film outdoors. Acontrast-control filteris used with black-and-white film to emphasize contrast in a picture. Anorthochromatic filteris a green or yellow-greenfilter used with black-and-white films to com-pensate the difference between the color sen-sitivity of films and the relative luminous effi-ciency of the human eye. Askylight filter re-

moves a portion of the blue light. Taking colorpictures in the shade under a clear, midday skyresults in an overall slightly bluish cast. Thisexcess blue can be corrected with a skylight fil-ter to produce a more natural effect.UV filterseliminate invisible ultraviolet light, renderingpictures with higher contrast, since ultravioletrays of shorter wavelengths are easily reflectedby vapors or floating dust, causing lower pic-ture contrast. Asoft focus filterdiffuses lightand imparts a slight flare to the image, provid-ing a soft-focus effect, which is ideal for portraitphotography.

filters, acoustic Devices that significantlydecrease amplitude of a complex sound in cer-tain frequency bands while the amplitude in oth-ers remains almost unchanged. A low-pass filtercuts off all frequencies above a critical one. Itcan be designed as a number of cavities of thesame form, arranged in a row and connected bynarrow tubes. A high-pass filter cuts off all fre-quencies below a critical one. It can be designedas a tube with holes at regular distances along thetube. A band-pass filter cuts off all frequenciesabove and below a definite band of frequenciesand can be designed as a combination of low-pass and high-pass filters.Acoustic filtersarewidely used in practice for noise reduction inair-conditioning systems, jets, and exhaust sys-tems of internal combustion engines in motorvehicles.

fish, hearing organs of The otolith organs,consisting of fine hairs. Theotolith organsal-low fish to have only a rough impression abouta received sound without detailed analysis. Theotolith organs can be considered as an early hear-ing mechanism in animals.

Fizeau fringes The interference pattern thatresults from the interference of light transmit-ted by wedge-shaped thin film. These are thefringes obtained with theFizeau interferometer,an optical arrangement in which light from aquasi-monochromatic source is collimated by alens and falls on a planar-wedge-shaped thin filmat nearly normal incidence. The light reflectedby both surfaces of the film are collected by thesame lens which converges it to an aperture inthe focal plane. To an eye or film immediately


behind this aperture, the Fizeau fringes are vis-ible over the entire area illuminated by the lenswhen lines of equal optical thickness are fol-lowed. The fringes may also be obtained froman optically thick film, provided the source issufficiently small. The method is used in opti-cal workshops to test the optical thickness uni-formity of transparent plane parallel plates.

Fizeau method (velocity of light) Themethod used in the first successful measurement(1849) of the velocity of light not involving as-tronomical observations. A brief flash of lightwas sent out and the time to travel to a distantmirror and back was measured. A toothed wheelor cogwheel with 720 cogs was rotated at highspeed so as to break up a light beam passingthrough its rim into a series of short flashes. Thelight was rendered parallel with a lens, and thenfocused with another lens onto a distant mir-ror (5.36 miles in Fizeau’s original experiment).After reflection, the light retraced its path andwas focused again onto the rim of the cogwheelby the first lens. If, during the time the light hastraveled, the wheel had turned enough for a cogto be interposed, the flash would be blocked out.A further increase in speed would cause the flashto reappear as the returning light could then passthrough an opening in the cogwheel. Fizeau wasable to say that light moved at 313,300 kilome-ters per second which is close to what we nowknow to be 299,792.458. The largest uncertaintyin this method is the determination of the con-dition of total eclipse by the cog. Young andForbes overcame this difficulty by placing anadditional lens and mirror at somewhat greaterdistance so that two images were observed si-multaneously, and the speed of the cogwheel de-termined at the time the images appear of equalintensity, a more accurate experiment since theeye is very sensitive to the detection of slight dif-ferences in intensity of adjacent images. Theseexperiments were soon improved by replacingthe cogwheel with a rotating mirror by Fizeauand Focault independently in 1850.

flame, singing A gas flame located at thelower end of a vertically positioned narrow tubeopened at the upper end, that generates sound.The gas flame causes and upward current of airand vibrations of the air column in the tube with

its natural frequency. This results in sound radi-ation from the open end of the tube. A singingflame is also called asigning tube.Signing tubesare often used in physics demonstration experi-ments.

flash A type of converter with extremelyrapid conversion times using comparators forcoding voltages to give binary outputs.

Flemming’s rule A rule relating the directionof force, velocity and magnetic field felt by amoving charged particle. It can be summarizedasF = q(V × B), where the direction ofF isgiven by the cross product ofV andB.

flip-flop A sequential logical circuit whoseoutput depends on the present values as well asthe history of its inputs. It is a basic buildingblock of larger and more complicated sequentialcircuits such as counters and shift registers.

The most important characteristic of a flip–flop is that it is capable of remembering binarysignals. Thus, it is viewed as a memory storageelement with control inputs. There are three dif-ferent types of control inputs:static, where theflip–flops output will change according to thepresent inputs (and their history),preparatory,which set up the flip–flop but are not able tochange the output directly, anddynamic,wherea “clock” signal is required to instigate anychanges of the output state as determined by thepreparatory inputs.

There are three basic types of flip–flops,which are identified by the way their outputsrespond to the inputs:

1. Direct R–S.This is the most elementaryflip–flop. It has two static control inputsS andR labelled forsetandreset.It also has an outputQ as well as an ancillary outputQ. The normalstate of the inputs isR = S = 0, andQ isallowed to be either 0 or 1. IfS is changed to1, thenQ = 1 and will hold this value even ifSreturns to 0. If, on the other hand,R changes to1, thenQ = 0 and will remain so whenR returnsto 0. The outputQ is not defined if bothR andS are 1. The operation of theR–S flip–flopcan be summarized in the accompanying tablewhereQprior refers to the prior state ofQ beforetheR, S values changed. Thus, by examining


the outputQ, one can determine the previousvalues ofR andS: a rudimentary memory.

The operation above can be illustrated byconsidering the realization of anR–S flip–flopusingNAND gates as shown in the figure.

Realization of an R–S flip–flop.

By using the truth table for theAND gate (seeconjunction; logic gates) as a starting point, theoutput value ofQ can be verified for any givenvalid input state. Observe that if bothR andSchange to 1 (from 0) simultaneously,Q is logi-cally indeterminable as both 0 or 1 are equallyvalid outputs, hence this ambiguous input con-dition is avoided.

2. D Flip-Flop. This device also has two in-puts, a preparatory input,D, and a dynamic in-put,clock.The output,Q, changes to the currentstate ofD when theclock signal makes a par-ticular state transition. Depending on the con-vention used by the particular circuit involved,the flip–flop will effect the change inQ whenclock changes 0→1 or 1→0; the former is themost common. If we assume this convention forillustration purposes, then the truth table for theD flip–flop can be determined as shown in theaccompanying table. Note that whenclock is 0(or even 1→0), the output does not depend onthe inputD, indicated in the table asX. Thus inoperation,D is sampled at the clock transitioninstant and held indefinitely; henceD for delay.

3. J–K Flip Flop. This device has twopreparatory inputsJ andK, and aclock input

to instigate a change in the outputQ based onthese inputs. The operation of this flip–flop isdefined as follows: IfJ = K = 0, then the out-put will hold its previous value, even on a properclock transition. IfK = 1 while J = 0 duringa clock transition, thenQ is set to 0; similarly,if K = 0 while J = 1 thenQ = 1. Thus, sofar theJ–K is operating in a manner similar toanR–S except that aclock is required. How-ever, ifJ = K = 1, an ambiguous condition fortheR–S flip–flop, thenQ = NOT Qprior (“tog-gled”) upon aclock transition. Assuming thesame previous conventions, the operation of theJ–K flip–flop is summarized in the accompa-nying table.

floating of circuit Conduction wherein a cir-cuit is not grounded or tied to an establishedvoltage supply.

flow cytometry A technique for counting,sorting or selecting individual cells as they flowthrough a tube of pipe.

flow impedance Flow impedance is a de-vice designed to regulate flow of a cryogenicgas in a low temperature apparatus. In a con-tinuous3He refrigerator, the condensed liquid3He must have its pressure throttled from thatat the condensation stage (of order 0.1 bar) tothe low pressure found in the3He pot (a few


mbar). This is usually accomplished by insert-ing a high impedance segment of the tube intothe flow path. The most common material forsuch constrictions is cupro-nickel tubing withthe proper conductance:

F =πa4

8η`Pa

wherea is the radius of the tube,` is the lengthof the tube,η is the viscosity of the liquid orgas in question, andPa is the average pressurein the tube. (The impedance is the reciprocal ofthe conductance.)

flow measurements The determination ofthe rate of movement of a fluid.

flow measurements, continuous The deter-mination of the rate of movement of a fluid atall times.

flowmeters Any device used to measure therate and direction of movement of a fluid.

flow pattern, cerebral fluid The particularmanner in which the cerebral fluid moves.

fluidity, cell membrane The degree to whichthe cell membrane is able to flow.

fluorescence Radiation caused by a transitionbetween two well-defined energy states of atomsof solids, liquids and gases due to absorption ofincident light. According toStoke’s law,thewavelength of the emitted fluorescent light isalways longer than that of the incident light.

fluorescent screen Screen coated with fluo-rescent material used, for example, to detect thepresence of ultraviolet light or X-rays. Fluo-rescent materials emit characteristic light whenexposed to ultraviolet radiation.

fluorescent spectroscopy, nanosecond Theanalysis of light given off by a sample dur-ing excitation by light of a higher energy. Forpulsed excitation, light emitted within about 10nanoseconds of the exciting pulse is consideredto be fluorescent. The fluorescent light is emit-ted after the energy of the original excitation hasbeen transformed by radiationless transitions to

different energy levels. Detailed informationabout the energy levels can be obtained by thistechnique.

fluorography Photography of an image pro-duced on a fluorescent screen by X-rays.

flutter Rapid fluctuation of frequency of re-produced sounds due to fluctuations in speedduring the processes of recording and reproduc-tion. These fluctuations in speed can be causedby variations in speed of a turntable, for exam-ple.

flux ball A test coil in which a series of coax-ial cylindrical windings of different lengths arewound to form a sphere. It is used to measuremagnetic flux density.

flux density, magnetic A vector quantity thatis used as a quantitative measure of a magneticfield; the force on a charged particle moving inthe field is equal to the particle’s charge timesthe cross-product of the particle’s velocity withthe magnetic induction.

flux gates These are used to determine themagnitude and direction of an external magneticfield. They generate electrical signals whosemagnitude and phase are proportional to the ex-ternal field acting along its axis.

flux, luminous In photometry, the time rateof flow of light per unit solid angle, weightedwith respect to its efficiency to evoke the visualsensation of brightness. Sources of light emit ra-diant energy at a certain rate, which flows at thespeed of light past any point, but only the part ofthis energy in the visible range of wavelengthswill excite vision. Flow orfluxcan be expressedin mechanical units, ergs per second, but whenvisual stimulation is to be measured, the flow isexpressed in luminous flux units (lumens). Theunit lumen is defined in terms of the flux from astandard luminous source (which implicitly in-vokes luminous sensation, as for example the In-ternational candle, black-body radiation at 2046K, etc.) into a unit solid angle. Equal luminousfluxes produce equal sensations of brightness,but different fluxes do not produce sensations ofbrightness in direct proportion. The luminous


flux density gives the intensity of illuminationin lumens per unit area.

flux, magnetic Lines used to represent mag-netic induction,B. Lines of flux are used torepresent the field in magnitude and directionat any point. The number of lines of flux perunit area of a surface perpendicular to the fieldis equal to the magnitude of the magnetic in-duction. The total number of lines of inductionthrough a surface is themagnetic flux.

flux, magnetic, changing This produces aninduced EMF in a conductor subject to thechanging magnetic flux that results from Fara-day induction.

fluxmeter An instrument to measure mag-netic flux. It consists of a moving-coil ballisticgalvanometer with a long period. A search coilis connected to the galvanometer and the changein the flux that results from the motion of thesearch coil that is detected by the galvanometer.

fluxoid The quantum of magnetic flux in asuperconductor is onefluxoid:

Φ0 = 2π/2e = 2.06785× 10−15 T m2 .

See alsosuperconductivity.

flux quantization The quantum mechanicalexpression for the current density in a supercon-ductor located in a magnetic field is

~j = −[2 e2

mc~A+

em

~∇Θ]|Ψ |2

where ~A is the magnetic vector potential,Θis the phase of the superconductor wavefunc-tion, andΨ is the amplitude of the supercon-ductor wavefunction. If the superconductor isin the form of a closed ring, it is possible for themagnetic field threading the ring to be non-zerowhile the magnetic field inside the superconduc-tor is zero. If we integrate the current densityalong a path inside such a ring, the integral sim-plifies to

2 e2

mcΦ +

2π n e m

= 0

whereΦ is the magnetic flux, andn is an integer.This further simplifies to the flux quantization

rule:

Φ =nh c

2 e= nΦ0

whereΦ0 is one flux quantum, or fluxoid.Seealsofluxoid; superconductivity.

flying spot A moving spot of light that iscontrolled by external signals.

focal length The distance along the opticalaxis between a focal point and its correspondingprincipal point, which, for a thin lens, is approx-imately the same as the distance of a focal pointfrom the lens.

focal lines For an off-axis cone of light, asa result of astigmatism, the fan of rays in thetangential (or meridional plane) and the sagit-tal planes come to focus on the so-called tan-gential focal lines and the sagittal focal lines,respectively. Thetangential(or meridional orprimary)focal lineis perpendicular to the merid-ional plane, and has a smaller image distancethan the secondary or sagittal focus. Thesagit-tal (or secondary)focal lineis in the meridionalplane.

Tangential and sagittal focal lines.

focusing, acoustic The convergence of soundwaves. Acoustic focusing is similar to optic fo-cusing. Due to diffraction, a sound wave cannotbe focused to a point; it is rather focused to aspot. The point where the sound intensity ismaximal is calledfocus. Since in most casessound waves are diverging, acoustic focusing isachieved only under certain circumstances. Itcan be achieved by specially constructed trans-ducers, by convergingacoustic lensesand mir-rors, and also as a result of random inhomo-geneities in a medium. Acoustic focusing is


widely used in technique and medicine for con-centrating acoustic energy.

focusing, magnetic In some opto-electronicdevices, magnetic fields are employed to pro-duce an effective focusing of accelerated elec-trons to form electron images, for example.

focusing of electron beam Focusing of anelectron beam by use of an electrostatic lens ina cathode-ray tube (CRT). In a CRT, electrostaticlens consists of a focus anode, accelerating an-ode, and so on. The focus anode and accelerat-ing anode are maintained at a positive potentialwith respect to the cathode. The potential ofthe accelerating anode is higher than that of thefocus anode. The focal length of this lens de-pends on the potential at the focus anode. Thepotential at the acceleration anode also affectsthe focal length.See alsoelectrostatic lens.

focus, tangential Seefocal lines.

folded cascode A cascode constructed withtwo different types of transistors such as ann–p–n and p–n–p bipolar transistor pair, ann–channel andp–channel FET pair, or a FET andbipolar transistor combination.Seeamplifier,cascode.

footprint (communication) This concernssatellite communication, and refers to down-ward beam covering a substantial fraction of theearth’s surface. It is therefore the area coveredby the satellite and reached by its radio beamson earth. This area can be broad or narrow, cov-ering up to hundreds of kilometers in diameter.

forced oscillations Oscillations that are dueto a driving force. The simplest model of thisis one-dimensional oscillations governed by theequation

Mdx2

dt2+R

dx

dt+ Sx = Feiωt .

Here,x is a coordinate of a physical quantityunder oscillations,t is time, F andω are theamplitude and angular frequency of the drivingforce, and other notations are the same as thosein the equation for one-dimensionaldamped os-cillations. The solution of the equation in ques-

tion is the sum of two terms. The first one cor-responds to free oscillations and exponentiallydecays in the course of timet. The second termis the steady state solution given by

x =F exp(iωt− iθ)√

(S − ω2M)2 +R2ω2,

wheretan θ = Rω/(S − ω2M). The ampli-tude of the steady state oscillations|x| reachesa maximum value at a given frequencyω =(S/M − R2/2M2)1/2. This phenomenon iscalledresonance.

format (1) Different software applicationssave data in certain ways. There are several stan-dard file formats; some examples of graphicalformats are.gif or .jpg.

(2) Hard drives need to be formatted afterpartitioning so that they can be used by the op-erating system. It allows the hard disk to beready for use. Floppy disks also need to be for-matted. There is also the SCSI low-level formatthat makes the drive ready to be used by a SCSIcontroller.

form factor The ratio of the effective value(root mean square value) of an alternating quan-tity to the average value during half of an interval(cycle). It compares the various kinds of peri-odic waveforms. The effective valueVeff ofalternating voltagev of intervalT is

Veff =

√√√√[ 1T

∫ T

0

v2

].

When the alternating voltage has a sinusoidalwaveform, the effective value is

Veff =

√√√√[ 1T

∫ T

0

(V cos

2πtT

)2],

=V√2.

The average value during a half of the intervalof this sinusoidal waveform is2V/π. The formfactor of sinusoidal waveform isπ/2

√2.

Forster critical distance The characteris-tic distance associated with the energy transfer


between two chromophores. This distance de-pends on the spectral overlap between the donoremission and acceptor absorption bands, the re-fractive index of the medium, the quantum yieldof the donor in the absence of the acceptor, anda geometric factor depending on the relative ori-entation of the donor and acceptor.

Forster dipole-dipole approximation Theinteraction between two chromophores is ap-proximated according to the interactions of theirassociated dipole moments. The success of thisapproach was first shown by the experiments ofStryer and Haugland (L. Strayer and R.P. Haug-land,Proc. Natl. Acad. Sci. USA98(1967) 719).

fountain effect Also called thethermome-chanical effect,thefountain effectis a manifes-tation of zero viscosity in Helium II. Considertwo containers of superfluid helium at a pres-sure,p, and temperature,T , connected by a nar-row tube or a tube filled with closely packedparticles. (See figure.) If we increase the pres-

Fountain effect. The thin connecting tube is filled with

packed powder.

sure in container A, the normal component ofthe helium will be unable to flow through thenarrow channel due to the large viscosity of thenormal fluid. The superfluid component, how-ever, will flow unimpeded through the orificeinto container B. This process will increase thetemperature in container A and decrease it in

container B such that

∆p∆T

= ρS ,

whereρ is the density of the fluid andS is theentropy of the normal fluid. Conversely, if webegin by increasing the temperature in containerB, superfluid flows into container B, producingthe hydrostatic pressure difference given above.These effects are quite large – at 1.5 K, a tem-perature difference of 0.001 K produces a 2 cmpressure head in the liquid helium.

Fraunhofer doublet One of the three typesof small aperture objective lenses used in mostprismatic binocular telescopes, low-power mi-croscopes and telescopes. The field of view inthese objectives is very small and the apertureis usually smaller than f/5. In this case the mostimportant aberrations are the primary spherical,coma, and chromatic. Cemented achromaticaplanatic doublets are commonly used. Imagequality deteriorates rapidly away from the axis,but there is little that can be done to improve it,so either the crown or flint component of the ce-mented meniscus lens pair can be placed on thelong conjugate side of the lens. If the crown is onthe long conjugate side, the achromatic doubletis of theFraunhofertype. If the flint is on thelong conjugate side, it is aSteinheiltype. Fraun-hofer types usually have shallower surfaces andthe advantage that the more stable crown glassis on the exposed side for a telescope objective.A third type is theGausstype, which consistsof uncemented meniscus lenses with an air gap,so it is possible to achieve better correction forspherical aberration at the expense of coma.

Fraunhofer lines The thousands of fine darkabsorption lines that cross the spectrum of thephotosphere (the glowing outer layer) of the sun.A few of the lines are due to absorption in theterrestrial atmosphere. The most prominent 600of these lines were observed and named by J.Fraunhofer in 1814, who recognized that theD lines coincided with the emission lines ofsodium. G.R. Kirchoff in 1859 gave the mod-ern interpretation of these lines. They are use-ful spectral benchmarks, used in the specifica-tion and measurement of refractive indices, forexample. Some of these benchmarks, together


with their origin and approximate wavelength inangstroms, are:

A terrestrial O2 7594-7621B terrestrial O2 6867-6884C H-alpha 6562.816Alpha terrestrial O2 6276-6287D1 Sodium 5895.923D2 Sodium 5889.953D3 He 5875.618E2 Fe 5269.541B1 Mg 5183.618B2 Mg 5172.699B3 Fe 5168.901B4 Fe 5167.491F H 4861.327D Fe 4668.140E Fe 4383.547F H 4340.465G Fe 4307.906G Ca 4307.741G Ca 4226.728H H 4101.735H Ca+ 3968.468K Ca+ 3933.666

The lines A, B, and alpha are oxygen bandscaused by absorption in the earth’s atmosphere.

free vibrations Vibrations that continue aftereliminating a driving force.

freeze resistance The ability of certain or-ganisms to function below the normal freezingtemperature by the use of specific molecules thatdepress the freezing point of their bodily liquids.

frequency The number of cycles completedby a periodic quantity, the acoustic disturbance,in a unit time. The units of frequencyf arehertz (Hz); 1 Hz corresponds to 1 cycle persecond. Frequencies audible to the average hu-man ear are in the range between 20 and 20,000Hz. The frequency is the inverse of the periodT, f = 1/T , the time interval during which oneoscillation cycle is executed.

frequency allocation Bands of frequenciesfor specified services are allocated by interna-tional agreement and refer to the frequency onwhich a transmitter has to operate within speci-fied tolerance.

frequency, angular For any oscillation, thenumber of vibrations per unit timef , multipliedby 2π, ω = 2πf . Also known asangular ve-locity or radian frequency.The units of angularfrequency are radians per second (rad/s).

frequency, audible The range of frequenciesthat the average human ear can hear, from 20to 20,000 Hz. Disturbances below the usefulfrequency range for human hearing (below 20Hz) are classified asinfrasound,and those with afrequency too high (above 20,000 Hz) are calledultrasound.

frequency band A continuous range of fre-quencies extending between two limiting fre-quencies, thenth band having a lower fre-quencyfmin(n) and upper frequencyfmax(n).The frequency scale is divided into contigu-ous bands, and is said to be proportional iffmax(n)/fmin(n) is the same for each band.The center frequency of a bandfc(n) isdefined as the geometric mean,fc(n) ≡√fmin(n)fmax(n). In an octave band the rela-

tion fmax(n) = 2fmin(n) holds. In noise con-trol the 1

3 -octave frequency partitioning schemeis used, for whichfmax(n) = 21/3fmin(n). Theprinciple of fixed frequency ratios is also ap-plied in the theory of musical temperament. Fre-quency ratios corresponding to classic music in-tervals that sound harmonious are 2:1 (octave),3:2 (perfect fifth), 4:3 (perfect fourth) and 5:4(major third).

frequency compatibility This refers to theability of television sets to receive color broad-cast in black and white without special adaption.

frequency conversion The shift from onefrequency band to another using multiplicationby a sine wave.Seemixer.

frequency, cut-off A frequency at whichaxially decaying modes develop (for examplein waveguides or horns). These modes de-cay exponentially with distance from the soundsource or the reflection location. For modes withfrequencies above the cut-off frequency, axialpropagation takes place, and the correspondingmodes are termedcut-on.


frequency discriminator A device used toprovide frequency – amplitude conversion whenfrequency demodulation occurs. The processrequires that output voltage or current amplitudeis linearly proportional to the frequency of theinput signal.

frequency, distress With respect to the S.O.S.signal, the international code signal used by air-craft or ships at sea when in distress.

frequency divider A unit or device that di-vides (reduces) an input frequency by a presetnumber. If the input frequency isfin, then theoutput of the circuit will be

fout = fin ÷N ,

whereN is the preset divisor. Frequency di-viders are usually based on binary counters,hence they will operate on and generate squarewaves. Programmable counters are often used,allowing a user to change the value ofN . Seealsofrequency synthesizer.

frequency doubler A circuit or device ca-pable of providing power at twice the input ACfrequency.

frequency, fundamental The lowest reso-nant frequency of the system. The lowest fre-quency in a complex wave. Other resonant fre-quencies are calledovertones.

frequency, maximum usable (MUF) In ra-dio wave propagation via the ionosphere, thehighest frequency that can be used between twopoints at a particular time. It therefore gives thebest frequency for long distance transmission.

frequency meter A device capable of mea-suring the frequency of either a sinusoidal ACinput voltage or a propagating electromagneticwave. Depending on the magnitude of the fre-quency involved, the device may either countthe number of waves in a predetermined dwell-time, or determine the frequency by means of acalibrated resonant cavity or circuit.

frequency multiplier A unit or device thatprovides an output frequency at some preset

multiple of the input frequency.Seefrequencysynthesizer.

frequency, resonant A frequency at whichsome measure of a physical system subjected toperiodic forcing develops a maximum. Threetypes are defined:phase, amplitudeandnaturalresonance.They are nearly equal when dissipa-tive effects are small. Also known asresonanceor natural frequency.

frequency response Frequency response ex-presses the relative gain of, for example, an am-plifier as a function of frequency for a pure sinu-soidal input signal. This information is usuallypresented in a graphical format.See alsohalf-power bandwidth.

frequency selective amplifier A frequencyselective amplifier combines a frequency- orphase-dependent network with a broadband am-plifying device to produce a narrow band-passor band-stop filter. Depending on the networkused and how it is connected, the frequency se-lective amplifier can be designed to either accepta very narrow range of frequencies (anaccep-tor) or reject a narrow range of frequencies (arejecter).

Rejecter and acceptor transfer functions of a frequency

selective amplifier.

frequency swing The total carrier frequencyshift between the lower frequency extreme andthe upper frequency extreme in telecommunica-tion transmission.

frequency synthesizer A frequency sourcewhose output is a programmable integer multi-ple of an input reference source. A block dia-gram of a simple frequency synthesizer is shownbelow. A frequency reference,fref, is sent to one


input of a phase difference detector whose out-put voltage controls a voltage control oscillator(VCO). The output of the VCO is feedback to theother input of the phase difference detector viaa divide–by–N counter. This counter generatesa pulse for everyN pulse of the VCO; hence theoutput frequency of the VCO isfout = N ×fref.The counter’sN value can be digitally con-trolled from, for example, a microprocessor ora simple thumb wheel switch array and the ref-erence frequency is usually derived from an ac-curate crystal controlled oscillator.

Operational block diagram of a basic frequency syn-

thesizer.

Frequency synthesizers are commonly usedin present day communications and radio sys-tems. However, the output frequencies in thesesystems are too high for common TTL or CMOSdivided–by–N counters. A technique calledheterodyne–down conversionis used to han-dle, i.e., systhesize, higher frequencies. In thistechnique, a second,offset or local oscillator(foffset) is mixed with the output frequency ob-taining the frequency differencefout − foffset.This frequency is then processed by the divide–by–N counter as before. This additional stepis shown in block form in the second illustra-tion. The output frequency is thus determinedby fout = N × fref + foffset.

frequency time sharing Feature of time di-vision multiplexing in which some of the timeinterval between adjacent pulses is used by otherindependent message signals. It allows for thejoint utilization of a common channel since sev-eral independent message signals can be sentwithout mutual interference.Seemultiplexing,time division.

Block diagram showing the heterodyne–down conver-

sion technique in a frequency synthesizer.

Fresnel approximation An approximationused in the theory of diffraction. Suppose that apoint source is located at the center of the Carte-sian coordinate system, and a receiver is locatedat the point(x, y, z). Then, the phase incrementof a wave propagating from the source to the

receiver is given byexp(ik√x2 + y2 + z2

),

wherek is a wave number.Fresnel approxima-tion is a replacement of this phase increment by

its approximate valueexp[ik(x+ y2+z2

2x

)].

This approximation is valid ifx √y2 + z2.

Fresnel approximation for other problems (e.g.,wave radiation by an aperture) employs an anal-ogous replacement.

Fresnel integrals These are the integrals thatcome from separating the error function into realand imaginary parts for a real variablex

1(1 + i)

erf(x) = C(x)− iS(x) ,

where the resulting real functions are theFresnelintegralsgiven by

C(x) =

x∫0

cos(πt2/2

)dt

S(x) =

x∫0

cos(πt2/2

)dt .

Fresnel integrals may be evaluated numericallyor found graphically by use of the ingeniousCornu spiral, which is a double spiral curveformed by plottingC againstS. As with anyvibration curve, the amplitude of the diffrac-tion pattern from Fresnel diffraction is computed


from distances on this curve, and the square ofthis length then gives the intensity.

Fresnel lines Fringed shadows that appear atthe edges of the geometrical apertures in Fresneldiffraction. A Fresnel fringeis a single band ina group of these light and dark bands that can beviewed in the periphery of the Fresnel diffrac-tion shadow. Unlike the Fraunhofer pattern, theminima in Fresnel diffraction do not go to zero.

Fresnel zones An explanation of Fres-nel diffraction effects obtained by dividing thewavefront falling on the obstacle into a num-ber of concentric annular zones, such that thedistances of the boundaries of the zones fromthe observation point increase by one half wave-length from zone to zone. By Huygen’s princi-ple, each point on the wave front can be con-sidered the source of a secondary wave, each ofwhich makes its own contribution to the lightreaching the observation point. Each Fresnelzone contains approximately the same area sothat each zone may be viewed as having thesame number of secondary sources, and thuscontribute the same amount of light at the ob-servation point. However, the contributions atthe observation are out of phase since each zoneis a half wavelength farther than the next. Thetotal contribution is the sum of a series of terms,one for each zone, which are alternately positiveand negative. While all the zones have aboutthe same area, the central zones point directlyat the observation point, while the outer zonespoint more obliquely. This causes the magni-tude of each term to decrease steadily from termto term. Then the sum of the series can bedemonstrated to be half the first term, and theamplitude reaching the observation point fromthe entire unrestricted wavefront is half the am-plitude that would result if all zones but thefirst were blocked off. The intensity (ampli-tude squared) is one fourth that due to the firstzone alone. For the case of a circular aperture inwhich the wave is now blocked off by a screenthat has a small circular aperture, the amount oflight that reaches the central point of the diffrac-tion pattern depends on the number of Fresnelzones that fit into the aperture. If the radius of theaperture is such that only the first Fresnel zonefills it, then the amplitude is twice and the inten-

sity is four times that of the unscreened wave.Increasing the radius of the aperture so that twozones can fit, the amplitude is just the differenceof the amplitude from these two zones, or practi-cally zero (the contributions are equal but out ofphase), and the intensity decreases even thoughthe aperture has been made larger. Further in-creases of aperture radius cause the intensity topass through maxima and minima each time thenumber of zones included becomes odd or even.If an odd number of zones fits the aperture, thenthe diffraction pattern has a central bright point;if an even number of zones fits the aperture, thepattern has a central dark point. The same ef-fect occurs by moving the point of observationcontinuously to or away from the aperture alongthe perpendicular, thereby varying the size ofthe zones, and producing minima or maximumalong the axis of the aperture. If the circularaperture problem is replaced by a circular ob-stacle problem, these methods lead to the sur-prising conclusion that there should be a brightspot in the center of the shadow. If the obsta-cle is made to consist of alternately opaque andtransparent optical zones, it is possible to ar-range for alternate Fresnel zones to be effectivefor a particular observation point. Then the re-sult is high intensity at that observation point,since arrivals from alternate zones are in phase.This type of obstacle is theFresnel zone plate,which produces a bright image of the source atthe observation point, and thus acts as a lens. Bythis arrangement, Fresnel lenses produce imagesof any small bright source by diffraction.

friction When the surface of one body slidesover the surface of another, each body exerts africtional force on the other. Frictional forcesare oriented parallel to the surface in the di-rection opposite to the motion relative to theother body. Thus frictional forces always op-pose the motion. They can exist when no motionis present. Static friction describes frictionalforces acting between surfaces at rest with re-spect to each other. Its magnitudeFs is propor-tional to the normal force on the surfaceN , re-lated through the coefficient of static frictionµs,asFs ≤ µsN . Forces ofkinetic frictiondevelopbetween surfaces in relative motion. The magni-tude of the force of kinetic frictionFk is relatedto the normal forceN through the coefficient of


kinetic frictionµk, Fk = µkN . Friction in flu-ids that deform under the action of shear stressis described by viscous forces. A fluid elementwhen subjected to a shear stressτyx experiencesa rate of deformation (shear rate) described bydu/dy, for Newtonian fluidsτyx = µdu

dy . Thecoefficient of proportionality is the absolute (dy-namic) viscosityµ.

Friction.

frictional coefficient (electrophoresis) Thefrictional coefficientff−1

o relates the Stokes ra-dius Rs of a macromolecule to its molecularmass M:

Rs = ff−1o (3vM)−1/3(4πNA)1/3

wherev is partial specific volume andNA isAvogrado’s number.

frictional electrification An electric chargegenerated by rubbing a material against a dis-similar material. For example, an electric chargeis generated on a glass rod rubbed against a silkcloth.

fringes Interference and diffraction phenom-ena are characterized by patterns of maximumand minimum intensity calledfringe systems.One of these alternate light and dark or colorbands, in a circular or rectilinear pattern, is abright or dark or color fringe. Depending on themeans of separation of the original beam intointerfering beams, interference fringes may beclassified as fringes from division of wavefront,as from a diffraction grating, or fringes from di-vision of amplitude, as in a semireflecting mir-ror. Fringes from plane-parallel plates as in theFabry-Perot interferometer are fringes of equalinclination orHaidinger fringes.Fringes fromother geometrical situations are fringes of equalthickness orFizeau fringes. Since the visiblespectrum occupies only one octave, fringes frompath differences of one wavelength are possiblein white light, and are calledwhite light fringes.With longer path differences, fringes are seenonly for nearly monochromatic light, and arecalledmonochromatic light fringes.

fuel cell A type of electrochemical cell wherethe reactants are not permanently contained inthe cell but are fed into the cell when power isdesired. The operation of a fuel cell is similarto a battery but no consummable electrodes areused.

full-duplex Refers to a communications sys-tem or equipment capable of transmission in twodirections, that is, transmitting and receiving si-multaneously. In pure digital networks, this isachieved with two pairs of wires. In analog net-works or in digital networks using carriers, it isachieved by dividing the bandwidth of the lineinto two frequencies, one for sending, and theother for receiving.

fullerphone System of telegraphy that usesdirect currents for actual transmission. Buzzersignals for keying and listening are also used,thus making the system more difficult to tap.

function generator (1) A device capableof producing a desired time varying waveform(e.g., sine, square, or triangle) with adjustablefrequency and amplitude.

(2) A circuit that accepts an input voltage asan independent variable,x, and generates a de-


A block diagram of a function generator accepting an

input voltagex and producing an output voltage f(x).

Also shown is an arbitrary function with an example of

a piecewise linear approximation.

pendent output voltage based on a prescribedfunction, f(x). Example functions includef(x) = log(x), f(x) = x2, or f(x) = sin(x).Depending on the quality of the circuit in use,the output may be a piecewise linear approxi-mation to the desired function.

fuse A device that is used to protect an elec-trical device or circuit from overloading. Whena loaded current exceeds a specified safe level,a fuse heats itself and melts to open the circuit.Some fuses consist of a wire enclosed in a smallglass or ceramic cartridge with metal terminals.


Ggain The ratio of the output variable of adevice to the input variable. For an electronicamplifier, it is the ratio of output voltage to theinput voltage. The gain is equivalent to the trans-fer function for passive circuits.

gain–bandwidth product A figure of meritthat determines the useful frequency range of atransistor or amplifier in general. It is the mid-band gain times the bandwidth. For transistors,it is sometimes expressed as√

availablepowergain × (bandwidth).

So, a high–gain, low–bandwidth amplifier isequivalent to a low–gain, wide–band amplifierby this algorithm.

gain, inverse In most feedback systems, theinverse of the total gain is approximately equalto the transfer function of the feedback network.See alsofeedback.

Galvanic cell It is believed that Luigi Galvanidiscovered that when two dissimilar metals arecontacted by a moist substance, a direct currentwill flow through the metals. The early elec-trochemical cell designed using this principle iscalled aGalvanic cell.

galvanometer An instrument used to deter-mine the strength and direction of an electriccurrent. It operates based on the fact that a mag-netic field is created by the current in accordancewith Ampere’s circuit law. This field interactswith the field of a permanent magnet, causingthe coil of the permanent magnet to deflect. Themagnitude and direction of the deflection is thenrelated to the magnitude and direction of the cur-rent in the coil.

galvanometer, ballistic A moving coil gal-vanometer used to measure charge by detectinga burst of current passing through the movingcoil. When an electric burst of current passes

through the coil, the initial maximum deflectionof the moving coil of the galvanometer is pro-portional to the total charge passed. The periodof the movement of the coil should be longerthan the duration of the transient current to bemeasured.See alsogalvanometer, moving coil.

galvanometer constant A factor by whicha galvanometer reading must be multiplied toobtain the current in a standard unit.

galvanometer, d’Arsonval Instrumentwhere the current carrying the coil is free to ro-tate in the magnetic field of a large permanentmagnet. The deflection of the coil is related tothe current it carries.

galvanometer, Einthoven Also known asstring galvanometer.A galvanometer that con-sists of a single conductive filament stretchedtightly between the poles of a powerful magnet.The current causes deflection of the filament.The deflection, which is observed through a mi-croscope, is proportional to the current strength.

galvanometer, Helmholtz A galvanome-ter that uses a magnetic field generated by aHelmholtz coil. The generated magnetic fieldis relatively uniform. A Helmholtz coil consistsof two coils. The two coils are identical and thedistance between the coils is as long as the radiusof the coil. The overlapped fields of the two coilscause a uniform magnetic field. The magneticdensity fluxB near the center of a Helmholtz

Helmholtz coil.


coil of radiusr is

B =25µ0I

8√

5r

(1 +O

(x4))

,

whereµ0 is the permeability andI is the inducedcurrent andx is the distance from the centeraround the axes of a Helmholtz coil.

galvanometer, integrating A galvanometerdesigned to measure very slow changes of theelectric flux generated in a coil in an electricfield.

galvanometer, moving coil A galvanometerthat measures a current passing through a lightcoil of many turns suspended or pivoted with aspiral spring in a fixed magnetic field producedby a magnet.

Moving coil galvanometer.

galvanometer, sine A galvanometer that hasa short magnetic needle suspended between twoHelmholtz coils. The current in the coil deflectsboth the coil and the scale. As a result the needlekeeps pointing zero scale when the current isloaded. The sine of the angle of deflection of coil(scale) is proportional to the current strength.

galvanometer, tangent A galvanometer thathas a short magnetic needle horizontally sus-pended at the center of a vertical coil. The cur-rent in the coil deflects the needle. The tangentof the angle of deflection is proportional to thestrength of the current.

galvanometer, vibrational A galvanometerthat measures an alternating current. The naturaloscillation frequency of the movable element of

the galvanometer is made equal to the frequencyof the current.

Galvini’s experiment The demonstrationthat electrical charge causes a muscle to con-tract.

gamma cameras A device for producingan image using gamma radiation, usually usedto locate radioactive substances within a livingbody.

gamma ray detector A device for detectingthe presence of a photon of gamma radiation.Such devices are used in nuclear medicine, radi-ation therapy, high-energy physics, and astron-omy.

gamma-ray spectrometry The measure-ment of the energy of specific photons of gammaradiation. Such experiments can yield valuableinformation about the source of the gamma ra-diation.

gate In digital electronics, a gate is anelectronic circuit that performs an elementaryBoolean function. The device is constructedsuch that it accepts one or more input voltageswhich are assumed to represent Boolean vari-ables given a predefined logic convention. Thegate then provides an output voltage (typicallyjust one) which represents the result of the oper-ation it was designed to implement. Examplesinclude theAND, OR, andEOR gates.

A gate is a combinational circuit, meaningthat the output state is the functional result ofthe present state of the input variables.Comparewith flip-flop, whose output depends on the his-tory of the input variables (seesequential logic).

The internal electronics of the device can bebased on one of many different schemes: TTL(transistor – transistor logic), RTL (resistor –transistor logic), ECL (emitter – coupled logic),CMOS (complementary metal oxide semicon-ductor), and so on.

Gauss This is the unit of magnetic flux den-sity and is equal to 104 webers per square meter.

Gaussian channel A Gaussian channelrepresents a communication channel model


wherein the transmitted signal is corrupted anddistorted only by additive Gaussian noise, atime-invariant amplitude attenuation and a time-invariant phase offset. As such, a Gaussianchannel possesses infinite bandwidth. TheGaussian channel often represents an adequatechannel model of a bandlimited channel for alimited time duration within the finite band-width over which the information-bearing signalis transmitted.

Gauss’s law (electric) Relationship betweenthe total electric flux through a closed surfaceand the charge within that surface. In integralform, it is

∫sE.da = Q/ε0, whereQ is the total

charge enclosed by the surfaces. If the chargeis distributed over a volume,ρ being the chargedensity, then by application of the divergence.

generator, current A device for generatingconstant currents. It is designed to make theoutput current independent of the impedance be-tween the output terminals.

generator, electrostatic A device for gen-erating high-voltage electrical charges. A vander Graaff generator is a kind of electrostaticgenerator.

generator, Hartman A device used to pro-duce powerful ultrasonic sound waves. Shockwaves induced by a supersonic gas jet at theedges of a nozzle resonate with the opening of asmall cylindrical pipe placed opposite the noz-zle.

generator, heteropolar An electric gener-ator whose conductors move through magneticfields of opposite direction successively. Usu-ally, a heteropolar generator has a conductor ro-tating in a non-uniform magnetic field. Mostgenerators used are heteropolar generators.

generator, impulsive Generator used to pro-duce pulses of high voltage and short duration.Usually achieved via parallel charging and se-ries discharging of capacitors.

generator, induction An AC generator thatconsists of an induction motor connected to anAC source. Most induction generators in com-

mon use are three-phase AC generators. The ro-tor of an induction generator is driven mechani-cally above the synchronous speed correspond-ing to the frequency of AC from the source. Themotor generates AC energy and sends the energyback to the source at the frequency of the source.

generator, reaction, AC A kind of AC gen-erator. Its rotor does not have any coils but hasa salient rotor. It is used for low speed rota-tion usage, e.g., hydroelectric power generation.The salient pole is designed to make a sinusoidalchange of the magnetic flux density. It requiresan AC current supply from another AC genera-tor. Therefore, it is connected in parallel withone or more synchronous generators. It is alsocalled asalient-pole synchronous generator.

A salient-pole synchronous generator.

generators, acoustic Transducers that con-vert electrical, mechanical or other forms of en-ergy into sound; they can be either constant vol-ume velocity or constant pressure. Both ver-sions can be impedance-type or mobility-type.The concept of acoustical generators is of im-portance in acoustical circuits.

generators, electrical A device for generat-ing voltages and currents. It usually converts aparticular mechanical power to electrical powerto generate voltages and currents.

generators, tandem A modified van derGraaff generator that is used as an elementaryparticle generator. It connects two generators ina series and a high voltage (positively charged)electrode between the pair of generators. Neg-


ative ions are accelerated from ground potentialtoward the electrode. The negative ions pass bythe electrode and then pass through low pressuregas to remove their surplus electrons from theions. The ions become positive and acceleratedagain to ground potential by the electrode in thesame direction. The ions are twice acceleratedin a tandem generator.

generator, van de Graaff Machine for gen-erating high voltages at low currents via electro-static charge separation. It consists of an insu-lating belt that is revolved at a high speed overseparated rollers, one of which is usually insu-lating while the other is neutral metal. Adjacentto the rollers are metal combs, one of whichis connected to the ground, while the other isconnected to the inside of the generator elec-trical conducting dome. As the belt rotates,charge separation occurs between the insulatingroller and the belt. The roller attracts oppositelycharged ions from the comb via the plasma cre-ated in the air due to the high voltage betweenthe comb and the roller. These are interceptedby the belt and carried away to the other roller,the charges being picked up by the other comband distributed over the generator dome. In thisway large potential differences can be achievedbetween the dome and the earth.

generic pump A device using various physi-cal principles to convert mechanical energy intofluid energy, usually energy of motion of thefluid. These devices typically work either bycompression or suction.

geophones A transducer used in seismicwork that responds to motion of the ground ata location on or below the surface of the earth.It is used in exploration seismology to analyzeacoustic waves reflected from different rock lay-ers in the earth’s subsurface.

germanium A group IV semiconducting el-ement. In its natural crystalline form, it has adiamond-like tetrahedral structure with covalentbonds between neighbors. It is an excellent can-didate for doping, thus it is technologically im-portant in the construction of semiconductingelectronic devices.See alsodoping; diode junc-tion.

glow discharge A device used for the pro-duction of charge particles or monochromaticlight. In electronic circuits, it can be used as avoltage reference.

The glow discharge consists of a dischargetube, two exposed conductors in the tube, and ameans of supplying a moderately high voltage tothe conductors. The discharge tube is an evac-uated vessel that is partially filled with the in-tended gas. Operating pressures are of the order100 microns. The conductors can be arrangedin many different arrangements. The simplestis the parallel plate geometry, as shown in thefollowing figure.

A basic arrangement for a glow discharge.

Considering only a DC voltage source, theconductor that is biased positively is called theanode. The negatively biased electrode is thecathode. The operation of the discharge pro-duces many separated electrons and ions: aplasma.The electrons and negative ions are at-tracted toward the anode, while the positive ionsare accelerated toward the cathode.

A typical I–V characteristic curve for theglow discharge is shown below. Normal operat-ing conditions are 10s to 100s of milliamperes ofcurrent and 10s to 100s of volts. The actual val-ues involved depend on many parameters, suchas the type of gas and electrode material as wellas gas pressure. Note that there is a region forwhich the voltage is constant for many differentcurrents. This behavior can be exploited in theconstruction of a voltage regulator. A ballast re-sistor is shown in the figure below to limit thecurrent drawn by the glow discharge.

The discharge is maintained from two mainprocesses:


A typical I–V curve for a glow discharge in arbitrary

units.

1. Positively charged ions colliding with thecathode will eject secondary electrons from thesurface. The yield of electrons depends on theaccelerating potential and the type of cathodematerial. These secondary electrons are accel-erated toward the anode.

The collisional production of secondary electrons at

the cathode surface.

2. As electrons are accelerated toward the an-ode they, with some degree of probability, willcollide with the neutral background gas atoms.If the electron’s energy is sufficient, then theatom can be ionized. This produces a positivelycharged ion that is accelerated toward the cath-ode. It also produces another free electron that,in turn, can ionize more atoms. If the electroncollisional energy is insufficient, the atom willonly become excited, which in turn emits pho-tons. This gives the discharge its characteristicglow.

Ionization of the background atom by electron impact.

If the secondary electron yield at the cathodesurface and the collisional probability of pro-ducing positive ions are sufficient, then the dis-charge will continue to “run” and current willflow. Otherwise, the discharge will “go out”and will not draw any current. Normally, thedischarge will not start by itself. A means ofproducing a few separated charge particles isrequired. Commonly, the discharge process isstarted simply from background radiation, e.g.,cosmic rays, ionizing a few of the atoms.

Goldman model This model relates potentialdifferences across a membrane in the presenceof more than one ion. The ions are assumed tomove independently of one another. The elec-tric field within the membrane is assumed to beconstant.

Gouy method A technique for determin-ing magnetic susceptibility that is based on themeasurement of the force exerted on a sampleby an inhomogeneous magnetic field. Whenany substance is placed in a magnetic field, thefield produced within the sample either is greaterthan or less than the applied field, depending onwhether the material is paramagnetic or diamag-netic. The method usually involves measuringthe weight of a substance in the presence andabsence of the magnetic field, making it one ofthe least expensive techniques for determiningmagnetic susceptibility.

gramophone An instrument for reproduc-ing acoustic signals, such as voice or music, bytransmission of vibrations from a stylus that is incontact with a groove on a flat disk. Also knownas aphonograph.


grating Any arrangement of diffracting bod-ies that imposes on an incident wave a periodicvariation of amplitude or phase or both. It is adevice for producing spectra by diffraction andmeasuring the wavelengths of incident waves.One of the simplest kinds of grating consistsof a number of identical equidistant slit aper-tures in an opaque screen, an idealization use-ful for mathematical analysis. Practical grat-ings may consist of equidistant diamond rulingson a plate or mirror or a replica made of theserulings. Making narrow slits the elements of agrating allows them to act as sources that ra-diate uniformly. Most of the important opticalcharacteristics of diffraction gratings, such asresolving power and dispersion, are concernedwith the interference effects from sources aris-ing from each of the corresponding elements ofthe grating, and are associated with the period-icity of the diffracting elements rather than withtheir individual shape. Diffracted light from agrating may be either reflected or transmitted,and produces maxima of illumination or spec-tral lines according to the equation

d (sin ı+ sin θ) = mλ ,

where d is the distance between correspond-ing elements= 1/N , whereN is the numberof grating lines per unit length of grating,ı isthe angle of incidence,θ is the direction of thediffracted maximum with respect to the normalcorresponding to orderm (m = 0 for central im-age), andλ is the wavelength. The concave orRowland grating removes the necessity of achro-matic collimation and telescopic objectives.Seealsoblazing of grating, echelette, echelon, grat-ing, Rowland.

grating, holographic The generation of agrating on a blank using the holographic pro-cess, in which a series of interference fringesare formed from the intersection of two coher-ent beams of light, corresponding to the groovesof the grating, are recorded in a photosensi-tive material. The spacing of these fringes isdetermined by the angle of intersection of thebeams and by the wavelength of light. Subse-quent chemical treatment, using the solubilitydependence of the photoresist that in a suitabledeveloper changes with exposure to light, formsa modulated profile on the surface of the blank

by selective dissolution. Since the grooves aredetermined by the interference of light, a gratingmade this way is free from the random and peri-odic errors present in gratings made with rulingengines. The idea of making gratings this waywas considered by Michelson as early as 1915,but it was not until 1960 that Burch made grat-ings this way. Gratings suitable for general spec-troscopic use did not appear until 1967, with theadvent of high power lasers, coinciding with thepractical realization of holography that was alsomade possible with the laser, and which was verymuch in vogue at that time, so the technique ac-quired the nameholographic grating,althoughit is more correct to speak of them asinterferencegratings. The most serious shortcoming of thistechnique is that there is little control over thegroove profile, usually producing gratings witha sinusoidal or a quasi-sinusoidal groove pro-file. It is also very difficult to control the blazeangle using this process. An advantage of holo-graphic gratings is that they can be made witha periodicity smaller than with a ruling engine.The periodicity in a holographic grating is lim-ited to 1/2 the exposing wavelength; e.g., usinga He-Cd laser at 442 nm, this limit is.221 mi-crons. Selecting holographic or ruled gratingsdepends on the requirements for grating period,blaze angle, and grating depth.

grating, radial A nonspectroscopic gratingused to measure angular displacement in whichthe wires or rods are set radially within a circu-lar structure, like the spokes of a wheel. Usu-ally radial gratings take the form of annulus be-tween 10 and 20 mm wide on a wider annularblank. Conventional circular dividing enginescan make such gratings with an accuracy of±1seconds of arc. Further reductions of resid-ual error are possible with multiple printing ofthe whole grating at all orientations. These areavailable with up to 43,200 lines or one line per0.5 min of arc.

grating, Rowland A concave grating. Theseratings are ruled on concave spherical mirrors ofmetal instead of plane surfaces. It diffracts andfocuses the light at the same time and eliminatesthe necessity of using lenses, thereby eliminat-ing chromatic aberration. It also has the ad-vantage of being used for regions of the spec-


trum that are not transmitted by glass lenses,e.g., the ultraviolet. A mathematical treatmentof the concave grating shows that ifR is the ra-dius of curvature of the spherical surface, then acircle of diameterR can be drawn tangent to thegrating at its mid-point which gives the locusof points where the spectrum is in focus, pro-vided the source slit is also on this circle. Mostmountings for concave gratings are based on thisRowland circle condition for focus.

gratings, crossed One of the techniques usedfor projecting a 1-D or 2-D grid of dots or lines.A series of dots is produced when the output ofa collimated laser is passed through a diffractiongrating. Using crossed diffraction gratings, theresult will be a 2-D grid of dots. If a laser linegenerator is used, the result on passing the outputthrough a single grating is a series of lines. Twosuch arrangements at right angles or a laser crossgenerator and crossed diffraction gratings willresult in a 2-D grid of lines. The spread of the in-dividual spots or lines is inversely related to thediffraction grating pitch. However, the bright-ness of the dots or lines may not be even closeto uniform since the intensity decreases with theorder of the diffracted beam. Lower densitygratings (fewer lines/mm) will result in a largernumber of more uniformly spaced higher orderspots or lines of more nearly equal brightness,but they will be dimmer and more closely spaced(not deflected as much). Crossed gratings canbe used to produceMoiré fringes. If the twogratings are identical transmission gratings thatconsist of alternating opaque and transparent el-ements, then when these gratings are placed faceto face with a small inclination angle, no lightwill be transmitted where the opaque parts ofone grating fall on top of the transparent partsof the other. This has the appearance of a set ofdark fringes called Moiré fringes. If one gratingis stationary, and the other moved perpendicu-lar to its rulings, the Moiré fringes will crossa point each time the gratings advance by thegrating interval. This allows measurement ofthe movement of the grating, by measuring theMoiré fringes, which magnify the movement ofthe gratings inversely as the angle between therulings. This has useful applications in automa-tion, where one grating may be attached to anobject being processed and is moved across a

stationary grating for controlled displacementof the object.

gratings, in series Gratings may be placedconsecutively one after the other in series. Oneadvantage of this set-up is the easy adjustmenteven in theIR region. The law of diffractionsuggests that by combining two grating orders,the beams with appropriate order numbers havethe same entrance and exit angles. Thus the se-ries set-up is invariant against tilt of the incidentbeams as long as both gratings are parallel.

grating spectrograph An optical instrumentthat uses a grating to diffract light into specificwavelengths so as to form the spectrum of alight source and record it on film or with pho-todetectors situated in the spectrum at positionspossibly corresponding to the lines of elementsor compounds whose presence is of interest.Early spectrometers, such as those developedby Fraunhofer and others, used Newton’s dis-covery of the dispersion of light by a prism in-stead of a diffraction grating. A concave gratingrequires no other means to form a sharp imageof the slit on the film, but a plane grating or aprism requires additional achromatic lenses orconcave mirrors for image-forming in additionto the diffracting element.

gravitational waves A propagating gravita-tional field, a distortion of spacetime predictedby general relativity, that is produced by somechange in the distribution of matter. It travels atthe speed of light and alternately produces out-of-phase contractions and elongations of spacealong two axes perpendicular to the propagationdirection. The strength of the field is character-ized by its strain, the fractional changes in thelengths along the two axes. Also know asgrav-itational radiation.

grazing incidence Light that strikes a surfaceat an angle nearly perpendicular to the normal.Similarly, grazing emergenceoccurs when anemergent ray is at an angle nearly perpendicu-lar to the normal of the emergent surface of amedium.

ground The point in a circuit that by def-inition has zero potential. In practice this is


achieved by connecting the circuit directly tothe earth via a conductor. This serves to cre-ate a fixed baseline potential from which otherpotentials can be measured.

ground glass Glass that has been frosted bygrinding or etching. It diffuses light by scatter-ing in directions close to the incoming beam; atlarger angles out from this direction, light fallsoff rapidly.

ground wave In transmission of radio waves,the path traveled by wave along the earth’s sur-face. It can interfere with sky waves to produceselective fading of radio signals.

group velocity The concept of group veloc-ity is used to describe the movement of acousticwaves in a moving fluid. Acoustic waves can,in their simplest form, be represented by planewaves, the direction of propagation~nperpendic-ular to the wavefronts, that move with the speedof soundc. The velocity of the disturbance in astagnant fluid isc~n. If the speed of the ambientfluid is ~v0, the velocity of the wave disturbanceregistered by an observer at rest, called the groupvelocity~vgr, is the vector sum~vgr = c~n + ~v0.Also describes the velocity of the envelope of

a group of waves having slightly different fre-quencies and phase velocities. Also known as(acoustic) modulation.

Grove cell A primary cell with a platinumelectrode in a nitric (or sulfuric) acid electrolyteinside a porous cup. Outside this cup is a zincelectrode in a sulfuric acid electrolyte. It wasinvented by William Grove.

guard wires A high-conductive connectionto a large conducting body. It is usually con-nected to electrical equipment for safety and cir-cuit completion.

Gunn effect Seediode, Gunn.

gun, sound from The sound wave generatedfrom the discharge of a gun, consisting of threeseparate signals. The first signal forms the en-velope of the waves emitted by a projectile thatmoves at a speed higher than the speed of sound,and it is called theheadorbow wave.This wavereaches the observer first, and is perceived asa sharp crack. The second wave is caused bythe explosion of the shell, and the third one bythe expanding gunpowder gases traveling at thespeed of sound.


Hhalf–adder Forms the basis of multi–digitaddition or subtraction of base–2 numbers. Itis the smallest operational block that performselementary binary addition or subtraction on asingle digit. The half–adder has two inputsXandY for the addend and augend. Normally acombinational circuit, it provides sum,S, andcarry,C, outputs based on the immediate inputconditions. The operation of the half–adder isdefined by

S = (X ⊗ Y )⊕ (X ⊗ Y )

≡ (X ⊕ Y )⊕ (X ⊗ Y )

C = X ⊗ Y ,

and is illustrated in the accompanying truth ta-ble.

Truth table of thehalf–adder

X Y S C

0 0 0 00 1 1 01 0 1 01 1 0 1

To perform basic addition of theith digitof two binary numbers (seedigital arithmetic),consideration must be given to the carry (or bor-row) of the next lower significant digit. Thefull–adderhas an extra input,Ci−1 in additionto the addend and augend,Xi andYi respec-tively. The full–adder is constructed from twohalf–adders; the first addsXi andYi and thesecond adds this subtotal withCi−1. A truth ta-ble illustrating the operation of the full–adder isalso presented.

half-duplexing Also known astwo way alter-nate. Data can be transmitted in network com-munication in either direction but only in one

Truth table of the full–adder

Ci−1 Xi Yi Si Ci

0 0 0 0 00 0 1 1 00 1 0 1 00 1 1 0 11 0 0 1 01 0 1 0 11 1 0 0 11 1 1 1 1

direction at any given time. This is done by useof a circuit to provide transmission alternatelyin either direction.

half–power bandwidth The frequency dif-ference between the two points in an amplifier’sfrequency response for which the power gainhas finally dropped to one half the center fre-quencyf0 power gain. This is equivalent to thepoints where the relative gain has dropped by3 dB. These two pointsfa, fb define the 3 dBbandwidth or pass-band(fa − fb).

half–power frequency The frequency atwhich an amplifier’s power gain has dropped byone half the center frequency power gain (i.e.,by 3 dB).See alsohalf–power bandwidth.

half tone A musical interval with a frequencyratio of 21/12 = 1.0595. In the theory of equaltemperament, any two half tones approximate amajor interval, any four a major third, any five afourth, any seven a fifth, any nine a sixth and anyeleven a seventh. Any twelve half tones forman octave. In just intonation (characterized bymathematically exact intervals) with referenceto the major key ofC, the frequencies for thekeysD,E, F,G,A,B andC are tuned to 9/8(major interval), 5/4 (major third), 4/3 (fourth),3/2 (fifth), 5/3 (sixth), 15/8 (seventh) and 2 (oc-tave) times the frequency of the firstC. Alsocalled half step or semitone.See alsofrequencyband and octave.

half-wave plate One of the simplest devicesfor production and detection of circularly polar-ized light is the quarter-wave plate, which in-troduces a 90 phase shift between the ordinaryand extraordinary vibrations. With the quarter


wave plate oriented at 45 with the plane of po-larization of the incident light, circularly polar-ized light is produced. A half-wave plate re-flects theE-vector about its axis. Light passingthrough a quarter-wave plate twice is equivalentto passing once through a half-wave plate. Likethe quarter-wave plate, these plates are usuallymade of thin sheets of mica or quartz cut parallelto the optic axis. The thickness is adjusted, de-pending on the wavelength (usually sodiumD isselected) to introduce a phase difference of 180

between the ordinary and extraordinary vibra-tions. Plane polarized light passing through theplate has its plane of polarization rotated through2θ whereθ is the angle between the axis and theincident vibrations. In certain instruments, suchas the Laurent Saccharimeter, it is desirable tocompare two adjacent fields of light polarized ata certain angle with respect to each other; thenhalf the field is covered with a half-wave plate,and the analyzer is rotated until two half fieldsare equally bright or dark.

half-width The full width at half maximum(FWHM) or half-width expresses the extent ofa function,y = f(x), given by the differencebetween the two extreme values,x2−x1, of theindependent variablex at which the dependentvariabley is equal to half of its maximum value,ym/2. For example, the half-width of the errorfunction integrande − y2 is 1.67. FWHM isfrequently applied to spectral width of sourcesused for optical communications. When appliedto pulse width where the independent variableis time, full duration at half maximum (FDHM)may be used.

haloes (1) In meteorology, the short lived andsometimes faintly hued circles or arcs that areseen to surround a light source viewed throughfog or light clouds. The theory attributingtheir formation to ice crystals was suggested bythe 17th century philosopher Descartes. Whitehaloes are formed by reflection from ice crys-tals, colored haloes from refraction. The size ofscattering ice determines the size of the ring.

(2) The ring surrounding a photographic im-age of a bright source and resulting from thescattering of light in random directions.

(3) The broad rings that appear as a result ofthe diffraction of monoenergetic beams of elec-trons or X-rays from crystalline powder.

hamming distance The number of bit posi-tions by which two binary codewords differ. Theerror detecting and error correcting properties ofa code depend on this distance.

hardware Physical devices, generally forthe interfacing of central processing units to thephysical world. Each piece of hardware willperform some specific and unique task. Thecomputer’s hardware will handle chores such asinputing and encoding data, by way of a key-board or scanner, for example. Output hardwareinclude printers, display devices, and roboticequipment.

harmonic analyzer Electronic device thatmeasures the frequencies and relative ampli-tudes of harmonic components in a complexwave. Also known asharmonic wave analyzer.

harmonic motion Motion in that the dis-placement of particles repeats itself in equal in-tervals of time, also calledperiodic motion.Pe-riodic motion can be described in terms of sinesand cosines. Since the termharmonic is usedfor expressions containing these functions, pe-riodic motion is also calledharmonic motion.Thus, harmonic motion along a line is given bythe functionx = a cos(kt + θ), wheret is thetime parameter, anda, k and θ are constants.Also known asharmonic vibrationor simpleharmonic motion.When frictional forces thatdissipate energy are present, the system will ex-ecute damped harmonic motion.

harmonics A series of sounds which havefrequencies that are integral multiples of somefundamental frequency.See alsofrequency, fun-damental.

harp, sound from Sound produced by pluck-ing strings spanned on a triangular frame. Theresulting vibration is a combination of severalmodes of vibration. When the string is pluckedat the center, the resulting vibration will consistof fundamentalandodd harmonics.When thestring is plucked at a point other than its center,


the spectrum of the constituent modes changes.For example, plucking the string at 1/4 of thedistance from the end suppresses the4th har-monic, etc. The string length determines thewavelength of the fundamental harmonic of thesound wave. The modern harp is equipped witha pedal mechanism that increases the span oftones that can be produced by the harp so that itcan exceed the power of keyboard instruments.

hearing The general perception and the spe-cific response to acoustic stimuli. Two ap-proaches are used to study and describe hearing.

1. The goal of auditory physiology is to un-derstand the structure, organization, and func-tioning of various components of the auditorysystem at different stages of processing of audi-tory signals.

2. Auditory psychophysics and psychoacous-tics deal with the way humans sense and perceivesound. Hearing involves the elements of inten-sity, frequency, pitch (sound rich in harmonics isperceived as having the pitch of the fundamen-tal frequency independent of the ratio of energycarried by the fundamental frequency and theharmonics), localization of sound, and the per-ception of complex spectra.

hearing, abnormal Impaired hearing is mostcommonly identified in pure tone audiometry byevaluating the auditory response of the individ-ual to sinusoidal signals at octave intervals from250 to 8,000 Hz sounded in a quiet room, usingthe audiogram. The impedance, or its inversethe admittance, of the middle ear is determinedthrough immittance measurements that are use-ful in establishing the site of the lesion withinthe auditory system. An important aspect ofhearing impairment is degradation in speech in-telligibility, evaluated by speech audiometry, todetermine the speech recognition threshold andword recognition score. The objectives of thetests are to establish the extent of hearing im-pairment and its cause (the site of the lesion).

hearing aids Miniature, portable prostheticdevices for individuals with impaired hearing.Conventional electroacoustic hearing aids arehead-worn sound amplifiers that consist of a mi-crophone, audio amplifier, earphone, and bat-tery. Assistive listening devices improve the

speech-to-noise ratio of conventional hearingaids, responsible for poor speech recognition,by moving the detached microphone closer tothe sound source. The microphone output signalcan be delivered to the amplifier either by wire orby means of radio frequency or infrared signals.Vibrotactile devices convert sound into an elec-trical signal to deliver it to the skin of the individ-ual as a pattern of mechanical vibrations throughvibrating mechanical contacts. Cochlear im-plants convert acoustical signals into electricalsignals, process these, and deliver them to thenerve fibers in the inner ear by electrodes in-serted surgically into the inner ear.

hearing loss Impaired general perception orspecific response to acoustic stimuli. Hearingloss can be congenital or caused by external fac-tors or illness. Approximately 9% of the pop-ulation in the United States is affected by hear-ing impairment of different levels of severity.Among the population above the age of 65, thispercentage is between 30 and 40%. The focus ofthe discipline of audiology is the diagnosis andrehabilitation of individuals affected by hearingloss. The aim of the diagnosis is to establishthe extent of hearing loss and its etiology usinga battery of tests. Depending on the site of thelesion, hearing impairment can be classified asconductive hearing loss, retrocochlear(involv-ing the lesion of the auditory nerve or neuralpathways in the brain), ormiddle ear pathology,as well assensorineural hearing loss(cochlearsite lesion).See alsohearing, abnormal; hearingaids.

heart-lung machine, artificial A deviceconsisting of a pump and an oxygenator usedto oxygenate the blood and to pump it throughthe body. Usually used in heart or lung surgery.

heat exchangers There are several types ofheat exchangers used at low temperatures. Ingeneral, heat exchangers consist of a fluid atone temperature,T1, and another fluid or bodyat a different temperature,T2. In most situa-tions, heat removal is the primary concern, soT1 > T2. A counterflow heat exchanger con-sists of two tubes with fluid flowing in oppositedirections. Usually made of concentric tubesof stainless steel, brass or a copper-nickel alloy,


this design allows heat flow across the wall ofthe inner tube. The temperature varies contin-uously along the length of the tube; therefore,this design is often called acontinuous counter-flow heat exchanger.At low temperatures, theKapitza boundary resistance increases, makingthe use of simple concentric capillary tubes in-effective. A second type of heat exchanger,astep heat exchanger,is used to increase the sur-face area at low temperatures. This exchangerhas a metal sinter to provide the necessary sur-face area. Often made of silver or copper pow-der, the sintered heat exchangers can providemany 10s of square meters of surface area. Ifthe heat needs to be exchanged between a liquid(e.g., helium-3) and a solid, then sintered metalsprovide the best exchangers available.See alsoKapitza boundary resistance.

heating effect of current This is also calledJoule heating. When a currentI is flowingthrough a resistor with resistanceR, the amountof energy loss per unit time (power loss) isI2R.

heat switches A heat switch allows a thermalconnection between two regions to be openedand closed upon demand by one of several tech-niques. Gas heat switches use the thermal con-ductivity of a gas, often helium or hydrogen, asthe mechanism to open and close the switch. Aclosed volume is connected to the two regionsof interest, and gas is introduced to close theswitch, then removed to open it again. Helium-4 can be used, but at temperatures below 2.17 K,the helium-4 film on the inner surface of theclosed volume becomes superfluid. The super-fluid film will continue to conduct heat after thegas is removed, so care to remove all of the he-lium must be taken. This problem is not presentif hydrogen is used, but hydrogen’s flamma-bility is a concern. In very low temperaturecryostats, the remaining hydrogen can undergoortho-para conversion causing substantial heatrelease. (Ortho-hydrogen is a spin I=1 moleculethat converts into para-hydrogen, a spin 0 state,at low temperatures.) Helium-3, a rare isotopeof helium, is an ideal choice, as it does not be-come superfluid until reaching temperatures be-low 2.5 mK, and does not have the problemshydrogen does. At temperatures above approx-imately 1 K, physical contact can be used. In

this technique, a spring, screw, or motor forcesa piston, attached to the first region, into contactwith the second region. A common techniquefor temperatures below 1 K is to use a super-conducting heat switch to provide the thermalcontact. In the normal state, a superconductingmetal carries heat through its conduction elec-trons. In the superconducting state, the elec-trons form Cooper pairs and cease to carry heateffectively. As a result, the thermal conductiv-ity in the normal state is 1000 times (or more)that in the superconducting state. In practice,the switch is closed by producing a magneticfield which drives the heat switch into the nor-mal phase, closing the switch. Foils of Al, Sn,Nb, and Ta are the most common metals usedfor superconducting heat switches due to theirsuperconducting transition temperatures, criti-cal magnetic fields, and ease of fabrication.

HeI A term synonymous with normal liq-uid 4He (in contrast to HeII).See alsohelium-4,normal; helium-4, superfluid.

HeII A term synonymous with the superfluidphase of liquid4He. See alsohelium-4, super-fluid.

helimagnetism A property possessed bysome metals, alloys, and salts of transition el-ements or rare earths in which the atomic mag-netic moments, at sufficiently low temperatures,are arranged in ferromagnetic planes, the direc-tion of the magnetism varying in a uniform wayfrom plane to plane.

helium-3/helium-4 mixtures Helium-3 andhelium-4 are miscible in all proportions at hightemperatures, but phase-separate at tempera-tures below 0.87 K. A schematic phase diagramis seen below.

Phase diagram for 3He/4He mixtures.


The superfluid transition temperature can beseen to decrease with increasing3He concentra-tion. Note that the3He dissolved in4He doesnot partake in the superfluidity of the surround-ing 4He. The3He-rich phase, located on thelower right of the figure is nearly pure, while the4He-rich phase contains some3He all the way toabsolute zero, seen in the figure by the non-zerox-intercept. Near 0 K, the dilute phase (4He-rich) has roughly 6.6%3He dissolved in it. Thisfinite solubility is due to the large binding energyof 3He atoms in4He, 2.8 K/atom (24 J/mol).The behavior of dilute solutions of3He in 4Heat low temperatures allows a cooling techniqueused in “dilution refrigerators” that can producesteady-state temperatures of 3–5 mK.

At temperatures below approximately 0.5 K,the liquid4He is deep in the superfluid phase andtherefore acts only as a “dense vacuum”. The4He is, under most circumstances, at thermalequilibrium due to its very high thermal conduc-tivity. The entropy and specific heat of the4Heis vastly smaller than that for the3He as well,so the3He atoms can ignore the4He atoms forthermal properties. The3He atoms do have tomove the4He atoms around in order to flow, sothe superfluid is said to increase the effectivemass of the3He atoms. The effective mass, m∗3,is about 2.4 times larger than the bare mass, m3.With this change to the3He atoms, the theory forinteractions between3He atoms and their col-lective behavior, Fermi liquid theory, providesan accurate description of the dilute gas of3Heatoms.

As mentioned above, the specific heat ofthe 4He is negligible compared to the3He, sothe specific heat of the dilute phase is constant(roughly 3/2 kB per atom as for an ideal gas)at moderate temperatures (0.25 K < T < 0.5 K)and varies linearly with temperature (as a Fermiliquid) at lower temperatures. At higher tem-peratures, the specific heat of the4He must beincluded for an accurate description.

The addition of3He atoms to a bath of super-fluid 4He changes the thermal flow propertiesby providing another path for heat conduction.At high temperatures, heat is carried by exci-tations of the superfluid, namely phonons androtons, and the3He atoms are carried along forthe ride. In addition, the thermal diffusion of the3He atoms and the heat flow through the normal

fluid contribute, particularly at lower tempera-tures. The viscosity is altered in a similar man-ner. The presence of3He decreases the meanfree path in the superfluid, thereby decreasingthe viscosity.

The enthalpy of the dilute phase (low3Heconcentration) is higher than that in the concen-trated (nearly pure3He) phase. In fact, for a6.6% solution, the enthalpy of the dilute solu-tion is roughly 8 times that in the concentratedsolution. Thus, if3He atoms diffuse into the di-lute phase from the concentrated phase, the con-centrated phase will cool by the enthalpy differ-ence per atom. This “evaporation” of the liquid3He in the concentrated phase to the “dense vac-uum” of the dilute phase is the mechanism usedin dilution refrigerators to cool down to severalmillikelvin. See alsorefrigerator, dilution.

helium-3, liquid Helium-3, the lighter stableisotope of helium, liquefies at 3.2 K atmosphericpressure. Between liquefaction and approxi-mately 0.1 K, liquid helium-3 behaves muchlike a dense classical gas. Thus, at tempera-tures above 1 K, the specific heat is constant, itsviscosity is also constant (roughly 25µPoise),and the speed of sound varies from 183 m/s atsaturated vapor pressure to 422 m/s at meltingpressure (34.4 bar).

At temperatures below 0.1 K, liquid helium-3 is well described by the Fermi liquid theory.This theory, described by Landau, replaces thebare3He atoms with quasiparticles that includethe effects of interactions and the quantum na-ture of3He. The specific heat is roughly linear,with the actual dependence found to be

CV /R = γT + ΓT 3 ln(T/Θc) ,

whereR is the gas constant, andγ,Γ, andΘc

are temperature-independent constants. Liquid3He has a specific heat much larger than typicalmetals in this temperature range. The viscosityfor 3He varies roughly asT−2 in this regime,reaching nearly 1 Poise, the viscosity of machineoil.

At very low temperatures, below 0.003 K, liq-uid helium-3 goes through a second order phasetransition into one of several superfluid phases.Due to the nuclear magnetic moment of3He(and hence its fermionic nature), superfluidity


in helium-3 can be described by a somewhataltered version of BCS theory, originally de-veloped to describe low temperature supercon-ductors. (Seehelium-3, superfluid.)See alsohelium-4, liquid; helium-4, superfluid; helium-3, superfluid.

helium-3, melting curve Solid 3He melts atpressures of 29.3 bars (at 0.315 K) to 34.4 bars(below 0.001 K). The coexistence curve for liq-uid and solid3He, the melting curve, is knownaccurately below the minimum at 0.315 K andcan, by measuring the melting pressure accu-rately, be used as a thermometer. The meltingcurve is shown in the figure below.

Heat Capacity of liquid 4He. The Lambda phe-

nomenon.

There are two major features of the melt-ing curve worth noting: first, the melting curvenever reaches atmospheric pressure —3He isone of two substances that do not solidify uponcooling at atmospheric pressure (the other be-ing 4He). The second feature is the pronouncedminimum in the melting curve visible at 0.315 K.The Clausius-Clapeyron equation:

dpmelt

dT=

∆S∆V

where ∆S is the entropy difference betweenthe solid and liquid and∆V is the differencein molar volumes. At the temperatures of in-terest, the molar volumes are roughly constant

and∆V is positive. The entropy of solid3Heis Ss = R ln(2) to within 1% at temperaturesabove 10 mK while the entropy of the liquidis S` = 4.56RT whereR is the gas constant.Thus, at temperatures below 0.315 K, the en-tropy of the solid islarger than that of the liquid,and the slope of the melting curve is negative.This remarkable fact means a compression of asolid-liquid mixture of3He below 0.315 K willproducecooling. This is in marked contrast to,for example, the response of a gas to being com-pressed.See alsorefrigerator Pomeranchuk.

helium-3, solid Liquid helium-3 becomesa solid only at elevated pressures. The min-imum pressure for solidification is 2.93 MPa(29.3 bars) at a temperature of 0.316 K. Thesolid at pressures below roughly 10 MPa is abody-centered-cubic structure, while at higherpressures the hexagonal-close-packed phase isstable. At very high pressures and tempera-tures (above roughly a kbar and 20 K), a face-centered-cubic structure is observed.

At temperatures above approximately 1 mK,solid helium-3 is a nuclear paramagnet with anentropy of roughly NkB ln(2) due to the N disor-dered spins. This entropy is actually larger thanthat in liquid helium-3 at temperatures below 0.3K. The Clausius-Clapeyron equation thereforeindicates that the slope of the melting (liquid-solid coexistence) curve is negative at low tem-peratures; a mixture of liquid and solid helium-3will actually cool as the pressure increases. Ata temperature of 0.93 mK, the nuclear spins gothrough a strongly first-order phase transitioninto an antiferromagnetic phase. This phase,known as the U2D2 phase, does not consist ofalternating spins as in a simple antiferromag-net. Instead, the system forms planes of spinswhich are ferromagnetic – the structure con-sisting of two such planes with the spins up,then two planes with spins down, etc.: henceU2D2 represents up-two, down-two. The en-tropy of the U2D2 phase varies asT 3, analo-gous to phonons. (In any system that has ele-mentary excitations with a linear dispersion re-lation, the entropy is expected to depend cubi-cally on temperature.) The U2D2 phase is sta-ble at low magnetic fields, below approximately0.45 Tesla. Above this field, the system goesthrough another first-order phase transition into


a different antiferromagnet, the canted-normalantiferromagnet (CNAF). This is a phase withan antiferromagnetic signature and a ferromag-netic signature at right angles to one another.A one-dimensional representation of the U2D2and the CNAF phases is shown below, respec-tively:

↑↑↓↓↑↑↓↓ · · · · · ·

The primary interactions between3He atomsin the low temperature solid is due to atomicexchange processes. There are many possibleexchange configurations, several of which con-tribute to the properties of solid3He. The sim-plest exchange is simply nearest-neighbor ex-change, but this is suppressed by the steric hin-drance of nearby atoms. (Steric hindrancede-scribes the fact that the atoms are not point par-ticles and must move past one another.) Multi-ple spin exchange in rings is therefore favoredover two particle exchange. Three, four, andsix particle exchange are all relevant for solid3He. This exchange produces an effective spininteraction through the Pauli exclusion principlerequiring the total wavefunction to be antisym-metric.

Above the transition temperature, the mag-netic susceptibility per unit volume behaves ac-cording to a Curie-Weiss law,

χ = λ/(T −Θ)

whereλ is the Curie constant per unit volumeandΘ is the Weiss temperature. The susceptibil-ity changes discontinously at the (field driven)transition between U2D2 and CNAF, increasingby roughly a factor of six.See alsohelium-3,melting curve.

helium-3, superfluid Liquid helium-3 be-comes a superfluid at 2.5 mK (at 34.4 bar). Since3He has one unpaired neutron in the nucleus,it has a net spin angular momentum of 1/2 (inunits of) and is therefore a fermion. As such,it cannot have more than one atom in a givenquantum state. This is in marked contrast tothe case for liquid4He which forms a superfluidwhen a large number of atoms form a interact-ing Bose condensate in the ground state. Su-perfluidity in liquid 3He can be described by

the formation of Cooper pairs, similar to theprocess in superconductors. Liquid3He canbe described as a “nearly ferromagnetic” liq-uid. One3He atom will tend to polarize nearbyatoms through the nuclear dipolar interactions.This cloud of polarized atoms will then “prefer”another atom of the same spin as the first one(e.g., spin up) over a spin of the opposite state(spin down). This produces a net interaction be-tween these two like spins that is attractive. Atsufficiently low temperatures, these atoms formCooper pairs of spin 1. Since the wavefunc-tion for fermions must be antisymmetric, theorbital wave function must therefore be in anodd angular momentum state and is found to bein an ` = 1 state. (In contrast, electrons in lowtemperature superconductors form Cooper pairsof total spin 0 and angular momentum= 0.)Thus the spin state of the wavefunction can be| ↑↑>, | ↓↓>, or| ↑↓> +| ↓↑>. This internalstructure of the Cooper pair leads to complexbehavior. At high pressures and relatively hightemperatures, superfluid3He-A is stable. TheA-phase consists of equal spin pairs (those pairswith Sz = 1). This pairing leads to3He-A be-ing very anisotropic, having preferred orienta-tions in both spin-space and real-space. Thisanisotropy produces complex magnetic behav-ior and liquid-crystal-like behavior. At lowertemperatures and pressures, there is a first orderphase transition to the B-phase. The B-phaseconsists of all three possible spin states avail-able for the Cooper pairs. The B-phase also hascomplex magnetic and orbital behavior. As themagnetic field increases, the volume of phasespace in which the B phase is stable decreasesuntil, at approximately 0.6 T, the A phase be-comes the stable phase. A third phase, the A1

phase, exists in magnetic fields between the Aphase and normal3He. The A1 phase consists ofCooper pairs of only one spin orientation (e.g.,| ↑↑>). This phase occupies a very small re-gion of phase space which is not visible on thescale of the figure, but which grows linearly withmagnetic field to be roughly 0.5 mK in a 10 Tfield. The specific heat of superfluid helium-3 shows a finite discontinuity at the transitiontemperature, indicative of a second order phasetransition. There is a slight kink at the transitionbetween A and B phases.


Phase diagram for superfluid 3He.

The specific heat varies roughly as T3 withdeviations due to strong-coupling effects. Theviscosity falls rapidly below Tc, with the limit-ing value being difficult to determine due to theanisotropic nature of the superfluid phases. Un-like 4He, liquid3He has a nuclear magnetic mo-ment and a corresponding response to magneticprobes. Nuclear magnetic resonance (NMR)measurements first indicated the complex or-der present in superfluid helium-3. In typicalNMR experiments, the net spin,S, is tippedaway from its equilibrium direction along anexternal field,B0. The precession ofS is sub-sequently measured. The frequency for super-fluid 3He-A differs from that for normal3He,ω0, by a temperature dependent termΩA. If,instead of tipping the spin away from the mag-netic field, the magnitude of the magnetic fieldis changed suddenly, the spin parallel toB0 os-cillates at a frequency equal toΩA. This parallelresonance does not occur for normal NMR sys-tems. Similar frequency shifts are seen in the B-phase under certain circumstances. In addition,superfluid helium-3 also exhibits sound modes(first sound, second sound, etc.) similar to thosefound in superfluid4He. There are also texturesin superfluid3He similar to those found in (roomtemperature) liquid crystals.See alsohelium-4,superfluid; helium-3, liquid; helium-3, meltingcurve; superconductivity, BCS theory; Cooperpair; liquid crystal.

helium-4, liquid Helium-4 has a critical tem-perature of 5.2 K and a normal boiling point of

4.21 K. Due to its large zero-point motion,4Heremains a liquid under its own vapor pressure allthe way to absolute zero (0 K). In fact, it doesnot solidify until the pressure exceeds 25.4 bar.At temperatures well above 2.17 K, liquid4Hebehaves much like a dense, classical gas. Thespecific heat is therefore roughly 3/2R whereR is the ideal gas constant. (Of course,liquidhelium-4 is not exactly a non-interacting gas,so the comparison is only approximately true.)The velocity of sound is low in liquid4He, ap-proximately 230 m/s. The thermal conductiv-ity near its boiling point is quite poor (roughly10,000 times worse than copper at similar tem-peratures). Upon approaching 2.17 K, how-ever, liquid helium-4 goes through a dramaticchange of character. The specific heat divergeslogarithmically as a critical temperature, Tc '2.1768 K, is approached. This divergence isa manifestation of a phase transition to an ex-otic phase, thesuperfluidphase. The shape ofthe specific heat, shown below, gives the transi-tion temperature its common name, thelambdapoint. The properties of the superfluid phase arediscussed in detail in a separate entry.See alsohelium-4, superfluid; helium-4, solid; bubbles,suppression of; lambda point.

Helium-3 melting curve. The minimum is located at

0.316 K and 29.32 bars, and the zero temperature

pressure is 34.39 bars.


helium-4, solid Unlike all other substances,liquid helium does not solidify at atmosphericpressure atany temperature. Liquid helium-4does not solidify at absolute zero until the pres-sure approaches 2.5 MPa (25 bars) and solidifiesat its normal boiling point, 4.2 K, when the pres-sure is 14 MPa (140 bars). Under most tempera-tures and pressures for which the solid is stable,helium forms a hexagonal-close-packed (hcp)structure with an in-plane lattice constant of ap-proximately 3.5. At very high temperatures,the stable phase is face-centered cubic, while abody-centered cubic structure is stable over alimited range of temperatures and pressures.

helium-4, superfluid Liquid 4He goesthrough a second order phase transition at a tem-perature of 2.1768 K into a superfluid state. Be-low this temperature, the viscosity is vanish-ingly small, the thermal conductivity is quitelarge, and the helium will support several newsound-like phenomena. Since the4He atomsare bosons, it is tempting to claim the superflu-idity is due to Bose-Einstein condensation, butthe truth is somewhat more subtle than that. It istrue that the general concepts of Bose-Einsteincondensation can be used in discussing superflu-idity in 4He, but the interactions in liquid4Heare much too strong to claim the bosons arenon-interacting. Thus, the term “Bose-condensedhelium” is not, precisely speaking, an accuratedescription of superfluid helium-4.

Superfluid helium is often discussed in termsof the two-fluid model which assumes the he-lium is composed of two interpenetrating, non-interacting fluids – the normal component andthe superfluid component. The superfluid com-ponent consists of the atoms that occupy theground state and therefore have zero viscosityand carry no entropy. The normal fluid consistsof any excited atoms, and these atoms carry all ofthe entropy and have finite, non-zero viscosity.Note that this model is adescriptionof super-fluid helium and should not be taken too literally.

The thermodynamic and transport propertiesof superfluid4He differ drastically from thoseof the normal fluid. The specific heat divergeslogarithmically (see the figure in the entry forliquid helium-4) as it approaches the phase tran-sition. Superfluid helium-4 supports severalwave phenomena not found in other states of

matter. Ordinary acoustic waves are known as“first sound” in superfluid helium. The veloc-ity of first sound at low temperatures is roughly240 m/s and shows a cusp at the lambda point.First sound is attenuated at temperatures abovethe lambda transition by the same processes asoccur in normal fluids, viscosity and thermalconductivity. At the lambda temperature, thereis a discontinuity in the attenuation due to thephase transition. At approximately 1 K, thereis a large peak due to a process specific to su-perfluids, second viscosity.Second viscosityiscaused by irreversible processes in superfluid4He involving multi-phonon and phonon-rotonscattering. (Rotonsare excitations of the super-fluid not present in normal fluids.)

In addition to first sound, superfluid4He alsosupports temperatures waves,second sound.If asinusoidally varying amount of heat is dissipatedat one end of a container of superfluid helium, itwill propagate nearly unchanged to the other endas measured with a thermometer. Unlikefirstsound,which consists of variations of pressurewith density, second sound is due to the varia-tion of the temperature with entropy while thedensity remains constant. In confined geome-tries, superfluid4He can also supportthird andfourthsound. Third sound occurs in films of su-perfluid and is effectively a second sound wavewhich propagates on the surface of superfluidhelium. If superfluid helium is confined to smalltubes or tubes packed with powder, the normalfluid is held in place by viscous forces while thesuperfluid component is free to move. Thermalwaves in such a geometry, fourth sound, propa-gate at a speed between that for first and secondsound.

The viscosity of liquid helium below thelambda point is the physical property most com-monly associated with the “super” in superflu-idity. If a torus of liquid helium above Tλ isrotated slowly, then cooled below the transition,the fluid will continue to rotate even if the con-tainer is slowed to a stop. In fact, the superfluidhelium will flow ceaselesslyas long as a limitingvelocity, the critical velocity, is not surpassed. Ifsuperfluid helium is set into motion with speedsgreater than this velocity, the system becomesdissipative. In practice, there are several crit-ical velocities, each one signaling the onset ofdifferent physical phenomena. The values of


these critical velocities depend on the type ofexperiment performed and the geometry of theexperimental cell. Numerically, these velocitiescan vary between a few millimeters per secondand several tens of meters per second. In mostcases, the critical flow velocity is the speed atwhich the flow becomes turbulent and dissipa-tion begins. This turbulence is caused by someform of vortex creation in the superfluid.

The thermal conductivity of superfluid he-lium is also quite spectacular in its deviationfrom that of normal fluids. In normal helium-4, the thermal conductivity is roughly that ofa high-density, ideal gas. Upon cooling belowTλ, the thermal conductivity increases by six or-ders of magnitude or more. This incredibly highthermal conduction results in a lack of bubblesin superfluid helium. Another aspect of super-fluidity which affects heat flow is the fountaineffect — the superfluid component will flow to-wards a heat source and, in a suitable appara-tus, will produce a pressure gradient that can bequite large.See alsohelium-4, liquid; helium-4, solid; bubbles, suppression of; lambda point;fountain effect.

helium, liquid Seehelium-4, liquid; helium-3, liquid.

helium, liquid, cooling power of Liquid he-lium has a latent heat of vaporization of 20.90kJ/kg (2.6kJ/). This translates into the ability tocool 1 kg of copper from 300 K to 4 K using 31` liquid helium in the process. If the enthalpy ofthe cold helium gas (roughly 200 kJ/`) is used toprecool the copper, the amount of helium neces-sary drops to 0.8 per kilogram of copper. Thelatent heat of liquid helium is nearly an order ofmagnitude lower than that of liquid nitrogen, soit is commonplace to cool most cryostats to 77 Kto decrease the volume of liquid helium needed.

helium, liquid, transfer tube A transfer tubefor liquid helium must be a more complicateddevice than a liquid nitrogen transfer tube dueto the small latent heat of vaporization of liquidhelium (23.9 kJ/kg compared to 199 kJ/kg forliquid nitrogen). Such a transfer tube consists ofan inner tube made of a low thermal conductivitymetal (often steel) surrounded by another metaltube with a vacuum space in between. The liq-

uid helium flows inside the inner tube while thevacuum space thermally isolates the inner tubeand the liquid helium from the room tempera-ture environment a few centimeters away. Thisvacuum-jacketed transfer tube allows the trans-fer of liquid helium into a cryostat with onlysmall losses.

helium, superfluid Seehelium-4, superfluid;helium-3, superfluid.

helmholtz coil (1) An apparatus for pro-viding a nearly constant magnetic field in asmall region for experimentation or measure-ment. Unlike a solenoid, the helmholtz coils aretwo many-turn loops, coaxial, and separated bya distance the same order of magnitude as thecoil diameters. The magnetic field generatedwhen current flows in the loops has low firstand second derivatives in the center of the coils,and thus is well behaved for many experimentaluses. The geometry allows insertion of non-magnetic equipment (such as evacuated tubesand phosphor-impregnated glass) in the centralregion of nearly constant magnetic field strengththat would be impossible in a solenoid.

(2) These comprise two parallel coils carry-ing equal currents separated by a distance equalto their radius. The magnetic field in the centerof the coils is uniform to within few percent.

hemodynamics The study of blood circula-tion and the forces involved.

henry An SI unit of inductance or mutual in-ductance, normally denoted by a symbol H. Onehenry is the inductance of a circuit in which acurrent changes at a rate of one ampere per sec-ond, inducing an electromotive force of one volt.Permeability is measured in henry per meter.

Henry’s function The mathematical formulaused in electrophoresis to account for the retard-ing force on the macromolecular ion of interestdue to counterions. Being of opposite charge,the counterions move in the opposite directionof the macromolecular ion. The interactionsbetween the macromolecular ion and the coun-terions with their associated solvent impedes themovement of the macromolecular ion.


high speed switching Seecircuit, switching.

high tension (1) High voltages, typically, 100kilo-volts (100 kV) or more.

(2) Anode voltages, typically in the range of60 to 250 volts.

Hodgkin-Huxley model A model describingthe electrochemical processes associated withnerve-cell discharges. The three basic steps ofthe model are:

1. activation of sodium ion conductance,2. subsequent inactivation of sodium ion con-

ductance, and3. activation of potassium ion conductance.

hole A vacant electron state that behaves likean electron with positive charge.

Electrons in semiconductors are only allowedto have certain specific energies. The allowedenergies are grouped into bands. The lowestband is the valence band and in semiconductorsthe available electron energy states are mostlyfilled. The next highest band is the conductionband and it is mostly empty. It is the electrons atthe top of the valence band and the bottom of theconduction band that are the most interesting asthese electrons are sufficiently close to vacantlevels and can readily change states.

The quintessential dispersion relationship(energyE vs. momentumk) for electrons in abulk semiconductor is shown below. The groupvelocity of an electron is given by

v =dE

dk

1~.

If one considers an electron in the semicon-ductor under the influence of an applied electricfield cE, then it is possible to show that

−ecE =~2

d2E/dk2

dv

dt.

Comparing this with Newton’s equationF =ma, then

~2

d2E/dk2= m∗

is the effective mass. Note, it is at the top of thevalence band,d2E/dk2, and hence the mass ofthe electron in this state, isnegative.

Typical dispersion relationship for an electron in a bulk

semiconductor.

The concept of an electron with negativemass can be simplified if one considers the smallnumber of vacant states in the valence band.First consider a completely filled band withnelectrons with velocitiesv1, v2, . . . , vn. Sincethe band is full and there is a net zero currentflow, there are as many electrons with velocity−v as there are with velocity+v. Hence,

−|e|∑

n

vn ≡ 0 .

If the ith electron is missing, there will be a netcurrent flow

−|e|∑n6=i

vn .

But since

−|e|∑n 6=i

vn +−|e|vi ≡ 0 ,

the net current flow is equal to+|e|vi and isconsidered to be due to a positive charge. It ispossible to show that the effective mass of thismissing electron, or hole, is positive at the topof the valence band. This idea of hole current isillustrated below as if there is an applied electricfield moving the electrons, successively fillingthe vacancy.

Therefore, holes can be considered as posi-tive charges with positive masses and hole con-duction can be thought of similarly as for elec-tron conduction. When an electric potential isapplied across a semiconductor, there are twocontributions to the conduction current:

1. electrons in the conduction band, and2. holes in the valence band.


Energy level diagrams are usually defined forelectrons. However, if desired, they could bedefined for holes as well provided the energy ismeasured downward from the top of the valenceband.

hole current Seehole.

hologram Image formation by the method ofwave-front reconstruction, as proposed by Ga-bor in 1947. A hologram is a special diffrac-tion screen that reconstructs in detail the wavefield emitted by the subject. Output from alaser is split into two beams one of which il-luminates the subject, the other is a referencebeam. The two beams form an interference pat-tern that is recorded on a high-resolution pho-tographic plate. The plate contains all the in-formation needed to reconstruct the wave fieldof the subject. After development, the plate isviewed with a single beam from a laser, and partof the diffracted wave field is a precise, three-dimensional copy of the original wave reflectedfrom the subject. The imagine is seen in depth,and moving the point of view changes the per-spective of the view.

homeostasis The ability of an organism tomaintain a stable internal condition (such asbody temperature) by regulating its physiologyin response to external environmental condi-tions.

homing adapter In automatic switching, thisdevice enables the automatic return of a sequen-tial selector to a predetermined unoperated po-sition upon its release.

homochronous Term applied to digital sig-nals whose corresponding significant instantshave a constant, but uncontrolled, phase rela-tionship.

Hooke’s law Used, together with Newton’slaw, to describe vibrations in linear systems.The elastic forceF acting on a spring is pro-portional to the length change of the springxdue to the forceF, F = −kx, wherek is theforce constant of the spring. This relation isknown asHooke’s lawand holds for moderatedisplacement amplitudes. The direction of the

force is opposite to the displacement of the end-point from the origin and it is a restoring forcethat points toward the origin. For a stretchedspringx > 0 and the forceF is negative, fora compressed springx < 0 and the forceF ispositive.

hopping (1) Method employed to force ajammer to cover a wider spectrum by randomlyhopping the data-modulated carrier from onefrequency to a next. Examples areslow fre-quencyandfast frequency hopping.

(2) It also refers to a small jump and, withreference to the internet, indicates the route thata computer takes in order to relay its informationfrom one point to the next.

horizontal scanning The scanning of anelectron beam over a phosphor surface to pro-duce a television image. The image is formed bymany horizontal scan lines. The number of linesper image depends on the signal encoding (525lines in American broadcast, 625 for European).The electron beam, therefore, must sequentiallyscan each line to form the image.

horns, sound from Sound radiated by acous-tic transducers consisting of a tube of vary-ing cross-sectional areas employed to match theimpedance of a relatively heavy vibrating di-aphragm to the light medium used to propa-gate the sound. Horns have different shapes(parabolic, conical, exponential, etc.) and theirdiameter is smaller than the wavelength of soundpassing through them. Horns are characterizedby their acoustic impedance as a function offrequency, cut-off frequency, resistance to re-actance ratio, and directional characteristics.

hot-wire ammeter An instrument that usesthe thermal expansion of a wire or bi-strip tomeasure the current passing through it. Certainmechanical devices are used to magnify the ac-tual increase in the length of the wire.

h–parameters Used in the circuit descriptionof a transistor and the inter-relationship betweenthe various currents and voltages in a transistor.The transistor can be considered as a two-port,four-terminal network circuit. Since the transis-tor has only three connections, there will be one


common to the two ports appearing in the fig-ure. Hence, depending on the model circuit, wecan have a common base, common emitter, orcommon collector and the scheme is designatedas CB, CE, or CC, respectively; an example isgiven later.

A two-port, four-terminal network representation of a

transistor.

There are four different variablesI1, I2,V1,andV2 external to the network as shown in thefigure. Any two of these can be chosen as inde-pendent variables and the other two expressed interms of them. For example, the currentsI1 andI2 can be chosen as the independent variables.Then, the voltagesV1 andV2 can be solved interms of the currents

V1 = f (I1, I2) ,

and

V2 = g (I1, I2) .

If these expressions are expanded in a Taylorseries, ignoring higher order terms;

δV1 =∂f

∂I1δI1 +

∂f

∂I2δI2 ,

and

δV2 =∂g

∂I1δI1 +

∂g

∂I2δI2 .

The quantitiesδV1, δV2, δI1, andδI2 are thesmall signal or incremental voltages and cur-rents. These are usually written in the lowercasev1,v2,i1, andi2. The AC component is as-sumed to be small compared with the DC values.Also,f andg are assumed to be linear functionsover the small range of the AC signals. Then,the above becomes

v1 =∂f

∂I1i1 +

∂f

∂I2i2 = z11i1 + z12i2 ,

and

v2 =∂g

∂I1i1 +

∂g

∂I2i2 = z21i1 + z22i2 .

Sincezij have units of impedance, it is said thatthese relations are in the impedance representa-tion.

If we had instead assumed thatV1 andV2

were the independent variables, then we wouldhave arrived at the admittance representation,

i1 = y11v1 + y12v2 ,

and

i2 = y21v1 + y22v2 .

There are a total of six different representa-tions corresponding to different choices of twoindependent variables from the seti1, i2, v1,andv2. A very useful representation is thehy-brid representation based oni1 and v2 as theindependent variables.

v1 = h11i1 + h12v2 = hii1 + hrv2 ,

and

i2 = h21i1 + h22v2 = hf i1 + hov2 .

Hybrid circuit representation of a transistor illustrating

the meaning of the h–parameters.

These h–parametershave especially usefulmeanings:hi = h11 is the input impedance with the

output short–circuited,hr = h12 is the reverse voltage ratio with

input open–circuited,hf = h21 is the forward current ratio with

output short–circuited,


ho = h22 is the output admittance with inputopen–circuited.

A circuit model of a transistor based on thehybrid scheme is shown in the second figure.This parameter formalism and circuit model isgenerally applicable to all schemes of modelingthe transistor (i.e., CE, CB, or CC); however, theh–parameters will be different for each case. Asecond subscript is used to indicate the schemeused.

For example, using the above model of a tran-sistor and assuming a common emitter scheme,hfe = IC/IE refers to the forward current gainandhie is the input impedance at the base ofthe transistor. The numbered parameters or la-bels in the circuit model can be replaced withthe appropriate lettered labels once the schemeis determined; e.g.,hrv2 becomeshrbvc in theCB scheme.

Huffman code The Huffman code repre-sents a very common variable-length memory-less code that attempts to match the average codeword length to the source entropy. Suppose theHuffman encoder is to encode a five stream of in-formation data symbols drawn from the symbolset a, b, c, d, e appearing with the respectiveprobabilities of 0.51, 0.06, 0.26, 0.11 and 0.06.This source’s entropy equals: -0.51log(0.51) -0.06log(0.06) - 0.26log(0.26) - 0.11log(0.11)- 0.06log(0.06) = 1.838. The Huffman encoderwould proceed as follows:

Step 1: Arrange the information symbols indescending order of probability. If two or moresymbols occur with equal probability, the orderamong them is immaterial.

Symbol Probabilitya 0.51c 0.26d 0.11b 0.06e 0.06

Step 2: Combine the bottom two entries intoa new entry, with a new probability equal to thesum of the two entries combined. Re-order thenew list in descending order of probability.

Symbol Prob. Symb. Prob.

a 0.51 a 0.51c 0.26 c 0.26d 0.11 b,e 0.12b 0.06 > b,e 0.12 d 0.11e 0.06 >

Step 3: Repeat Step 2 until only two entriesare left.

Symb. Prob. Symb. Prob.a 0.51 a 0.51c 0.26 c 0.26d 0.11 d 0.11b 0.06 > b,e 0.12c 0.06 >Symb. Prob. Symb. Prob.a 0.51 a 0.51c 0.26 c 0.26b,e 0.12 > b,d,e 0.23d 0.11 >Symb. Prob.a 0.51b,c,d,e 0.49

Step 4: Starting with the right of the table,assign the most significant bit of the code word.Then move left in the table and assign anotherbit if a split occurs. The assigned codewords arewritten in parenthesis below:


Symb. Prob. Symb. Prob.

a 0.51 a 0.51c 0.26 c 0.26d 0.11 d 0.11b 0.06 (1110) > b,e 0.12e 0.06 (1111) >

Symb. Prob. Symb. Prob.

a 0.51 a 0.51 (0)c 0.26 c 0.26 > (10)b,e 0.12 > b,d,e 0.23d 0.11 (110) >

Symb. Prob.

a 0.51b,c,d,e 0.49

Thus, a→ 0, b→ 1110, c→ 10, d→ 110, e→ 1111. The average length equals:1×0.51+4x0.06+2×0.26+3×0.11+4×0.06 = 1.940,only 5.5% above the theoretical minimum spec-ified by the source entropy of 1.838. If blockcoding is used, the average length would haveequaled 3.

The Huffman code self-punctuates — itneeds no explicit punctuating symbols as doesthe Morse Code. Successive blocks of the Huff-man encoder output may be directly concate-nated and uniquely recoverable by a Huffmandecoder.

human voice Sound generated by the humanvocal system, including the organs for humanspeech. Vocal sound is produced by forcing airfrom the lungs (by contracting the chest mus-cles) through the larynx and the vocal chords.The vocal chord is a muscular organ shaped likea diaphragm with a slit-like opening that allowsmodulating the air stream and controlling thefundamental frequency of sound by changingthe tension of the lips of the slit. The cavities andorifices of the throat, mouth and nose togetherwith the tongue form an acoustic network andcontribute to the formation of different soundsby varying the shape of the oral cavity. In thisway some of the harmonics formed by the vocalchords are emphasized while others suppressed.The sound intensity at a distance of two metersaway from the speaker in a normal conversationis 2.10−8 W/m2 and the frequency range of thehuman voice is 40 to 12,000 Hz.

Huygens’ principle A method of analy-sis used for problems of wave propagation thatavoids the difficulties of a rigorous mathemat-ical computation of waves in inhomogeneousmedia or near obstacles. The principle makesonly general assumptions about wave propaga-tion and states that each point of an advancingwavefront is itself the center of a new distur-bance, the source of a new train of waves. Italso notes that the entire advancing wave as awhole can be considered to be the result of thesecondary waves that arise from points in themedium already traversed. This view of wavepropagation facilitates the study of various phe-nomena such as diffraction, where the light onreaching a slit or edge of an obstacle is regardedas being the source of wave fronts that proceedfrom there so as to interfere and give the maximaand minima observable as diffraction fringes.

hybrid coil Used in telephony to interface a2–wire communication system with combinedreceive-and-transmit signals to a 4–wire systemwith independent receive-and-transmit circuits.

Most local telephone exchanges use a 2–wirecommunication system to service the commontelephone. However, physical separation of thetransmit and receive signals are required in theswitching networks. Separation of the two sig-nals is also desirable since it is easier to transmitvarious types of data (e.g., digital or voice) andit is possible to insert amplifiers in the transmis-sion circuit for enhanced performance. There-fore, a circuit that interfaces the 2–wire and 4–wire systems is needed; referred to generally asahybrid circuit. The hybrid coil is a simple andconventional way to accomplish this task.

hybrid communication network A hybridcommunication network represents a commu-nication system capable of transmitting and re-ceiving both analog and digital signals.

hydrogen, liquid Hydrogen molecules, H2,liquefy at 20.3 K at 1 bar. Hydrogen remainsa liquid down to 14.0 K, at which point it so-lidifies. In the past, liquid hydrogen was usedas a refrigerant. Since hydrogen combines withoxygen explosively if present in sufficient con-centrations, most laboratories use liquid heliumand or cryocoolers instead of liquid hydrogen.


hydrophones Devices that receive underwa-ter sound waves (underwater microphones) andconvert them into electric signals. Hydrophonestake advantage of the piezoelectric properties ofcrystals such as quartz or the magnetostrictiveproperties of materials such as nickel. They candetect sound emitted by a sound source and thedirection of the sound source (for example, anunderwater vehicle) by passive listening. Hy-drophones are also used to measure the distanceof underwater objects by measuring the timeneeded for the emitted sound beam, reflectedby the object, to return in the form of an echo.They can operate in the sonic or ultrasonic fre-quency range. Hydrophones are characterizedby their directivity and the frequency range.

hyperchromism An increased absorption ofelectromagnetic radiation (usually in the ultra-violet region) caused by geometry-dependentinteraction between two or more parts of amolecule.

hyperfine structure A set of very closelyspaced lines making up a spectral line or para-magnetic resonance line. There are at least twotypes of hyperfine structure:

1. The splitting of an element’s spectral lineinto doublets, triplets, etc., can be the result ofthe interactions between the electron spin andthe spins of adjacent magnetic nuclei via thecoupling of the total angular momentum of theorbital electron with the nuclear spin.

2. The presence of several isotopes in thesample being tested, in which each isotope con-tributes one or more components of the spectralline (this type of hyperfine structure is termedisotope structure).

hyperopia Seeeye, far-sighted.

hypersonic waves Sound waves of frequen-cies above 500 megahertz.

hypochromism A decreased absorption ofelectromagnetic radiation (usually in the ultra-violet region) caused by geometry-dependentinteraction between two or more parts of amolecule.

hysteresis Occurs in ferromagnetic materi-als. When a changing magnetic fieldH pro-duced by a changing electric currentI is appliedto ferromagnetic material it exhibits a magneti-zationB that is not a simple linear function ofthe applied field.B will eventually show satu-ration for high values ofH. WhenH is reducedthe magnetization is less than what occurred asH was increased, i.e., the magnetization lagsbehind the magnetizing field. On reducing, re-versing and then increasingH, theB field goesthrough a complete loop. The phenomenon isknown ashysteresis.

hysteresis loop The hysteresis loop is thecomplete functional relationship betweenH andB which forms a closed loop.Seehysteresis.

hysteresis loss The area inside the hysteresisloop representing the loss of energy occurringin each cycle of the changing currentI is knownas thehysteresis loss.

hysteresis tester An instrument for the rapiddetermination of the hysteresis loss of a givenspecimen of magnetic material. The version at-tributed to Ewing comprises a specimen builtup out of strips of a prescribed shape and sizeand is rotated by hand between the poles of ahorseshoe magnet which is suspended and bal-anced so as to be free to turn on an axis in thesame straight line as the axis of rotation of thespecimen. The latter is rotated sufficiently fastto produce a steady deflection of the horseshoemagnet, which is read off. The hysteresis lossin the specimen is proportional to the sine of theangle of deflection.


Iignitron A three terminal device capable ofswitching extremely large currents (hundreds ofamperes) at high voltages. It has electrical char-acteristics similar to an SCR. It works by con-trolling a gas discharge between an anode andcathode. The anode and cathode are well insu-lated, so when the device is off, it can separateseveral thousand volts. The device is turned onby initiating a gas discharge. This is accom-plished with a minute arc to the cathode withthe ignitor terminal. Once the discharge is initi-ated, the current flow between cathode and an-ode maintains the discharge. Ignitrons are usedin high current regulated voltage supplies or forcontrolling automated spot welding equipment.

illuminance Luminous flux density on a sur-face, i.e., luminous flux incident per unit area ofa surface, when the latter is uniformly illumi-nated. Synonymous with illumination, a moregeneral term. Also synonymous with the inten-sity of illumination. Practical units of measure-ment are lumen per square meter or meter-candleor lux, lumens per square foot or foot-candle, lu-mens per square centimeter or phot. 1 phot=10,000 lux= 929.03 foot-candles. Note thatluminance of a source is the number of lumensemitted per solid angle (steradian) by a unitsource area. Visual acuity and other propertiesof vision depend on the illumination, and min-imum values are tabulated for various occupa-tions in lighting codes. The eye has maximumefficiency between 10 and 100 foot-candles.

illumination Synonymous with illuminance(illuminanceis the preferred usage, sinceillumi-nationhas more a general meaning). A surfaceis illuminated when a luminous flux is incidenton it. The flux per unit area at any point onthe surface is the illumination (orilluminanceor intensity of illumination) at that point.

image analysis The extraction of scientifi-cally useful information from an image.

image converter A device that converts tele-vision video information between different en-coding formats. This device is sometimes re-ferred to as an image buffer.

There are many different video encoding for-mats for the transmission and display of images,e.g., scanning frequency and number of scanlines constituting the image. This is becausedevelopment of television occurred simultane-ously in different countries and horizontal scan-ning in the display of the television image issyncronized with the local power frequency. So,an image converter would be used to convert a625–line, 50 Hz signal (European) to a 525–line,60 Hz signal (American).

image dissector An early video tube for theencoding of light images to electrical signalsthat formed the basis of early television cam-eras. The device is obsolete and no longer usedin modern cameras.

image distance The distance from the vertexto the image. The vertex is the point on thesurface of the lens where the optical axis crosses.From the thin-lens equation, the image distances for a thin-lens, whose focal length isf , can becalculated from the object distances′ as

1s

=1f

+1s′,

1f

=(n′ − n)

n

(1R

+1R′

),

where the radius surface of the lens isR andR′.

Image distance.

Focal length, image distance, and object dis-tance of a thick lens should be measured fromthe principal planes of the lens.


image duplication A 1:1 reproduction of anobject by light rays. An image forming opticalduplication gathers light from an object pointand transforms it into a beam that converges to-ward another point (real image) or diverges fromanother point (virtual image).

image enhancement electron micrographA signal-processing operation performed to in-crease the quality of an image.

image, primary A real image formed by theobjective of optical instruments such as a micro-scope or telescope. The primary image is alsoknown as the first real image. (Seeimage, real).The eyepiece (ocular) magnifies the primary im-age. It can be defined as an image that includesthe point where each ray of light from the ob-jective passes through. If astigmatism exists,the primary image of the point source can notbe a perfect point. In a compound microscope,the objective lens forms a real, inverted image ofthe object. The image is formed in space on theplane of the field stop of the eyepiece within themicroscope tube. The eyepiece observes thisreal image. In telescopes, the primary imageis formed by the objective lens or the objectivemirror. For projection with a microscope, theprimary image is formed ahead of the first focallength of the ocular, which forms a real imageagain. The magnification of the eyepiece shouldbe treated as a lateral magnification. For visualobservation, the primary image is generated in-side the focal length of the ocular as a magni-fier. The magnification of the eyepiece shouldbe treated as an angular magnification.

Primary image.

image, real An optical image such as thatformed by the light from an object that actu-ally passes through the image. The real imageis luminous and is generally visible. It can beprojected on a screen and the projected image issharp.

images, acoustic Geometric figures formedin space by acoustic mirrors, lenses or otheracoustic equivalents of optical system compo-nents. Also know asimaging. Acoustic imag-ing deals with the generation of real-time imagesof the internal structure of metallic or biologi-cal objects that are opaque to light by irradiatingthem with sound. Also known assonographyorultrasonic imaging.

image, ultrasonic The use of ultrasound toproduce an image of the internal structures ofthe body.

image, virtual An optical image where raysof light only appear to diverge. A virtual imagecannot be projected on a screen. The image of anobject in a plane mirror is a virtual image. Thevirtual image is not luminous actually. The raysof light do not actually focus on a virtual image.With a converging lens, the object closer to thelens than the first focal point will form a virtualimage, not a real image. The image formed by aconcave lens is also a virtual image. The imageformed by (viewed in) a plane mirror is a virtualimage. A stereoscopic image generated by ahologram is a virtual image.

imaging, dark-field An imaging techniquein which illumination conditions are regulatedby apertures and stops to permit only certainelectrons or light photons to reach the lens. Thisallows for the illumination of certain parts of thesample while the remaining parts of the sampleremain dark.

imaging, medical The use of X-rays, ultra-sound, and other techniques to make images ofinternal structures of the body.

imaging, NMR The use of nuclear magneticresonance (NMR) to produce images of internalstructures of the body.


imaging, radionucleotide The use of ra-dioactive decay to visualize internal structuresof the body.

immunofluorescence A process in whichfluorescently labeled antibodies are attached tocells. These cells are then examined and sep-arated by an optical microscope while shiningultraviolet radiation on the sample.

impedance The relationship between a si-nusoidally varying quantity, e.g., force, voltage,electric field strength, to a second quantity thatmeasures the response of the system to the si-nusoidal quantity, e.g., velocity, current, mag-netic field strength. The response of the systemwill often depend on the frequency of the ap-plied disturbance. The most common usage ofthe term is in alternating current circuits wherethe impedanceZ relates the voltage mV (ex-pressed as a complex number) to the currentmI Z = V/I = V0/I0e

iφ, whereV0 and I0are the voltage and current amplitudes, andφis the phase difference between them. Similarrelationships are found in other systems, e.g.,mechanical systems, whereZ relates an appliedsinusoidal force to the resultant velocity of abody.

impedance, acoustic The complex ratio ofthe sound pressurep on a given surface to thesound flux through that surface, the volume ve-locity of the fluidU , ZA(ω) = p/U , expressedin units kg/(m4s). It was introduced as the anal-ogy to mechanics where the impedance is theratio of the force amplitude to the velocity am-plitude. The real and imaginary parts ofZ,RandX, respectively, areacoustic resistance(as-sociated with the dissipation of acoustic energy)andreactance(resulting from the effective massand stiffness of the medium).

impedance, characteristic The impedanceof a waveguide/line to a transmitted wave of aspecific frequency assuming the guide is of in-finite length. For a lossless line, it is equal to√CL, whereC andL are the capacitance and

inductance per unit length. If a guide is termi-nated with an impedance equal to the character-istic impedance, there will be no reflection from

its far end, thus simulating a guide of infinitelength.

impedance diagram A diagram used to an-alyze the steady-state behavior of a circuit or toanalyze the response of a circuit to a particularinput. It shows the relationship between resis-tance, reactance and impedance in a circuit ora portion of a circuit. The real axis indicatesresistanceR and the imaginary axis indicatesreactanceX of a circuit.

Z2 = R2 +X2 ,

Z = R+ iX ,

X = X L −X C ,

whereX L andX C are reactance caused by in-ductance and capacitance, respectively.

The impedance diagram of a circuit can beobtained by dividing each of the voltage phasordiagrams by the magnitude of the current. Theimpedance diagram applies linear circuit theo-rems to the AC circuits. It means the ratio ofcomplex voltage to complex current for each re-sistive, inductive, and capacitive element of thecircuit is constant. It comes from Ohm’s law.The phasor diagram is also used for similar pur-poses. The phasor diagram of each branch of thecircuit is used to create the impedance diagramof the circuit. The phasor diagram is based onthe equation of complex voltageV and complexcurrentI:

V = RI + jωLI − j1ωC

I ,

whereomega, R, L, andC are the frequencyof the input sinusoidal signal, resistance, induc-tance, and capacitance, respectively. When thephase angle of the current of a branch isφ to thecurrent of another place of the circuit, it can bedescribed as

V =(RI + jωLI − j

1ωC

I

)ejφ .

In such case, the phasor diagram is used as ro-tated by phase angleφ.

impedance, driving point The ratio of thecomplex componentVi of the applied alternat-ing voltage to the complex componentIi of thealternating current.


Phasor diagram.

Impedance diagram.

impedance, dynamic The impedance of anelectric component or an electric device with anAC signal input.

The dynamic impedanceZ includes not onlyresistanceR but also reactanceX:

Z2 = R2 +X2 ,

Z = R+ iX .

The resistance (the real part of the impedance)indicates the loss of power, The reactance (theimaginary part of the impedance), which iscaused by capacitance and/or inductance, indi-cates the phase difference between the voltageand the current. Dynamic impedance causedby capacitance and/or inductance indicates fre-quency dependency.Seeimpedance diagram.

impedance image A pair of impedances ofa quadripole that satisfies two conditions:

1. when the first pair of terminals of thequadripole is terminated with an impedanceZ1,the impedance of the second pair of terminals isZ2, and

2. when the second pair of terminals of thequadripole is terminated with an impedanceZ2,the impedance of the first pair of terminals isZ1.

impedance, input The impedance presentedto an input source of a device by the device.

Impedance image.

impedance, iterative The impedance which,when connected to two terminals of a four-terminal network, produces the equivalent im-pedance across the other two terminals.

impedance matching The method of maxi-mizing the power transmitted from a source toa sink. Maximum power transmission occurswhen the resistances of the sourceRo and sinkRi are equalRo = Ri, and the reactances,Xo

andXi, are equal but opposite:Xo = −Xi.The standing wave ratio is a minimum when theimpedances are matched in this way.

impedance, membrane The degree of diffi-culty of the flow of current (usually in the formof ions) through a membrane.

impedance, membrane, measurementsTechniques used to determine quantitatively thedifficulty with which current flows through a cel-lular membrane.

impedance, output The impedance pre-sented to the load of a device by the device.

impedance, reflected The input impedanceof a transformer is given by the sum of two termsThe impedance of the primary winding and thereflected impedance due to the secondary wind-ing. The reflected impedance is equal toω2 ∗M2/Z, whereω is the angular frequency of theapplied voltage,M is the mutual inductance be-tween the windings, andZ is the impedance ofthe secondary winding.

impedance, source The impedance of thesource (be it a supplier of charge, heat, light,etc.) as seen by the rest of the device.

impedance, synchronous The impedance ofan alternator while it is operating at a fixed fre-quency. It depends on the stationary impedanceof the alternator and its armature reactance.


impedance, transfer The complex ratio ofthe applied sinusoidal quantity, such as voltage,force, etc., to the corresponding response (cur-rent, velocity etc.) between any two points ofthe device.

impulse function The electrical potentialalong an axon during the generation of a nervesignal.

impulsive sound Sound caused by a short du-ration disturbance. An impulse excitation can becaused by a force that is applied for a very shortor infinitesimal length of time and is nonperi-odic. Impulsive sound is of importance in archi-tectural acoustics, for example, in determiningthe reverberation time of a room. The reverber-ation time of a room for impulsive sound (suchas a hand-clap) can be considerably greater thanthat computed using methods for sustainedsound.

impulsive voltage/current A waveform thatrises rapidly and reaches a short duration of volt-age/current plateau and then falls rapidly to zero.In practical usage, the ringing of the impulseshould be considered.

Impulsive signal.

impurity A substance intentionally added toa semiconductor to change the available numberof charge carriers in the crystalline lattice.Seedoping.

inactivation The decline or stoppage in cur-rent of a particular ion through the cellular wall.

inactivation, kinetic interpretation Theconductance of ions through the cellular mem-brane is dependent on the potential difference

across the membrane. In the case of sodiumions involved in nerve pulses, the movement ofsufficient numbers of ions causes the potentialdifference to change to the state of inactivation.

inactivation, voltage dependence The pro-cess of stopping the flow of ions through thecellular membrane because of the potential dif-ference across the membrane.

incandescence The emission of visible lightradiated by a substance with a high temperature(> 3000 K). The radiation itself is sometimescalledincandescence.Electric lights are incan-descent.

inclination, magnetic The angle betweenthe magnetic field vector of the earth’s magneticfield and the horizontal plane. Same asmagneticdip.

incoherence Absence of a fixed phase differ-ence between two sinusoidal waves. The phasedifference from two incoherent sources mayvary rapidly and irregularly with time. For opti-cal sources, the interference fringes of the resul-tant disturbance changes as the phase differencechanges, and at any given instant the maximaand minima change positions faster than can beresolved and the result appears to be a uniformillumination — i.e., superposition of incoherentlight waves gives an intensity equal to the sumof the intensities of the individual waves. Whenfluctuations in phase of beams from differentsources are completely uncorrelated, the beamsare incoherent. When the beams are from thesame source, the fluctuations are correlated andthe beams are completely coherent or partiallycoherent depending on how complete the corre-lation is. The degree of correlation is measuredby the degree of distinctness of the interferencefringes when the beams are superimposed.

incubator A device in which the environ-mental conditions can be carefully controlledfor the purpose of sustaining living organisms,such as premature babies, developing eggs, andcultured microorganisms.

index of refraction (refractive index, refrac-tive constant) A dimensionless quantity with


the symboln. The refractive index of mediais equal to the ratio of the speed of an electro-magnetic wave in two mediums. The absoluterefractive index of a mediumn is the ratio of thewave speed in free space toc wave speed in themediumv:

n =c

v.

The index of refraction depends on the wave-length. Usually, the index of refraction for yel-low light (sodium D-lines, wavelength 589.3 nm)is used. Optical lengths is a product of the in-dex of refraction and the lengthl of the path inthe free space:s = nl. The reduced distancel′ is equal to the optical path length in the airldivided byn.

induced charge When a charge is broughtnear an uncharged conductor, charges of the op-posite sign in the conductor will move to theparts nearer the charge and those of the samesign will move away from it. These charges onthe conductor are known as induced charges.

induced magnetic fields Magnetic fields ac-quired by some magnetic materials when placedin an external magnetic field.

inductance General term given to the cre-ation of an inducted potential difference in acircuit due to a changing magnetic field whichthreads the circuit. It is a result of Faraday’sinduction law.

inductance, leakage The inductance in atransformer that results in leakage reactance,and is a result of flux linking only one coil ofthe transformer.

inductance, mutual The mutual inductanceM between two circuits is defined asM = Γ/I,whereΓ is the flux linking one circuit as a re-sult of the currentI flowing in the other. FromFaraday’s induction law, one determines that thevoltageV induced in the first circuit is given byV = −MdI/dt.

inductance, self The self inductanceL of acircuit is defined asL = Γ/I, whereΓ is theflux linking the circuit as a result of the currentI flowing in it. From Faraday’s induction law,

one determines that the voltageV induced in thecircuit is given byV = −LdI/dt.

induction balance Invented by A.G. Bell.Two coils (a transmit coil and a receive coil)work on the principle of eddy current genera-tion and the inductive imbalance between thetwo coils. A very low frequency current causesthe transmitting coil to create an electromagneticfield where polarity is pointed to an object. Ifthe object is metallic or ferromagnetic, an eddycurrent induced inside of it creates its own mag-netic field. This field is then detected by thereceive coil. Some metal detectors use an in-duction balance.

induction coil A coil used to produce an in-termittent high voltage from a source of low,constant voltage. The low voltage source is con-nected to a primary coil of few turns that sur-rounds an iron core via a switch which interruptsthe current. Around this coil is a secondary coilof many turns. The rapid variations of the fluxlinking the primary coil cause a correspondinglylarge voltage to be induced in the secondary coil.

induction, electromagnetic The setting upof an electric field by reason of the variation inmagnetic flux density with time. Any current soinduced is in such a direction as to oppose thechange in magnetic flux.

induction heating Method of heating a con-ductor via the application of an alternating mag-netic field. This creates circulating eddy cur-rents in the conductor, as a result of Faraday’sinduction law, which heat the conducting ma-terial via the Joule effect. With this method ofheating, the material is not contaminated withcombustion gases. It also allows one to primar-ily heat only the surface by the use of a high fre-quency field which, due to the skin effect, willproduce heating currents only in the surface.

induction machine A device that produceshigh-voltage electrical charges by electrostaticinduction. It is also known as anelectrostaticgenerator. The van der Graaf generator andWimshurst machines are well-known examples.


induction meter A motor meter that uses akind of induction motor. It is also used as a watt-hour meter for AC current because the loss ofpower is during measurement and it is reliable.

inductive reactance A part of the reactanceX normally denoted by symbolXL, measuredS (Siemens) orOhm. It is caused by the exis-tence of inductance. It is considered as a positiveimaginary numberXL = j2πfL, wheref is thefrequency of input signal,L is the inductance ofthe component, andj is the unit imaginary num-ber. In a purely inductive reactance circuit, thecurrent lags the applied voltage in its phase byπ/2.

inductor A coil (turns of wire) introduceselectromagnetic inductance. It is measured inhenry. Usually the symbolL is used to indi-cate an inductor. The eddy current induced inan inductor causes the power loss of the induc-tor to increase with frequency. The effectiveinductance is affected by the stray capacitancesbetween the turns of the existing coil. The in-ductance of inductors in a series is an algebraicsum of each inductance of the inductor. The in-verse of the inductance of the inductors in par-allel is the algebraic sum of the inverse of theinductance of each inductor.

inductor, stored energy in With a varyingcurrentI passing in an inductor of inductanceL, it is necessary to provide energy to drive thecurrent against the induced electromotive force.The electromotive forceVemf is equal toLdI/dt.Therefore, the electromagnetic energy stored inthe inductorU is:

dU

dt= VemfI ,

= LIdI

dt,

dU = LdI/dtdt

U =12LI2 .

U is stored in the magnetic field of the inductor.

information Evaluated facts and judgmentswith application; the summarization of data.Technically, data are raw facts and figures thatare processed into information.Dataandinfor-

mationare terms often used synonymously andinterchangeably. Information can be said to bestructured in the following manner: data, text,spreadsheets, pictures, voice and video. Dataare discretely defined fields. Text is a collectionof words. Spreadsheets are data in matrix (rowand column) form. Pictures are lists of vectorsor frames of bits. Voice is a continuous streamof sound waves. Video is a sequence of frames.Databases can store all kinds of information.

information channel That by which data istransmitted from input point to output point. Italso refers to the communication link connectinga PC or server to a hub in the wiring closet.

information entropy This provides a quanti-tative measure of the degree of randomness of asystem and is a measure of the average informa-tion content per source symbol; it can be quan-tified by the probabilities of the source symbols.

information, mutual When noise is intro-duced into the channel, the symbol at the chan-nel output will not always be identical with thestate at the channel input. A measure of theaverage information rate at the receiver output,and thus the average information rate throughthe channel, is given by the log of the ratio ofthe final and initial uncertainties regarding thesource. If there is no noise in the channel, themutual information is 1 and if the noise is sogreat that the output states are independent ofthe input states, then the mutual information iszero.

information transmission, substrate forThe medium through which information is trans-mitted, such as electromagnetic waves for radio.

infrasonic waves Sound waves below thefrequency range of human hearing, covering thepart of the acoustic spectrum below 20 Hz. Thecompressibility of air is responsible for acousticoscillations above the frequency of about0.003 Hz. Below this frequency, transverse os-cillation can develop, and buoyancy acts as arestoring force in a stratified medium. At verylow frequencies (characteristic periods of theorder of hours) the transverse oscillation com-ponent is dominant, and such waves are called


gravity waves.Infrasound accompanies naturalphenomena (avalanches, powerful storms, erup-tions of volcanoes, earthquakes, wind, oceanwaves) and can also be generated artificially (ex-plosions, rocket launches). Infrasound can prop-agate to very large distances due to refraction inthe atmosphere and low absorption.

injection loss The loss of light that resultswhen two fibers are joined at a connection point.

input The signal, current or voltage appliedto a circuit or device or the terminal at whichthey are applied.

input characteristics The electrical proper-ties (impedance, capacitance, etc.) associatedwith the input channel of a device; in a FET(field effect transistor), the dependence of theinput current on the voltages between the sourceand gate and between the drain and gate.

insects, sound from Sound produced forcommunicative purposes by many insect speciesusing a wide variety of mechanisms. Frictionalmethods are predominant, and sound productioncan occur during locomotory, cleaning or feed-ing movements. Stridulation, the rubbing of onebody part against another (a file with a series ofpegs or teeth can be rubbed against the scraperformed by a single edge or ridge), is the mostcommon. Another common method of soundgeneration is by a vibrating membrane driven bymuscles. Sound frequency and patterns vary andtheir true nature cannot be appreciated by hu-mans because of the narrow frequency responseand the long time-constant of the human audi-tory system.

insert (in antenna) Also known asaerialinsert. In a buried cable run, it refers to the rais-ing of the cable followed by an overhead runusually on poles. The cable is subsequently re-turned to the ground. This procedure becomesnecessary where it is not practical to run cablesunderground such as in areas with rivers.

insertion gain In transmission line theory,this refers to a negative insertion loss where theinsertion of a line or network between a gener-ator and a load introduces dissipative elements.

If the impedance matching is reduced, the powerdelivered to the load is reduced. The insertion ofa line or network in this case causes an increasein load current representing a negative loss.Seeinsertion loss.

insertion loss The insertion of a line or net-work between a generator and a load may im-prove or diminish the impedance match betweenthe source and load and introduce dissipative el-ements. Increased power delivered to the loadat the receiver leads to this type of loss. It can bequantified by the number of decibels by whichthe current in the load causes a change, by theinsertion.Seeinsertion gain.

insulated conductor A conductor that sur-rounds a non-conducting material itself and isseparated from other conductors.

insulation, acoustic Materials used to di-minish the energy of sound that passes throughthem or strikes a surface, which is of con-siderable importance, for example, in archi-tectural acoustics. It is customary to differ-entiate between airborne (human speech, mu-sic) and structure-borne (mechanical excitationsof structures by machines or people walking)sound excitation. The sound transmission coef-ficient τ , defined as the ratio of the transmittedand incident sound, and the sound reduction in-dex,R = 10 log(1/τ), expressed in decibels,provide a measure of sound insulation.

insulator A substance that is electrically non-conductive. One or more energy bands of an in-sulator are full and other bands are empty. Onlyan electron having an energy of enough electronvolts can jump to a conduction band. The energyis higher than room temperature (2.6×10−2 eV)and sufficient to disrupt.Seesemiconductors.

integrated communication system (ICS)An integrated communication system (ICS)represents a communication system that joinsand interoperates two or more originally au-tonomous communication systems, which haveconsequently lost their initial independence buthave become one single interdependent system.


Insulator.

integrator A circuit that takes a single inputand gives the integral of the input signal as theoutput signal.

intensity The radiant energy per unit time(flux) or the number of photons per unit time,flowing through a unit area, through a surfacenormal to the direction of propagation. For me-chanical waves, the intensity is proportional tothe square of the amplitude of the wave. Fora traveling light wave, the intensity is propor-tional to the average energy flux per unit time,or the mean square value of the optical distur-bance. The optical disturbance varies with timetoo rapidly to be observed directly, so it is thelight intensity that measures the observable ef-fects of light.

intensity modulation Image reproduction byvarying the intensity of an electron beam andthus the light output of a cathode-ray tube inaccordance with the magnitude of the signals itreceives.

intensity of sound Average rate at whichacoustic energy is transmitted in a specified di-rection through a unit area of a surface perpen-dicular to the direction of propagation~n, ~I =~np2

ρc . The unit of I is watt per square meter

(W/m2). Acoustic energy travels with the speedc in the direction~n; p is the pressure andρ thedensity. Also calledacoustic energy fluxoracoustic intensity.

interaction, exchange Since electrons arefermions, they obey thePauli exclusion princi-plewhich disallows more than one fermion in agiven state. Examining a two electron system,

we find the singlet state, an anti-symmetric spinstate withS = 0, has a lower energy than anyof the triplet states, those states with symmetricspins andS = 1. This difference leads to an ef-fective spin interaction, the so-called exchangeinteraction. It is possible to recast the exchangeinteraction into a spin Hamiltonian. For the twoelectron case, the spin Hamiltonian has the form

Hspin = −− JS1S2

whereJ is the exchange coupling constant (theenergy difference between spin states) and theSi, the spin of the i-th electron. In an N-electron(or N-spin) system, the spin Hamiltonian gen-eralizes to include all pair interactions (repre-sented above) plus higher order interactions. Inmany systems, only the two-particle interactionsare relevant, and the spin Hamiltonian is theHeisenberg Hamiltonian,

HHeisenberg= −− ΣJijSi

Sjwherethesumisoverallpairsofspins.

interfacial tension The force exerted onmolecules at the interface between two bound-aries, such as surface tension.

interference (light) The systematic attenu-ation and reinforcement of the amplitudes overdistance and time of two or more overlappinglight waves that have the same or nearly thesame frequency. The maxima and minima oflight wave interference cannot be described bythe ray approximation of the wave equation.From Huygen’s principle, interference occurswhen there are two or more paths of differentlengths from a source to the observation point.The interference is constructive (destructive) ifthe phases and amplitudes increase (decrease)the resultant amplitude squared relative to thesum of the squares of the amplitudes. Sinceenergy is conserved, the energy that is missingfrom the destructive interference zones or darkspots in the interference pattern is found at theconstructive zones or bright spots. Interferencecan also occur when there is more than a singlesource, provided there is a fixed phase relation-ship between the sources, i.e., the sources arecoherent. The interference of light was first dis-covered by Thomas Young in 1801 using a single


light source and twin pinholes or slits. Fresneland Young explained the resulting fringes usingthe wave theory of light.

interference, acoustic The variation overdistance or time of the amplitude of a wave thatresults from a superposition (algebraic or vectoraddition) of two or more waves. A medium cansimultaneously transmit any number of waves,which propagate independently of the other. Thedisplacement of the medium at any point andany instant of time is the algebraic sum of thedisplacements caused by the individual wavesat that instant of time. The terminterferenceiscommonly used to describe this effect, althoughthe termsuperpositionwould be more accurate.Unlike light that requires coherent light for in-terference, in acoustics, separate sources will becoherent and can give rise to interference effects.See alsomodulation, acoustic.

interference, constructive The case of su-perposition of waves arriving simultaneously ata point so as to give a resultant intensity that isgreater than the sum of the squares of the am-plitudes of individual disturbances. Since theequations of the amplitude wave motion is a lin-ear equation, the sum of any number of solu-tions is also a solution. The intensity, which isthe observable quantity for light phenomena, isthe square of the amplitude. Thus the resultantintensity is not merely the sum of the intensitiesof the individual waves, but can be greater orless than the sum of individual intensities.Seeinterference.

interference, destructive The case of su-perposition of waves arriving simultaneously ata point so as to give a resultant intensity that isless than the sum of the squares of the amplitudesof the individual disturbances.Seeinterference,constructive.

interference fringes The maxima and min-ima of intensities seen during the optical inter-ference of light waves. The maxima occur as aresult of constructive interference of light waves,which arrive in phase, to give a bright spot, whilethe minima occur as a result of the destructiveinterference of light waves, which arrive out ofphase, to give a dark spot.Seeinterference.

interference, heterodyne The mixing of twosignals of different frequencies in a nonlinear de-vice resulting in the generation of two new fre-quencies that are the sum of and the differencebetween the two original frequencies. The ef-fect is used in the heterodyne receiver where twosignals having slightly different frequencies arecombined to form an audio-frequency beat sig-nal that can be heard with a loudspeaker. Undercertain conditions, a steady, high-pitched audiotone, known asheterodyne interferenceor het-erodyne whistle,can be heard in the amplitude-modulation radio receiver as a result of the het-erodyne action.

interference, radio frequency An unwantedsignal that enters the transmission line from ra-dio and television transmitters at a level suffi-cient to degrade the performance of the channelby a significant amount. With this type of inter-ference, the cable acts as an antenna.

interference, thin film Interference phenom-ena that occur from the reflection of light fromthe two surfaces of a thin transparent film. Themaxima in the easily observed interference oc-cur when the thickness of the film is an oddmultiple of a quarter of the wavelength of theincident light, and the minima occur when thethickness is an even multiple. If the thicknessvaries across the film, then an image of the filmusing a lens will show a different brightness indifferent places. Using the eye to form an imageof the thin film directly on the retina, instead ofa lens and screen, shows a system of interfer-ence fringes, with brightness varying accordingto whether the quarter wavelength is an even orodd multiple of the thickness. Lines of equalthickness appear as lines of equal brightness.These results apply equally well to films withan index of refraction greater than the surround-ing medium (e.g., a thin plate of glass) or withan index of refraction less than the surround-ing medium (e.g., an air gap between two thickplates of glass).

interference with diffraction Interferenceisthe modulation of wave amplitude into reinforc-ing maxima and canceling minima produced bythe superposition of a finite and usually smallnumber of beams.Diffraction is the modifica-


tion of amplitude determined by a superposi-tion via integration of infinitesimal elements ofa wave front. The double slit pattern is thusa combination of interference (superposition ofbeams from each of the two slits, yielding nar-row maxima and minima) and diffraction (inte-gration over the wave front from each slit yield-ing a modulation of the interference maxima andminima).

interferogram The pattern of interferencethat results when two waves of the same wave-length are brought together. Extremely accuratemeasurements of distance can be made usingthis technique.

interferometer, acoustic A device for mea-suring the velocity and attenuation (absorptioncoefficient) of sound waves in a fluid by the inter-ference method. In the acoustic interferometeran electrically driven crystal oscillator is usedto induce longitudinal vibrations in a column offluid. A movable reflector plate is placed par-allel to the radiating surface, to allow standingwaves to form in the fluid column. Varying thespacingl between the reflector and the sourcecauses various modes of resonance of the fluidcolumn, leading to changes of the driving cur-rent of the crystal oscillator circuit. The periodicmaxima of the driving current (that resemblepatterns registered by optical interferometers)correspond to the resonance patterns in the fluidthat are one half acoustic wavelengthλ/2 apart.Plotting the crystal driving current vs. the re-flector position yields information regarding theacoustic wavelengthλ. If the frequency of thesound wavesf is known, the velocity of soundcan be determined asc = λf . In an absorb-ing fluid the relative amplitude of the reflectedwave decreases as the spacing between the os-cillator and the reflector increases, and this ismanifested as the decrease of amplitude of thecurrent peaks of the interferometer pattern. Theabsorption coefficient is measured by measuringthe shape of the current peaks as a function ofspacingl.

interferometer, Fabry-Perot Optical instru-ment that utilizes the interference fringes pro-duced by multiple reflections of light from abroad source in the air gap between two plane

parallel plates that are thinly silvered. A lens,which may be the lens of the eye, is used to bringtogether for interference the parallel transmittedrays from break-up by reflection of the incidentray on the first silvered surface. The conditionfor reinforcement of the transmitted rays is thesame as that for the Michelson interferometer,2d cos θ = mλ, which is satisfied by the pointson a circle with the center given by the intersec-tion of the axis of the lens with the screen, somaxima are a series of concentric rings, whichare not images of the source, with spacing thatchanges with the air gap distance,d. To varyd,one plate is fixed, and the other can be moved viaa slow-motion screw attached to an accuratelymachined carriage arrangement. The rings arevery narrow, so light from a source that consistsof two closely spaced wavelengths produces twoclearly separated sets of rings. Fabry-Perot in-terferometers are thus useful as spectrometerswith high resolution for nearly monochromaticlight. If the light is not nearly monochromatic,the interference pattern becomes too difficult tointerpret, unless another instrument is used todo a preliminary wavelength separation.

interferometer, Michelson-Morely An op-tical interference apparatus based on amplitudedivision of a wave front into two beams that aresent in different directions against plane mirrors.When the beams are recombined, interferencefringes are formed. The main optical parts con-sist of two plane mirrors and two plane platesof glass, one of which is sometimes slightlysilvered. Light from an extended (not a pointor slit) source encounters this slightly silveredglass and is divided into a reflected and transmit-ted beam of equal intensity. A mirror is used toreflect this beam back to the glass plate, while asecond mirror reflects the transmitted beam backto the glass plate where they are recombined.The secondary glass plate is inserted along thetransmitted beam in the compensating plate usedto give equal pathlengths in glass to the reflectedand transmitted beams. The compensating plateis not necessary for fringes from monochromaticbeams, but it is required for fringes in whitelight. The mirror reflecting back the reflectedbeam from the first glass plate is mounted on amoving carriage that rides on an accurately ma-chined track and is attached to a slowly turning


screw that is calibrated for distance measure-ments of the mirror’s motion. Adjustments onthe two mirrors allow them to be made accu-rately perpendicular to each other. When thepathlengths are the same for each beam and theimages coincide, interference fringes will beseen. The fringes will be circular if the mirrorsare exactly in adjustment, with maxima given byangles relative to the axis given by2d cos θ =mλ, whered is the separation of the virtual im-age of the fixed mirror from the moving mirror.Since the fringes are determined by a phase dif-ference determined by an angle of incidence,these are fringes of equal inclination. Unlikeother types of fringes, these fringes may retaintheir visibility over very large path differences.Also, unlike many other types of interferome-ters, the two beams traverse widely separatedpath lenses, which make possible many applica-tions where a beam is required to traverse a sub-stance to be compared with the reference beam.Small differences in the index of refraction oftwo substances can be accurately measured thisway. The Michelson interferometer is also usedto set the standard length of one meter in termsof the wavelength of the red line of cadmium.The Michelson interferometer was used in theMichelson-Morely experiment to establish theabsence of an ether drift, where the distancedwas made as large as 11 m by reflecting the lightback and forth between 16 pairs of mirrors.SeeMichelson-Morley experiment.

interferometers An optical instrument thatuses the interference of light waves originat-ing from a common source, with various practi-cal applications depending on its design. Theprevalent interferometer designs are Michel-son, Twyman-Green, Fabry-Perot, Lummer-Gehreke, Jamin, Mach-Zehnder, and Fizeau.TheMichelson interferometerhas widely sepa-rated beams and a path difference that is readilyvaried, with refracting material commonly in-serted into the path of one of the beams, so as tomeasure distances in wavelengths, and refrac-tive indices of the inserted material.Twyman-Greenis used to test the accuracy of optical sur-faces.Fabry-Perotis used to accurately measurewavelengths and hyperfine structures.Lummer-Gehrekeis used in theUV . Jamin is used tomeasure the refraction of gases.Mach-Zehnder

is used to study slight changes of refractive in-dex over a considerable area, as in the flow pat-tern in wind tunnels.Fizeauis used to test theuniformity of optical thickness of plane-paralleltransparent plates.

intermediate state The intermediate state ofa superconductor occurs when the magnetic fieldnears the critical field. At fieldsH < Hc, themagnetic flux is expelled from the superconduc-tor via the Meissner effect. This exclusion ofthe flux enhances the magnetic field near thesurfaces of the superconductor, however, andnearHc, this is sufficient to drive a portion ofthe superconductor normal. The stable form forthis state is a state in which there are alternat-ing strips of superconducting and normal metalaligned parallel to the magnetic field.

intermodulation Process in nonlinear deviceor system whereby the components of a complexwave modulate each other to produce new waveshaving frequencies equal to the sum and differ-ences of the frequencies of the various compo-nents of the harmonics of the input wave. Thiscauses distortion in nonlinear devices.

intermodulation, acoustic Modulation ofthe components of a complex wave by eachother, generating new waves whose frequenciesare equal to the sums and differences of inte-gral multiples of the frequencies of the originalwaves.See alsomodulation, acoustic; interfer-ence, acoustic; interference, heterodyne.

International Commission on Radiation Pro-tection The international body that providesguidance about all issues related to safety ofionizing radiation. First organized in 1928 asthe International X-ray and Radium ProtectionCommittee,its name was changed to theInter-national Commission on Radiation Protectionin1950. This body makes recommendations aboutthe basic principles of radiation safety and leavesthe detailed recommendations to the various na-tional regulatory bodies.

International Telecommunication Union(ITU) An agency of United Nations thatis responsible for standardizing internationaltelecommunications. Its sectors are concerned


with allocating radio frequencies worldwide tocompeting interest groups and with telephoneand data communication systems.

internodal segment, electrical characteristicsLarge nerve cells in vertebrates are covered witha membrane wrapped around the axon (myelin).Gaps between the myelin are called nodes andthe myelin-wrapped regions are internodal seg-ments. The myelin greatly impedes the flow ofions in this region of the axon, resulting in littlecurrent flow.

internodal segment, equivalent electrical net-work The presence of the myelin modifies theelectrical properties of the internodal segments.In an electrical analysis of the nervous system,the internodal segments have both resistive andcapacitive aspects of their electrical properties.

interocular distance The separation of thetwo eye pupils when the observer is viewing dis-tant objects, approximately 65 mm. For the caseof two photographs taken by identical camerasfrom positions representing an observer’s eyes,a stereoscopic effect is seen if the camera lensesare separated by the correct interocular distance,and correct viewing distance.

intervals, musical The spacing in pitch orfrequency between two sounds. Two notesforming the musical interval of one octave havetheir frequencies as the ratio 2:1. The frequencyinterval is expressed as the ratio of the frequen-cies or the logarithm of this ratio.See alsooc-tave; frequency band; pitch, acoustic.

intrinsic conductivity The conductivity of apure semiconductor material as opposed to theextrinsic conductivity due to the presence of im-purities in the semiconductor material.

invariance of charge The charge of an elec-tron or proton that appeared in the equationsgoverning electrostatics and electrodynamics isinvariant under Lorentz transformation. In otherwords, the charge of a particle is independent ofits speed.

inverse square law The law stating that forany propagating wave, the rate of flow of energy

across a unit area perpendicular to the directionof propagation (intensity) from a point sourcevaries as the inverse square of the distance be-tween the source and the receiver. The inversesquare law is also one of the two fundamentallaws of photometry, and states that illuminanceor irradiance falls off at the inverse square ofdistance from the source.

inverter An electronic device that inverts theinput, i.e., produces a high output for a low inputand vice versa.

ion channels (cell) Molecular structures onthe surface of cells that regulate the flow of par-ticular ions through the cell membrane.

ionic current (cell) The movement ofcharged species both inside and outside of livingcells.

ionic current (cell), measurements The de-termination of the movement of charged speciesinside and outside a living cell.

ionization chamber An instrument used todetect the presence of ionizing radiation by mea-suring the current due to the ionization of themedium inside the chamber (usually a gas) bythe ionizing radiation.

ionography The study of the ions that moveacross cellular membranes.

ionophore Any molecule that transports aspecific ion across a cellular membrane.

ion pump A vacuum pump in which the re-maining gas molecules are ionized and drawnout by electric fields (it is usually used after aroughing pump).

ion transport (cell membrane) The move-ment of charged species through the membraneof cells.

irradiance A measure of the time rate oftransfer of radiant energy (radiant power) perunit area that flows onto or across a surface. Ra-diant energy is any energy transferred by electro-


magnetic waves without a corresponding trans-fer of mass. Also calledradiant flux density.

irradiation Application of radiation (any-thing that propagates as a ray, such as electro-magnetic waves, and the emission of radioactivesubstances) to a material body.

isochromatic line Lines that are of the samecolor, as in the interference fringes producedin birefringent materials. For a pencil beam ofplane-polarized rays incident obliquely on a uni-axial crystal cut perpendicular to the axis, therewill be two emergent pencils, plane-polarized atright angles. For a thin crystal, the two beamsare not separated on emergence and will inter-fere, giving a series of concentric light and darkrings. If white light is used, the rings will becolored, and due to symmetry about the axis,the color is constant around any circle centeredon the axis; such lines are termedisochromaticlines.

isochromatic surface Surfaces that give thelocus of points of constant phase difference

between the ordinary and extraordinary rays inuniaxial and biaxial media, which depend onthe direction of the light. Sometimes referredto asBertin’s surfaces.Observed isochromaticlines superficially resemble the intersections ofisochromatic surfaces with the crystal face, andthese intersections can provide qualitative ex-planation of the isochromatic line forms.

isoelectric point The pH value (hydrogen ionconcentration) of a solution at which the colloidparticles in the solution have zero net charge.The solution has minimum viscosity, conduc-tivity and osmotic pressure at this pH value.

isomagnetic lines Lines connecting points ofequality in magnetic properties within magneticmaterials.

isoplanatism In the diffraction theory ofaberrations, this refers to regions that are freeof coma, which is one of the five aberrations ofa lens with spherical surfaces. Skew rays from apoint object meet at the same point on the imageplane, instead of a pear shaped spot (coma).


Jjammers Units producing specific types ofjamming waveforms, e.g., a single-tone orpulsed noise jammer.Seejamming.

jamming Waveforms that are used on somefraction of transmitted symbols creating burstsof errors at the receiver output. A spread spec-trum system is particularly susceptible to it, andrelies on error-correcting codes combined withinterleaving to combat it.

jitter Type of analog communication linedistortion caused by the variation of a signalfrom its reference timing positions. This cancause data transmission errors, particularly athigh speeds. It also can create a short time lineor circuit instability.

Josephson junction A thin insulator sepa-rating two superconducting materials throughwhich electron hole pairs tunnel.

junction The interface where two types ofmaterials meet. Two different bandgap materi-als give a heterojunction (e.g., GaAs/AlGaAs),and different dopings in the same material com-ing in contact give a homojunction.See alsop-njunction.


Jjammers Units producing specific types ofjamming waveforms, e.g., a single-tone orpulsed noise jammer.Seejamming.

jamming Waveforms that are used on somefraction of transmitted symbols creating burstsof errors at the receiver output. A spread spec-trum system is particularly susceptible to it, andrelies on error-correcting codes combined withinterleaving to combat it.

jitter Type of analog communication linedistortion caused by the variation of a signalfrom its reference timing positions. This cancause data transmission errors, particularly athigh speeds. It also can create a short time lineor circuit instability.

Josephson junction A thin insulator sepa-rating two superconducting materials throughwhich electron hole pairs tunnel.

junction The interface where two types ofmaterials meet. Two different bandgap materi-als give a heterojunction (e.g., GaAs/AlGaAs),and different dopings in the same material com-ing in contact give a homojunction.See alsop-njunction.


Kkaleidophone A thin metal bar of rectangu-lar cross section carrying a bead at the upper endand clamped in a vice used to generate vibra-tions of prescribed frequencies in the differentplanes of vibration, thus forming characteristicpatterns. The frequency of vibrations is the samefor the two planes when the cross section of thebar is square or circular. The stiffness of thebar is greater in the plane of greater thicknessin case of a rectangular cross section, leadingto higher vibration frequency in this plane whencompared to the side of smaller thickness. Theratio of vibration frequencies in the two planescan be adjusted to the desired value by appropri-ate selection of the dimensions of the cross sec-tion. The kaleidophone was invented by Wheat-stone. A modification of the original design,with the bar divided into two parts, allows thecontinuous variation of the frequency ratio bychanging the location where the bar is clamped.

Kaleidoscope An optical toy consisting ofa tube and between two to four plane mirrors.Most kaleidoscopes consist of two or three mir-rors, and the mirrors are placed at an angle of45 or 60. It produces symmetrical patternsby multiple reflection by the mirrors. Front-surface-mirrors are used to generate a clear im-age. Objects are illuminated at one end of thetube and the image is observed through a smallhole at the other end.

Kalman filter A method of recursively es-timating a state from a series of measurements.The Kalman filter is not a filter in the sense of acircuit, but rather an estimator algorithm basedupon linear relationships of measurements to thestate being estimated, as linear transitions in thestate from measurement to measurement. Thepower of the kalman approach is seen when anindividual measurement does not allow full ob-servability of the state, but when all measure-ments are sufficient to observe the state beingestimated.

Kapitza boundary resistance The Kapitzaboundary resistance is the resistance that occursbetween liquid helium (either3He or4He) andsolids. Acoustic mismatch theory, taking intoaccount the different velocities of sound, pre-dicts the thermal resistance should beRK ∝(AT 3)−1 whereA is the area of the interface.This result is in reasonable agreement over alimited temperature range, but in marked con-trast to the boundary resistance between two di-electric solids; acoustic mismatch theory doesnot work very well for solid-helium interfacesabove roughly 1 K and below roughly 10 mK.In fact, depending on surface treatment, the ther-mal resistance can be several orders of magni-tude smaller than the theory predicts. It is alsofound that the resistances for liquid4He, liquid3He, and solid3He are all roughly the same atT = 1 K. The Kapitza resistance between3Heand metals with magnetic moments (even as im-purities) is even smaller than that of ultra-puremetals andRK ∝ T−1orT−2. There is evi-dence that this is due to dipole-dipole couplingbetween the3He nuclei and the magnetic mo-ments in the metal, but the question is still notcompletely resolved.

Kerr effect A nonlinear electro-optic effectthat makes certain substances behave like a uni-axial crystal (doubly refracting with a single op-tic axis) when placed in an electric field. Theoptic axis is parallel to the lines of force, andthe magnitude of the effect is proportional tothe electric field squared. The effect was firstobserved by Kerr in 1895 for glass, and is alsoseen for gases and liquids (nitrobenzene). TheKerr cell consists of a glass cell containing aliquid for which the effect is strong, located be-tween the plates of a capacitor. Polarized lightpassing through the medium across the field canbe interrupted at high frequency, and is usefulas an electro-optic shutter.

keying (1) Entering data by typing on a key-board.

(2) The process of changing some character-istic of a direct current or other carrier betweena set of discrete values in order to carry infor-mation.

(3) The process that causes modulation at atelegraph or radiotelegraph transmitter.


keying frequency, maximum The highestrate at which it is possible to key, and can bedependent on the response time of the system.

keying, frequency shift (FSK) A commonlyused method of frequency modulation in whicha one and a zero (the two possible states) are eachtransmitted on separate frequencies for the trans-mission of binary messages. The amplitude andphase are held constant in this technique.

keying, multiple frequency shift (MFSK)The same process as for frequency shift keying(FSK) but applied to the scenario where a mod-ulator in a coded digital communication systemprovides a one-to-one mapping of the channelsymbols into a set of signals.

keying, multiple phase shift (MPSK) Amodulation technique to convert binary data intoan analog form comprising a single sinusoidalwave. The phase of a carrier changes byπ ra-dians or 180. In this modulation technique,frequency and amplitude are held constant. Themodulator in a coded digital communicationsystem provides a one-to-one mapping of thechannel symbols into a set of signals.

keying, phase shift (PSK) Seekeying, mul-tiple phase shift.

kidneys, artificial A device for filtering wa-ter and wastes from the blood and producingurine.

Kirchhoff’s rules The rules found by G.R.Kirchhoff (1824–87). The first law (thecurrentlaw) states that the algebraic sum of the currentsthat meet at a point is zero. It is useful in parallelcircuits.

The second law (thevoltage law) states thatthe algebraic sum of the electromotive force inany closed path is equal to the sum of the prod-ucts of current and resistance in the path. Thesum of the voltage drop is equal to the sum ofthe voltage sources, which is called theeffectivevoltage.It is useful in series circuits.

Kirchoff’s law (emission and absorption)TheKirchoff radiation law,as derived from ther-modynamics, states that the ratio between “ab-

sorbtivity” (absorption) and emissive power isthe same for each kind of ray for all thermal ra-diators in thermal equilibrium, and is equal tothe emissive power of a perfectly black body atthe same temperature. The ratio depends onlyon the wavelength and the temperature. Theabsorption determines the loss of light on trans-mission through the material and there is not asimple relation with the absorptance that mea-sures the loss of light on a single reflection. Kir-choff’s law is a very general relation between theabsorption and emission of radiation by surfacesof different bodies. If the absorption is large, theemission must also be large. Black bodies ab-sorb all wavelengths completely, and also givethe largest amount of radiation at a given tem-perature.

klystron An electron tube in which the veloc-ity of the electrons is regulated/modulated. Dis-covered by Russel and Sigurd Varian in 1937,it is composed of an electron gun, a modulat-ing cavity (buncher), and a collecting cavity(catcher).

Block diagram of a klystron. The electrons are emitted

at the electron gun. Their velocity is modified by the

field generated byVb in the buncher to collect them into

bunches and they are then collected in the catcher.

Knight shift When measuring the nuclearmagnetic resonance frequency of a metal, thefrequency is found to differ from that of a freeatom of the same metal or that of a salt of thesame metal. This frequency shift, theKnightshift, is due to a change in the local magneticfield at the nucleus. The conduction electronsin the metal are polarized by the external mag-netic field,B0. These polarized electronic spinsinteract with the nucleus (the hyperfine interac-


tion), producing a magnetic field at the nucleus,thereby changing the resonance frequency. Themagnitude of the Knight shift is generally a fewpercent or less of the unshifted frequency.

Kondo effect The presence of magnetic im-purities can have drastic effects on the resistanceof metals at low temperatures. Near such animpurity, the spin of conduction electrons be-comes polarized by the impurity magnetic mo-ment, and other conduction electrons inelasti-cally scatter off the cloud of electrons surround-ing the impurity. This scattering flips the spin ofthe electrons involved. This increased scatter-ing rate produces an increase in the resistivityof the metal at low temperatures. TheKondoeffect resistivity,−ρK ln(T ), dominates at lowtemperatures and produces a minimum in the to-tal resistivity at the temperature when the effectbecomes important.

Korringa relation The spin-lattice relax-ation time,τ1, is related to the temperature ofthe lattice and electrons in a nuclear paramag-net according to the Korringa relation:

τ1T ≈ κ ,

whereκ is the Korringa constant. This rela-tionship is only approximate, as the Korringa

constant actually varies slowly with the mag-netic field and with temperature at extremelylow temperatures.See alsoBloch’s equations;nuclear magnetic resonance.

Kundt’s tube A tube used to measure thespeed of sound. It is a wide tube closed at oneend by a piston and at the opposite end by a di-aphragm attached to a rod clamped at its center.The tube is filled with air or other gas and con-tains a light powder. When longitudinal vibra-tions are excited in the rod, they are transferredto the gas in the tube through the diaphragm.The position of the piston is adjusted so that acertain number of standing waves forms in thetube. These waves are visualized by the pow-der in the tube that becomes lumped at nodes,giving the length of standing waves generatedin the tube. By knowing the frequency of thesound generated in the rod and the wavelength,the speed of sound can be determined.

Kundt’s tube.


Llagging current In a seriesRLC circuit,if the capacitive reactanceXC is less thanthe inductive reactanceXL, the current is lag-ging behind the voltage by a phase angleφ =tan−1[(XL −XC)/Z].

Lalande cell The zinc-copper oxide cell in-vented by F. de Lalande and G. Chaperson in1881 is known as theLalande cell. It consistsof a zinc anode and copper oxide cathode in aKOH electrolyte. It is a primary cell employingthe following chemical reaction:

Zn + CuO ——– Cu+ ZnO .

lambda leaks The bane of scientists work-ing at low temperatures, lambda leaks are leaksthat only appear when the apparatus is immersedin liquid helium below the lambda temperature,2.17 K. Such a leak occurs when the source ofthe leak (a crack, hole, etc.) is small enough toprevent gases and normal liquids from enteringdue to viscous drag on the gas or liquid. Theviscosity falls to zero below the lambda temper-ature, allowing lambda leaks to occur.

lambda phenomenon Seehelium-4, super-fluid; lambda point.

lambda point The lambda point is the tem-perature at which pure4He becomes a super-fluid, 2.1768 K. The name is due to the partic-ular shape of the specific heat vs. temperaturecurve at the phase transition.See alsohelium-4,liquid; helium-4, superfluid.

lambert A unit of luminance, equal to1/πcandle/cm2.

laminated core An iron core made up by thelamination of sheet iron or steel generally usedin transformers.

A laminated core used in a transformer.

lamp, arc An electric lamp in which light isgenerated by an arc. The arc is a spark producedwhen current flows through ionized gas betweenthe two electrodes. Carbon electrodes are oftenused. The electrodes are vaporized by the heatof the arc.

lamp, fluorescent A lamp emits light byfluorescence (luminescence). It contains gas(sodium vapor, mercury vapor, and so on) at alow pressure. The gas is excited by collisionswith electrons emitted by a cathode. When gasthat is raised to an excited state returns to aground state, ultraviolet light is emitted. Flu-orescent substances are coated on the inner sur-face of a fluorescent lamp and emit visible lightwhen they absorb the ultraviolet light emitted bythe gas in the lamp.

lamp, incandescent A lamp uses incandes-cence raised by an electrically heated filament.Incandescence is an emission of the light by sub-stances heated to a high temperature (> 3000K).

lamp, tungsten An incandescent lamp usestungsten as its filament. It is used as a standardlighting.

LAN Acronym for local area network.A communications network that serves userswithin a confined geographical area in the samebuilding or group of adjacent buildings. It ismade up of an interconnection of servers, work-stations, and a network operating system usinga communications link.Serversare high-speedmachines that hold programs and data shared bynetwork users. Theworkstationsor clientsarethe users’ personal computers, which can accessthe network servers. They can retrieve all soft-ware and data from the server. In small LANs,


which are easier to install and manage, the work-stations can be used as servers, and users cantherefore access data on another computer. Aprinter can be attached to a workstation or toa server and be shared by network users. Forlarge networks, dedicated servers are required.LANs run on a network operating system such asNetWare, UNIX, or Windows NT. The messagetransfer is managed by a transport protocol suchas TCP/IP or IPX. The physical transmission ofdata is performed by an access method such asEthernet or Token Ring, which is implementedin network adapters that are plugged into the ma-chines. Network adapters are interconnected bytwisted pair or optical fiber cable.

Langevin function This is a mathematicalfunction that is important in the theory of para-magnetism. Analytically it is given by:

L(x) = cothx− 1/x ,

x !, L(x) ∼= x/3, the polarizability ofmolecules having a permanent electric dipolemoment and the paramagnetic susceptibility ofa classical collection of magnetic dipoles givenby a function of this type.

language, programming/machine Thesedescribe a problem solution as a source pro-gram. Each language has a translator or com-piler to convert the source program to an ob-ject program. Machine language programminginvolves writing instructions directly in objectcode with only binary symbols 0 and 1.

laser An acronym that denotes the processof light amplification by stimulated emission ofradiation. In this process, stimulated emissionfrom an atom or molecule is used to amplify anelectromagnetic field, and this amplifier may becombined with a resonator to make an oscillator.The two-mirrors of the Fabry-Perot interferom-eter provide the optical resonator and atoms ormolecules in a metastable excited state, form-ing an inverted population of energy levels, andprovide the gain medium. The excited state isattained with an optical pump, or an electricaldischarge may provide the energy source. Theresonator mirrors feed photons belonging to thelaser modes back into the resonance cavity sothat their number can grow through repeated

interaction with the gain medium and providestimulated emission. One of the cavity mirrorsis the output mirror and is allowed to transmit 1to 2% of the light produced. This is a loss mech-anism that must be overcome if the gain mediumis to lase. The amplitude of the laser field willthen grow until a steady state is reached wherethe laser radiation rate is the same as the netrate at which energy is supplied. Many differ-ent kinds of lasers have been developed coveringthe spectrum fromIR toUV , some of which aretunable with a selectable wavelength, and someof which operate at more than one wavelength.Unlike ordinary light sources, such as glowingwires, the radiation from lasers is highly co-herent, highly collimated, extremely monochro-matic, and intense. The degree of control of vis-ible light with lasers approaches that of radiofrequency oscillators and microwave sources.Since the light from lasers is concentrated inone or a small number of modes, the photonoccupation number in each mode is very large,making the radiation field more classical thanconventional light sources.

laser beam, directionality of Laser beamsare exceptionally narrow. The width of a laserbeam is determined by the size of the open-ing provided by the partially silvered mirrorthrough which the beam exits. The main sourceof spreading out of the beam as it exits is fromdiffraction around the edges of the opening, sovery little spreading-out occurs. Photons emit-ted at an angle relative to the laser tube axis arequickly reflected out the sides of the tube via thesilvered ends, which are carefully arranged to beperpendicular to the tube axis. Since the emit-ted photon in the stimulated emission processtravels in the same direction as the stimulatingincoming photon, the net effect of arranging thecavity mirrors this way is to make the beam fromthe laser highly directional.

laser beam, monochromaticity of In stimu-lated emission, an incoming photon induces anelectron in an excited state to change energy lev-els, but only if the incoming photon has an en-ergy that exactly matches the difference of en-ergy between the two states. This makes stimu-lated emission similar to a resonance process, inwhich the incoming photon triggers an electron


to change energy states only if energies, and thusphoton wavelengths, exactly match. When thestimulated emission only involves a single pairof energy levels, the output beam has a singlewavelength and the radiation in the laser beamis monochromatic. The early lasers each hadtheir own characteristic wavelength, dependingon the material used, and could not be tunedvery far, just the width of the laser line. Theoutput wavelength is determined only roughlyby the amplifying medium. It is more preciselydetermined by the tuning of the laser resonator.By applying a magnetic field or changing thetemperature of a solid laser material, more tun-ing could be obtained. With the development ofdye lasers, it is possible to cover the range ofwavelengths from 350 to 950 nm continuously,throughout the entire visible spectrum.

laser, cavity of In a laser, the two-mirrorFabry-Perot interferometer serves as the opticalresonator or cavity. Laser processes begin whenan atom or molecule in the metastable excitedstate spontaneously emits a photon parallel tothe axis of the laser cavity. This photon causesother excited atoms or molecules to emit a pho-ton via the stimulated emission process such thatthe emitted photon travels in the same direc-tion as the original one and exactly in phase.To ensure that more photons are created in anavalanche of stimulated emission from a multi-ple reflection process, both ends of the tube aresilvered to form mirrors that reflect the photonsback and forth. So that some of the photons mayescape the tube and form the laser beam, one ofthe mirrors is only partially silvered and servesas the output window for the laser beam.

laser communications Laser communica-tions represent a form of optical communicationwith a laser as the light source. A laser rep-resents a single-frequency phase-coherent lightbeam. Telecommunications using laser beamsis distinguished by lasers’ very wide frequencyspectrum, lasers’ efficient use of transmissionpower, and the laser beam’s precise spatial di-rectivity. The visible spectral band over whichlasers operate spans over half a trillion mega-hertz; thus, one laser beam may theoreticallyoffer a transmission rate surpassing that of theentire radio-frequency spectrum. Because of the

precise spatial directivity and spectral coherencyof laser beams, they are highly power-efficientand useful for long distance communicationssuch as satellite or spacecraft telecommunica-tions, where the satellites or spacecrafts typi-cally have very limited power resources. How-ever, laser beams are highly vulnerable to ob-struction by fog, rain or snow in outdoor atmo-spheric channels. Laser beams are often trans-mitted within protective pipes for earth-boundtelecommunications. Laser fiber optics trans-mission is typically in baseband, with the in-formation signal represented as a sequence ofon-and-off light pulses. Semiconductor photo-diodes represent the most common optical com-munication receivers.

laser, effect on biological tissues The ab-sorption of electromagnetic energy by biologi-cal tissues means that energy is deposited in tis-sues when irradiated. For the specific exampleof lasers, the physiological response depends onthe wavelength and intensity of the laser light aswell as the degree of focusing of the light.

laser, efficiency of Ratio of the output powerto the input power. The overall efficiency is aproduct of efficiency factors, which are individ-ually defined, depending on the mechanisms ofenergy transfer in the laser. Thepumping effi-ciencyis the ratio of the total pump power ab-sorbed by the gain medium to the electrical inputpower into the pump source, and this efficiencyis composed of efficiency factors, such as theratio of lamp radiation within absorption bandsof the gain medium to electrical input power.Another factor is the fraction of the electricalinput power that results in potentially useful ra-diation. Still another is the efficiency of transferof the useful radiation from the pump source tothe gain medium, and so on. Theenergy extrac-tion efficiencyis the ratio of actually extractedpower to available power.

laser, gain in Increase in signal power intransmission from one point to another. Unitsfor power gain in common engineering usageare decibels or db. An active medium ex-hibits gain rather than absorption at a certainfrequency. The bandwidth of an active mediumis the full distance between frequencies at which


the power gain has fallen to 1/2 its peak value,corresponding to 3 dB down from peak gain.This amplification bandwidth is considerablysmaller than the atomic linewidth, especially athigher gains — an effect calledgain narrowing,which reduces useful bandwidth of high gainamplifiers. Gain is synonymous withamplifica-tion.

laser induced fluorescence The spontaneousre-emission of radiation that occurs after a gasis put into an excited state by illumination witha high-power laser. The amount of spontaneousemission or fluorescence from an atomic tran-sition is proportional to the upper-level popula-tion of the transition. Since the upper levelsare initially sparsely populated, the observedlaser induced fluorescence is a measure of thelower level number density. This provides a use-ful technique for measuring the population ofmetastable levels, which cannot decay by giv-ing off a photon. Laser induced fluorescence(LIF) diagnostics operate by pumping electronsout of the metastable state to a higher excitedstate which can decay radioactively; measure-ment of the LIF can be used to determine thecross-section for excitation into the metastablestate.

laser materials, in general The active lasermedium that emits radiation from stimulatedelectronic transitions to lower energy states.Laser operation has been demonstrated in a widevariety of media, but only a few types of mate-rials have been developed for commercial use.For a material to be useful as a laser there arecertain optical, chemical, thermal, and mechan-ical properties it must have. The ions providingthe optical emission must be able to efficientlyabsorb pump energy, and emit efficiently at thedesired frequency. Often, there is not a singleion species that can do both efficiently, and com-binations of ions are used in the same host mate-rial: one to absorb pump energy (sensitizer ion)and one to provide the lasing (activator ions),with a strong overlap in spectra of both types ofion. For a laser material to be useful commer-cially it must also be economically produced insufficient quantity at high quality. To be usefuloutside the laboratory, the material must be sta-ble and robust in its environment. It should be

chemically stable and preserve the ion valencestate and resist ion diffusion out of the opticalpath, while resisting internal stresses that maybe either thermally or optically generated. Therequirements for lasing materials are often mu-tually contradictory and no single material canmeet all of these criteria simultaneously. Thusfor successful design of a laser system, a widevariety of materials must be considered and se-lection made based on a thorough understandingof their optical and other physical properties.

laser, medical application The use of laserradiation for a therapeutic purpose, such assurgery on the retina.

laser mode The field pattern of light inan optical resonator. The optical resonator ofmost lasers consists of two reflectors facing eachother, aligned so that multiple reflections takeplace. An analysis of interference by multiplereflections of light in this resonator reveals thenature of the axial and transverse modes. Asin the Fabry-Perot interferometer, there is fulltransmittance at a series of discrete wavelengthsseparated by a free spectral range determined bythe wavelength and the separation of the mir-rors. Each of these frequencies is a spectralmode or axial mode. These modes representresonant frequencies that are exactly an integralnumber of half-wavelengths along the resonatoraxis between the mirrors. In a real cavity, anywave as it bounces between mirrors will alsospread transversely due to diffraction, and willalso distort in transverse amplitude and acquirediffraction ripples in even a single pass throughthe laser cavity. Analytical or computer calcu-lations may be carried out to find the change ofthe transverse field pattern, with repeated passesthrough the laser cavity, as pioneered by Fox andLi. Usually these are carried out with the lasergain omitted for simplicity; for any given cavitywith finite diameter and mirrors, there will bea distinct set of transverse amplitude and phasepatterns, which self-replicate in form, thoughare reduced in amplitude after each round tripthrough the cavity. These are termedtransverseeigenmodesor transverse cavity modesand de-pend on the detailed shape and curvature of theend mirrors. They are analogous to the trans-


verse modes of electromagnetic waves in closedwaveguides.

laser, pulsed A laser with an emission wave-form that consists of short duration bursts, eachcharacterized by a rise and decay time. Betweenpulses, the laser is inactive, in contrast with thecontinuous waveorCW laser, which has a con-stant output. A widely used technique to shortenthe pulse duration and allow the laser pumpingprocess to build up a larger than usual popula-tion inversion isQ-switching,in which one ofthe end mirrors is effectively blocked to removefeedback, which is subsequently restored to itsusual large value, dumping the entire populationinversion in a single short laser pulse.

laser, safety The knowledge of operating alaser in a manner that results in no injury to theuser or any bystanders.

laser surgery The use of laser light to ac-complish a surgical procedure, such as remov-ing growths on the skin.

laser therapy The use of laser light for ahealing purpose.

lattice vibration Periodic oscillation of theatoms in a crystal lattice about their equilibriumpositions.

law of mass action The law of mass actiondescribes the equilibrium behavior of a varietyof chemical systems in solution and gas phasesby stating that for a reaction of the type,

aA+ bB → cC + dD ,

the thermodynamic equilibrium constant isgiven by

Keq =[C]c[D]d

[A]a[B]b.

The coefficientsa, b, c, d are stoichiometric co-efficients andA,B,C,D represent chemicalspecies. The square brackets indicate that theconcentration of the chemical inside is at equi-librium. Therefore, the value of the equilibriumconstant at a given temperature can be calcu-lated only when the equilibrium concentrationsof the reaction components are known. Notice

that the unit forKeq depends on the reaction be-ing considered because it is determined by thepowers of the various concentration terms.

Knowledge of the equilibrium constant fora reaction allows for the prediction of severalimportant features of the reaction. Namely, thetendency of the reaction to occur (but not thespeed of the reaction), whether or not a givenset of concentrations represents an equilibriumcondition, and the equilibrium position that willbe achieved from a given set of initial concen-trations.

The tendency of a reaction to occur is indi-cated by the magnitude of the equilibrium con-stant. IfKeq is much larger that 1, the equi-librium lies to the right and the reaction systemwould mostly consist of products. IfKeq is lessthan 1, the equilibrium lies to the left and thereaction system would consist of mostly reac-tants. This would mean that the given reactiondoes not occur to a very significant extent.

Seevan’t Hoff’s law.

leader stroke A thin, highly ionized, andhighly conducting channel that grows from oneelectrode toward another of opposite polarity ina gas (usually air) during the initial stage of aspark discharge. It is neutralized when the tipof the leader reaches the second electrode, trig-gering the next phase of the discharge known asthe return stroke. The leader stroke is clearly de-tectable in spark discharges through large gapssuch as lightning flashes which begin with thegrowth of a leader from cloud to earth or earth tocloud. See alsolightning flash; lightning stroke.

leading current In anRLC circuit, if the ca-pacitive reactanceXC is more than the inductivereactanceXL, the phase angleφ = tan−1[(XL−XC)/Z] is a negative quantity. In this case, thecurrent reaches its maximum before the voltagedoes and the current is called the leading current.

leakage current The undesirable current that“leaks” through an insulator.

leakage fields Fields that extend beyond theregion over which they were designed to exist.

least distance of distinct vision Conven-tionally, the near point of a normal eye is a


distance (nearest distance of distinct vision) ofabout 25 cm (10 in). This is called theleast dis-tance of distinct vision.It is the average of theleast distance between the eye and an object suchthat the object can be seen in focus by an unaidedeye. It is used to compare the magnifying pow-ers of microscopes. For a simple microscope(magnifier), the angular magnificationM is de-scribed by the least distance of distinct visionLand the object distances:

M =L

l

(=

25l

),

where distances are measured in cm. Conse-quently, if the image is viewed at infinity,

M =L

f,

wheref is the focus length. If the image isviewed at the near point of the eye (image dis-tances′ = −L),

s =Lf

L+ f,

M =L

f+ 1 .

These angular magnifications are estimated bythe least distance of distinct vision. However,actual magnifications depend on the particularobserver. It is also known as thedistance ofmost distinct vision.

lecher wires (1) Two parallel wires usedto measure high radio frequencies. The twowires, which are a few wavelengths long for thefrequency to be measured, are either adjustedby sliding a shortening bar along them, or ter-minated at their far end and varied electricallyby tuning a capacitor that is in series with thewires. When connected to the high frequencysource, standing waves will be generated in thewires when their lengths are multiples of half-wavelengths. Measurement of the correspond-ing node or anti-node positions allows the fre-quency to be calculated.

(2) Two parallel straight wires, as in a two-wire transmission line, with a sliding short cir-cuit copper strip between them. The wires canbe tuned to a specific frequency of an oscilla-tory electrical wave by moving the strip along

the wires. Generally, lecher wires are used in themicrowave frequency range as part of a waveme-ter for determining wavelength. They can alsobe used as a tuned circuit or an impedancematching device.

Leclanche cell The zinc-carbon cell origi-nally invented by Georges Leclanche is knownas aLeclanche cell. It consists of a zinc an-ode and a manganese dioxide cathode. TheLeclanche cells are noted for their low cost andgood shelf life. It is a primary cell employingthe following chemical reaction:

Zn + 2MnO2 → ZnO · Mn2 O3 .

Lenard spiral A spiral of bismuth wire,mounted between mica plates, that is used tomeasure magnetic field strength. It has smallor negligible inductance while its resistance isstrongly dependent on the strength of the mag-netic field directed orthogonally to the axis ofthe spiral. An increase in the magnetic field re-sults in an increase in the resistance and viceversa. Thus a measurement of resistance can berelated to a magnetic field strength.

lens A piece of isotropic, transparent mate-rial that has two surfaces with a common axis.The common axis is called theoptical axis.Thepoint on the surface of the lens where the opti-cal axis crosses is called thevertex of the lens.The geometrical center of the lens is called theoptical center. A light ray that passes throughthe optical center will not be deviated. A lensis used for refraction of light. Usually, polishedglass or molded polymer is used as the material.There are various kinds of lenses and their sur-faces are usually spherical (seelens, spherical).The surface of others are aspherical, cylindrical,parabolic, toroidal and so on. The curvature ofthe surfaces affect the functions of the lens: fo-cus length, aberration, astigmatism, and so on.The surface of a lens, calledconcaveor convex,curves inward or outward, respectively. Lensesare divided into positive or negative (converg-ing or diverging). A positive lens causes paral-lel rays of light to converge and a negative lenscauses them to diverge. Theshapeof the lensrefers to the shape of the periphery of the lens.


Theformof the lens indicates the relative alloca-tion of the curvatures. Lenses are classed as thinor thick. A thin lensis a lens whose thickness issmall enough to be neglected in the calculationof optical quantities of the lens. A lens shouldbe treated as athick lenswhen a precise solutionof a lens problem is required. A typical problemis the design of a camera lens. Thefocal lengthof a thin lens is the distance between the opticalcenter and the focal point of the lens. The im-age distance and the object distance of a thin lensare measured from the vertices. Solving a lensproblem, the thin-lens focal length, the imagedistance, and the object distance of a thick lensshould be measured from the principal planes ofthe lens.

Lens.

lens, achromatic An optical system that iscorrected for chromatic aberration. Usually,a combination of positive lenses and negativelenses of different refractive indices is used. Anachromatic lens is designed so that the disper-sions of the two lenses neutralize each other,and their refractions do not neutralize each other.The simplest achromatic lens consists of a pos-itive lens and a negative lens, and is called anachromatic doublet.It is used as an objective ofa telescope and is also known as anachromat.

An achromatic lens (achromatic doublet).

lens, antiflex An antiflex objective is usedto visualize structures that would be invisible innormal bright–field microscopy. An example isthe observation of cells grown on the bottom of aPetri dish visualized by incident-light-reflectioncontrast techniques.

lens, apochromatic An optical system thatis highly corrected for both spherical and chro-matic aberration for two or more colors and isused as a microscope objective.

lens, astigmatic A toroidal (toric) lens thatis used in eyeglasses to correct astigmatism. Itis also known as anastigmat.

lens, Barlow A lens system of one or morenegative lenses that are placed just ahead of thefocal plane of the objective, between the objec-tive and eyepiece, in a telescope. It is used toincrease the effective focal length and therebyincrease magnification. Plano-concave lensesare usually used.

lens, blooming of The process of coating atransparent, thin film on a lens to reduce the re-flection of light at the surface of the lens. Asubstance, such as a magnesium fluoride, is de-posited on the lens to form a thin film with athicknesst of one quarter wave lengthλ c =λ/n c. The thickness of the coatingt is

t =λ c

4,

=λ

4n c,

where the refractive index of the substance isn c.Usually evaporation is used for the deposit. Thesubstance should have a lower refractive indexthan the lens to be bloomed. The reflectivityR of the interface of the different media with


indicesn1 andn2 is

R =(n2 − n1

n2 + n1

)2

.

The desirable refractive index of the coatingmaterial is dependent on this relationship andthe number of layers to be deposited on the lens.For example, with one layer coating, the desir-able refractive index of the coatingn c is

n c =√n2n1 ,

wheren2 andn1 are the refractive index of freespace and one of the lens, respectively.

Lens blooming.

lens, concentric A lens with two sphericalsurfaces. The spherical surfaces have the samecenter.

lens, condenser A lens system that is usedto concentrate as much light from a source aspossible. Also known as acondensing lens,it is used in an instrument of illumination forvarious kinds of optical systems. A condenserlens should be free from aberration. Usually adouble plano-convex condenser lens is used. Itis also used in an projection system. AnAbbecondenseris well known. It consists of a pairof lenses and has a variable large-aperture. AnAbbe condenser is used as a microscope objec-tive.

lens, converging (lens, positive) A lens thatcauses parallel rays of light to converge on aprincipal focus on the axis of the lens. The cen-ter of a converging lens is thicker than the edgeof the lens. A converging lens has a positivefocal length. In a converging lens, the secondfocal point lies on the opposite side of the light

A condenser lens.

source. (See alsolens.) With a converging lens,the object closer to the lens than the first focalpoint will form a virtual image, not a real image.A biconvex lens, a plano-convex lens, and a con-verging meniscus are well known as converginglenses.

lens, crossed A kind of spherical lens de-signed with particular radii of the curvature ofthe two surfaces in order to realize a minimumspherical aberration for parallel incident rays.The radii are dependent on the refractive indexof the lens.

lens, crystalline An elastic, jelly-like lensof the eye that is elastic biconvex. The refrac-tive index of the crystalline lens is high (about1.4), and it is highest in the center and lowest atthe equator. It lies between the anterior cham-ber, which is filled with aqueous humor, and thevitreous, filled with vitreous humor. Its highrefractive power can be altered by varying itsthickness which is altered by the ciliary muscle.

lens, cylindrical A lens in which one or bothof its curved surfaces are a portion of a cylin-der. It has axial astigmatism and is used in thecorrection of visual deficiencies. A planar cylin-drical lens is used for correcting astigmatism ofthe eye.

lens, decentered A lens whose optical centeris different from the geometrical center of therim of the lens. A decentered lens works as alens combined with a weak prism.


Crystalline lens.

Cylindrical lens.

lens, diverging (lens, negative) A lens whosefocal length is negative. A diverging lens causesrays of parallel light to diverge. Biconcave,plano-concave, and diverging meniscus areknown as a diverging lens.See alsolens.

lens, equation, Newtonian form One of thelens-users equations,this form uses image dis-tance and object distance measured not from ver-tices but from the focal points of a lens. An equa-tion that treats the relationship between the dis-tances between two conjugate points and theirrespective foci. The product of the distances isequal to the square of the focal length of thelens. The distance between an object and a fo-cal pointxo, the distance between an image andanother focal pointxi, object sizeyo, and imagesizeyi, and lateral magnificationm, satisfy theequation;

m =yi

yo=

f

xo=xi

f,

where the focal length isf . These relations de-rive from the Newtonian form lens equation as

xoxi = f2 .

It can be used with both thin and thick lenses.

Newtonian form lens equation.

lens, equivalent A lens or a system of lensesthat forms almost the same image as a given lensor a given system of lenses.

lens, landscape A simple meniscus lens oran achromatic doublet (see lens, achromatic)with its stop rate greater than f/11. The angleof the field is limited to about 40 because ofthe oblique astigmatism of a landscape. It isalso called anachromatic meniscus.A menis-cus of which the concave side facing the objectis corrected for both astigmatism and coma stillhas spherical and chromatic aberration and dis-tortion. A combination of two landscape lenseswill improve its aberration.

Landscape lens.

lens maker’s equation Also called thelensequationor lens maker’s formula,this equationis used with thin lenses. The focal powerP andthe focus lengthf of a thin lens can be calcu-lated by the equation. It uses the fact that therefractive powerP of a thin lens is the algebraicsum of the pair of surfaces of the lensP1, P2;P = P1 + P2. When a thin lens of indexnl issurrounded by a medium of indexnm, the pow-ers of the surfaces, whose radii areR1 andR2,


are calculated by the surface power equation

P1 =nl − nm

R1.

Therefore, the power of the lensP is obtainedas

P =1f

= (nl − nm)(

1R1

− 1R2

).

It is used to decide the two radii of the surfacesof the lens. It is important for lens makers todecide how to grind the lens.

lens, negative Seelens, diverging.

lens, objective The lens of an optical sys-tem that is nearest to the object to be observed,it usually has a short focal length. The objec-tive lens of a microscope forms a real air image(seeimage, primary). For projection with a mi-croscope, the primary image is formed ahead ofthe first focal length of the ocular which forms areal image again. For visual observation, the pri-mary image is generated inside the focal lengthof the ocular as a magnifier.

Image formation in a telescope.

lens, positive Seelens, converging.

lens, power of (focal power) The power of alens shows the ability of a lens to converge theparallel rays of light. It is equal to the recip-rocal of the focal length, the distance betweenthe optical center of a lens and its principal fo-cus. The powerP of a thin lens is the algebraicsum of the pair of surfaces of the lensP1, P2;P = P1 + P2. The angular magnification of alensMθ and power of the lensP have a relation-shipMθ = 1

4P . This equation is known as thequarter-power equationand it applies to the lensthat is used without accommodation; an image

is seen at infinity and the lens is set close to theeye. With accommodation,Mθ = 1

4P + 1.

lens, speed of The speed of a lens indicateshow much light the lens can gather and transmit.The extent of the energy of light gathered by thelens from a light source is proportional to thearea of the lens. The gathered energy is inverselyproportional to the area of corresponding imagethrough which the gathered light passes. Theenergy flux density passing through the imageis proportional to the(D/f)2, for a lens of focallengthf and diameterD. Thef-stop(f-number)is described asf/#(= f/D). D/f is calledthe relative apertureof the lens. Thespeedofa lens is measured by the f-number of the lens.The f-numberof a lens is equal to the ratio ofthe focal length to the diameter of aperture ofthe lens. A lens with a lower f-number (morerapid lens) can gather and transmit more rays oflight. Thedepth of fieldof a lens is equal to theratio of the focal length to the speed of the lens.

lens, spherical A lens whose surfaces formportions of spheres. It is easier to make sphericallenses than other kinds of lenses. Therefore,this kind of lens is widely used. Aspherical ordeformed spherical lenses are used, because aspherical lens has aberrations. A spherical lenssometimes refers to a lens of complete sphereform.

lens, split (a billet split lens) A lens that iscut into two parts along the optical axis of thelens. It is used in an interferometer.

lens, Stokes A variable-power compoundlens made up of cylindrical lenses of equal powermounted so that the angle between their axes canbe varied.

lens, telephoto The telephoto lens is a kindof camera lens, especially for asingle-lens reflex(SLR) camera. It has a long focal length, usu-ally longer than 80 mm and is used for taking aphotograph of an object far away from the cam-era. Its field of view is very narrow. To avoidmaking a telephoto lens too long, a negative lens— of which the focal length is shorter than theother positive lens — is used as the second lens.


Telephoto lens.

lens, thick A lens whose thickness is suchthat the effect of the thickness cannot be ne-glected to consider the optics of the lens or thelens system. Whether a lens is regarded as thinor thick also depends on the precision required.The principal planes of a lens are a pair of planesthat are perpendicular to the optical axis of thelens. The image of any object on another planeis formed on the other plane with unity lateralmagnification.

Principal planes of thick lenses.

With a thick lens, the focal length, image dis-tance, and object distance of the lens should bemeasured from the principal planes of the lens.

lens, thin A lens whose thickness is insignif-icant enough to be neglected in the calculationof the optical quantities of the lens. The thick-ness of a thin lens is sufficiently smaller thanthe focal length of the lens, radii of the pair ofthe curvatures, the image distance, and the ob-ject distance. Whether a lens is regarded as thinor thick also depends on the precision required.The focal length of a thin lens is the distancebetween the optical center and the focal pointof the lens. The image distance and the objectdistance of a thin lens are measured from thevertices. The rays of light, which pass throughthe first focal point of a lens, will form paral-lel rays just after the rays pass through the lens.The second focal point is formed by a set of par-

allel rays of light which pass through a thin lens.With a positive lens, the second focal point lieson the opposite side of the light source. Usually,“the focal length” of a lens means the second fo-cal length of the lens. With a converging lens(positive lens), the object closer to the lens thanthe first focal point will form a virtual image,not a real image.

lens, thin, combination of The focal lengthand power of the combination of thin lensesf ,P can be calculated from the focal length of thelensesfi, Pi, i = 1, . . . , n as:

1f

=n∑1

1fi,

P =n∑1

Pi .

The combination of thin lenses can be describedas a product of the optical matrix of each thinlenses.

lens, toroidal Also known as atoric lens,this is a lens whose surface forms parts of toricsurfaces. A toroidal lens is used for the correc-tion of astigmatism on both of the perpendicularmeridian planes. Theplano-cylindrical, sphero-cylindrical, andspherotonicare troidal lens.

Lenz’s law of induction This law states thatwhen a conductor moves with respect to a mag-netic field, the currents induced in the conductorare in such a direction that the reaction betweenthem and the magnetic field opposes the motion.

Leyden jar An early form of capacitor thatwas first thoroughly investigated by Pieter vanMusschenbroek of the University of Leyden in1746. It consisted of a stoppered glass jar filledwith water and a nail piercing the stopper anddipping into the water. Holding the jar in onehand then touching the nail to an electrode of anelectrostatic machine and finally disconnectingit caused the jar to acquire and store charge. Anelectric shock was experienced when the freehand touched the nail. John Bevis modified theLeyden jar so that inner and outer surfaces werecovered with metal foil. This was a closer ar-rangement to modern capacitors.


lidar Acronym for light detection and rang-ing. Method for determining positions of distantobjects by use of a laser beam. The process in-volves reflection of laser light from an object andthe determination of time required for a beam toreach the object and return to the emitter. Dis-tance is then computed from the product of lightspeed and time. Similar to radar, but based onelectromagnetic waves in the visible part of thespectrum.

light, absorption of The process wherebylight is absorbed by the atoms or molecules of amaterial. The energy of absorbed light in gen-eral serves to excite electrons into higher energystates in the material, from which they can sub-sequently decay by emission of electromagneticradiation with less energy (frequency) than thatof the absorbed radiation. Energy retained in thematerial appears as thermal energy in the atomsand molecules of the matter.

light, corpuscular theory Early theory oflight which asserted that light consisted of astream of particles referred to ascorpuscles.The theory (alternatively known as “emissiontheory of light”) was most successfully ex-pounded by Issac Newton (1642–1727), whopostulated the existence of different kinds of“light-particles” that could excite a materialether filling all of space into different types ofvibrations so as to produce the sensation of dis-tinct colors. The notion of light as a streamof localized entities, inherent in the corpus-cular theory, was revived in the quantum the-ory of light, which interprets light on a sub-microscopic scale in terms of “particles” of en-ergy calledphotons.

light, emission of Process whereby light isemitted by the atoms or molecules of a material.Sources of light emission are

1. accelerated charges (electrons or ions), and

2. excited atoms and molecules. Light emit-ted by accelerated charges results from the con-version ofkinetic energyof charges into radia-tion energy. Light emitted by excited atoms andmolecules is produced (primarily) by the con-version ofpotential energy(in excited states)into radiation energy.

light emitting diode A semiconductor deviceconverting electrical energy into light, whichis heavily used as a light emitter in displays,etc. Commonly, an LED is a forward biased p-njunction diode in which the light is emitted whena hole and electron recombine. The most recentLEDs in use are GaN, giving blue/green light,and SiC LEDs.

light, energy of Energy associated with lighton the basis of its capacity to exert forces (whichcan do work) on electric charges. In the electro-magnetic wave theory of light, the energy trans-ported across a unit area per second by a lightwave is represented by the magnitude of a vec-tor quantity,S, referred to as thePoynting vector,and defined in terms of the electric and magneticfields of the wave,E andB, via the relation

S =1µ

(E×B) ,

where µ is the permeability constant of themedium. Alternatively, in the quantum theory ofradiation, the energy of light is said to reside indiscrete entities termedphotons,each of whichis associated with a quantized amount of energyhf , wheref is identified with the frequency ofthe radiation andh is a fundamental constantof nature known as Planck’s constant, with the(approximate) valueh = 6.626075 · 10−34 J.s.

light, momentum of Linear momentum as-sociated with light on the basis of its capacityto exert forces that transfer momentum to (theelectrons, atoms and molecules of) material me-dia. In the electromagnetic wave theory of light,the momentum transported across a unit area persecond by a light wave is represented by the ratioof the Poynting vector,S, divided by the speedof light, c. Alternatively, in the quantum the-ory of radiation, the momentum of light is as-sociated with discrete entities termedphotons,each of which is characterized by a quantizedmomentum equal tohf/c, wheref is the “fre-quency” of the light andh is Planck’s constant,(approximately) equal to6.626075× 10−34 J.s.Seelight, energy of.

light, monochromatic Light consisting ofelectromagnetic waves (or photons) character-ized by a single frequencyf . The term, sig-


nifying single color, derives from connectionbetween different frequencies of light and dis-tinct colors in the visible spectrum. It con-trasts with non-monochromatic light consist-ing of a combination of electromagnetic wavesof different frequencies. Since in practice,all physical light beams (of non-infinite ex-tent) must consist of mixtures of electromag-netic waves with frequencies within some non-zero range, monochromatic light is in prac-tice quasi-monochromatic— meaning that itconsists of electromagnetic waves having onlya small spread of frequencies about a central(dominant) frequency.

light, monochromatic, biological actionCertain monochromatic single–wavelength lightseems to have a biostimulation and sometimestherapeutic effect when applied to cells in tis-sue. Accelerated tissue repair, stimulated bythe monochromatic light, is useful in fields ofdentistry, dermatology, and neurology. Theseeffects are sometimes called photo–stimulation.

Wavelengths used are usually in the 630 to950 nm range. The character of the light source,continuous or pulsating, seems to have an effecton cell functioning. For example, with contin-uous light, relief of pain, relaxation of musclefibers, and reduced swelling may be observed.This may be due to an increase in cellular bloodflow. On the other hand, a pulsating light sourcemay stimulate protein production and calciumaccumulation, which might result in acceleratedhealing of damaged tissue.Seered light, healingeffect.

lightning arrester (surge protector) A de-vice used to protect electronic and electricalequipment from damage due to voltage spikesin power lines, communication lines, as well asother long wires during lightning surges. Thearrester is a shunt device with an impedancethat changes from high to low during the spikeand thus provides a bypass circuit preventingdamage to the attached apparatus. Examples oflightning arresters are short air gaps with a spe-cific breakdown voltage, or semiconducting el-ements with a resistance that becomes very lowabove a predetermined voltage.

lightning conductor (lightning rod) A rod,usually made from copper, mounted as high aspossible above buildings, trees, power lines andother structures to be protected against lightningstrokes. The upper end of the rod consists ofone or more sharp points while the lower endis firmly connected to conductors embedded inthe ground. Lightning rods primarily serve toneutralize the charge on a nearby cloud by con-ducting negative charge to or from the groundand through the atmosphere at a relatively slowrate. As a result, the probability of a direct light-ning stroke is reduced. As an approximate rule,the rod acts as a shield over a cone with a radiusat ground level equal to the height of the rod.

lightning flash (or discharge) Refers to allthe phenomena associated with electrical dis-charges between clouds, and between cloudsand the earth. Its main constituents are the leaderstroke and the return stroke. The latter producesthe highly luminous part of the discharge. Onaverage, a lightning flash lasts for 200 millisec-onds and consists of several pulses each of some10 milliseconds separated by 40 milliseconds.See alsolightning stroke.

lightning stroke (return stroke) A highlyluminous channel triggered by the leader strokeof a lightning flash. The luminosity travels fromground to cloud as a large current travels fromground up to neutralize the highly charged leaderchannel at a velocity of 0.1 to 0.3 of the speed oflight. The most dangerous effects of the light-ning stroke are connected with the current peakwhich can reach values of 100 kA.

lightning surge Large voltage transient intransmission lines, communication lines, andother long wires induced by lightning strokes.The damage from a lightning surge to any ap-paratus is minimized by an installed lightningarrester.See alsolightning stroke; lightning ar-rester.

light pipe A flexible transparent polymer rodthat transmits light from one end to the othereven when it is bent. It is used to guide a lightbeam. The principle of total internal reflection isapplied to light pipes. The angle of the incidenceof the rays of light is larger than a specific critical


angle, and the interface between two differentmedia acts a mirror. The refractive index of thelight pipen should be higher than the refractiveindex of the surrounding medianm. The criticalangleφc is

φc = arcsin(nm

n

),

wherenm andn is the refractive indices of thesurrounding media and one of the light pipe, re-spectively. Usually,φc = arcsin

(1n

). It is used

in various fields, as an instrument for internalhuman organs, communications system, and soon.

Optical fiber.

light, pressure of Pressure associated withlight on the basis of its capacity to exert forceson electric charges that produce pressure (de-fined asforce per unit area) on a material ob-ject. In the electromagnetic wave theory of light,pressure exerted by light on an object is directlyproportional to the magnitude of the Poyntingvector of the light wave,S, divided by the speedof light c. The relationship between pressureand the Poynting vector follows from equiva-lence between rate of transfer of linear momen-tum across the unit area of a surface and theforce per unit area on surface.Seelight, energyof; light, momentum of.

light pulse Propagating an electromagneticwave (with a frequency in the visible part of

spectrum) that has a finite (usually short) limit intime,∆t, and a corresponding finite limit alongthe direction of propagation. A light pulse cor-responding to a length in time∆t must consistof a combination of “wavelets” with a range offrequencies,∆f , approximately equal to1/∆t.

light, speed of The rate of propagation oflight (and other electromagnetic radiation). Thespeed of light in a vacuum is a fundamental con-stant of nature (denotedc) with a value (approxi-mately) equal to2.997925×108 m/s. The speedof light in a macroscopic medium,v, can neverexceedc, and is expressed in terms ofc and theindex of refractionof the medium,n, via therelation

v =c

n,

where the value ofn in general depends on boththe properties of the medium and the frequencyof the light.

light, unpolarized Light consisting of an (ef-fectively) uniform mixture of light waves havingall possible polarizations. Corresponds in prac-tice to light in which the state of polarizationvaries randomly within a time interval less thanthe minimum time required for a measurement,such that no (resultant) state of polarization canbe detected.Seepolarization of light.

light, wave theory of Theory in which light(and other electromagnetic radiation) is inter-preted to be a type of wave. Current theory ofelectromagnetism interprets light to consist ofoscillating (time varying) electric and magneticfields that propagate through space as waveswith a speed dependent on the properties of thematerial medium that occupies the space.

light-year Length (abbreviatedly) equal tothe distance that light travels through a vacuumin one year. It represents a convenient unit oflength for the specification of astronomical dis-tances, and has a value given by the product ofthe speed of light in vacuumc times the numberof seconds in a year, (approximately) equal to9.461× 1015 m (= 1 ly).


limbs, artificial Artificial body parts thatfunction like their natural counterparts. Usedin the substitution of limbs for people who havesuffered accidents leading to the loss of suchextremities.

The artificial limbs form a subset of the gen-eral termprothesis.

limiter A circuit or component used to limitthe amplitude of a signal to a predeterminedlevel. For example, a resistorR is used as acurrent limiter in a circuit. The current limit fora bias voltageV is V/R.

limit of resolution The smallest angular sep-aration between two point sources of light (sub-tended at the position of the detector) for whichthe images of the two sources are seen to be sep-arated. The limit of resolution of light sourcesis most commonly determined by the so-calledRayleigh criterion,which states that the lightfrom separate sourcesS1 andS2 will be resolvedas distinct images only if the peak of the diffrac-tion pattern produced by one source (at the ob-serving screen or retina) is displaced from thepeak of the diffraction pattern produced by thesecond source by a distance equal to or greaterthan the half-width of the diffraction patterns.

Limit of resolution.

It follows from this criterion that the limitof resolution of light from two sources can beshown to be approximately equal to the ratio ofthe wavelength of the light divided by the di-mension of the aperture through which the light

passes en route to the surface on which the lightis detected.

linear circuit A linear circuit is a circuit towhich the superposition theorem can be applied.Namely, if there areN independent sources pre-sented in the circuit, any branch voltage or cur-rent is composed of the sum ofN contributionseach of which is due to each of the independentsources acting individually when all others areset equal to zero.

linear code A code is a linear code only iffor any two elementsa1 and a2 chosen fromthe alphabet and any two valid codewordsC1

andC2, a1C1 + a2C2 also represents a validcodeword. A linear code must thus contain anall-zero codeword.

line, artificial A network that simulates theelectrical characteristics of a line over a givenfrequency range. For example, a transmissionline can be simulated with a network of induc-tors, capacitors, and resistors.

line broadening An increase in the naturalrange of frequencies of the radiation emitted orabsorbed by a source or absorber of radiation.Corresponds to a “broadening” of the graph ofthe intensity of the emitted or absorbed radiationplotted versus the frequency of the radiation.Primary mechanisms resulting in line broaden-ing are:

1. Doppler-broadening,produced by thespread in the velocities of the emitting or ab-sorbing atoms or molecules, and

2.collisional broadening,resulting from col-lisions between the emitting or absorbing atomsor molecules that produce a change in the al-lowed energy states of the atoms or molecules,giving rise to an increase in the possible frequen-cies of the emitted or absorbed radiation.

line profile Plot of the intensity of emit-ted or absorbed radiation as a function of thefrequency (or wavelength) of the radiation.The “natural” line profiles associated with non-interacting atoms or molecules are well repre-sented by Lorentzian functions of the frequencyof the forms shown in the figures below, where


Figs. (a) and (b) correspond to emission and ab-sorption line profiles, respectively.

lines of force (electric) Continuous linesdrawn to represent the direction of an electricfield. The number of lines passing through asmall area of fixed size, oriented at right anglesto the lines, gives the magnitude of the electricfield.

line width Width, at half maximum intensity,of the peak in a graph of intensity of emittedor absorbed radiation vs. frequency (or wave-length) of radiation. (Seeline profile and ac-companying figures, in which the line width isdenoted∆ f.)

Line profile.

linkage magnetic The product of the numberof turns in a coil and the magnetic flux passingthrough the coil.

liquefaction coefficient The liquefaction co-efficient is a measure of the efficiency of the liq-uefaction of a (usually) cryogenic liquid. Forisenthalpic processes (in Joule-Thompson liq-uefiers, for example), the efficiency is

E =Hf −Hi

Hf −H`,

whereHi,Hf are the enthalpy at the beginningand end of the process andH` is the enthalpy ofthe liquid state.

liquefier, Claude This is associated with ex-pansion engine liquefiers. Claude’s first ma-chines used isentropic expansion with liquid airbeing produced in the engine. Joule-Thompsonexpansion for the final liquefaction stage waslater used. The operating efficiency is similar tothe Linde system. This liquefier is often usedin conjunction with Philips-Stirling and turbineexpanders.

liquefier, Collins This is a Joule-Kelvincombination liquefier that does external work.There are two pistons in cylinder expansion en-gines whose respective working temperaturesare about 60 K/30 K and 15 K/9 K. The in-coming helium gas enters via a series of heatexchangers and is distributed in the followingmanner: hotter engine, 30%; colder engine,55%; Joule-Kelvin stage, 15%. The mechan-ical components have to be carefully designedso as to enable them to operate at temperatureswhere conventional lubrication methods are im-possible. Gas purification is essential since evensmall traces of air can solidify and cause seizureof the piston.

liquefier, Hampson, air This is a singlestage air liquefaction process utilizing a heatexchanger that makes it possible to liquefy airby Joule-Kelvin expansion alone starting fromroom temperature. At first some cooling occursbut no liquid is produced as the high pressureincoming air expands at the Joule-Kelvin ex-pansion valve. The resultant colder, low pres-sure air flows back through the heat exchanger


and cools the incoming air stream. As a resultthe whole system gradually cools. Eventuallysteady operating conditions are reached and acertain fraction of the incoming air then lique-fies as it expands. This liquefier is also popularlyknown as aLinde-Hampson system.

liquefier, Kapitza, helium This system em-ploys the Heylandt principle. Helium at a pres-sure of 30 atm is cooled to 65 to 70 K with liquidnitrogen. This then passes through the heat in-terchangers, expands to 2.2 atm in an expansionengine, and leaves the liquefier via the heat in-terchangers. A further amount of helium at 15 to18 atm is cooled by the gas from the expansionengine and then expands at a valve where partof it liquefies. The final temperature drop to 4.2K is obtained by the Joule-Thompson effect.

liquefier, Kapitza, hydrogen Hydrogen isliquefied in a similar manner as for helium in theKapitza liquefier.Seeliquefier, Kapitza, helium.

liquefier, Linde The main components ofthis liquefier are a compressor, heat exchanger,and expansion throttle used for liquefying air. Itis similar to the Hampson which only employsa different design of a heat exchanger. The effi-ciency is improved by letting the gas expand intwo stages; further improvement occurred whenLinde introduced a liquid ammonia pre-coolingstage so that the compressed gas entered theexchanger at about−48 C. See alsoliquefier,Hampson air.

liquefier, Philips, air This is a single stage airliquefaction process that uses the Stirling cycle.The working fluid is operated on a closed cyclewith the regenerator performing the functionsof a heat exchanger by separating the colder andhotter ends of the system. Air condensation sur-faces are attached to the outside of what corre-sponds to the colder cylinder.

liquefier, Simon (expansion) This is a singleexpansion helium liquefier. Helium gas is com-pressed isothermally into a chamber to a pres-sure of 100 atm. A temperature of 15 K is main-tained. This temperature can be reduced to 10 Kby reducing the pressure above the evaporatinghydrogen to well below its triple point pressure.

The gas is eventually adiabatically cooled to thefinal temperature.

liquid crystal States of organization of atomsor molecules showing less symmetry than a crys-tal, but not as disordered as a liquid. The scien-tific term for this ismesomorphic phase.Thereare at least 9000 known molecules giving riseto mesomorphic phases. They are used in sometypes of displays (e.g., 7-segment display). Ap-plication of an electric field causes a large in-crease in light scattering creating a bright region.

liquid refrigerant level, surface detectionThere are several techniques for detecting thelevel of a liquid cryogen. One common solutionis to immerse a superconducting wire in the liq-uid and to drive current through the wire. If thecurrent and wire are chosen carefully, the cur-rent will drive any of the superconductor abovethe liquid normal while leaving the portion ofthe wire immersed in the liquid superconduct-ing. The resistance of the wire is therefore ameasure of the distance to the surface of theliquid. A technique that does not dissipate somuch heat in the refrigerant is to use a capacitorto measure the liquid level. Often, a capaci-tor made of two long concentric cylinders willbe partially immersed in the liquid and attachedto an AC bridge. As the liquid level rises andfalls, the amount of dielectric (i.e., refrigerant)in between the plates of the capacitor changes,and the bridge measures the concomitant capac-itance changes. In another technique, a thin tubewith a rubber diaphragm on one end is inserted,open end first, into a storage dewar of liquid he-lium. As the tube is cooled, oscillations will be-gin in the diaphragm. These oscillations, calledTaconis oscillations,are described by a com-plicated, non-linear set of hydrodynamic equa-tions which must include the heat deposited intothe helium gas and liquid by the tube, the heatconducted along the tube, along the gas, andother effects. If these equations are solved nu-merically or approximately, the frequency ofthese oscillations depends on whether the bot-tom of the tube is in contact with liquid helium orgaseous helium. Experimentally, the surface ofthe liquid is found by finding the point at whichthe frequency of oscillations changes discontin-uously. Of course, visual location of the fluid


and or a float can also be used if the apparatusallows direct viewing of the fluid.

Lissajous’ figures Stable patterns showingthe path of a particle moving in a plane forced bytwo harmonic oscillations whose frequenciesω1

andω2 are related asmω1 = nω2, wherem andn are integers. Lissajous’ figures are obtainedwhen cosω2t is displayed againstcosω1t fordifferent values of the integersm andn. Theyare calledLissajous’ figuresin honor of JulesAntoine Lissajous.

Lloyd’s mirror A mirror arranged relative toa (point) source of light and a viewing screen soas to produce an interference pattern on screenresulting from the superposition on the view-ing screen of part of the wavefront of an inci-dent light beam which reaches the screen di-rectly from the source and part of the wavefrontthat arrives at the screen after reflection from themirror surface.

load, electric The electrical impedance con-nected to the output of an electrical energysource such as an active electrical circuit, elec-trical generator, battery or amplifier. It is alsoused in reference to the electrical power con-sumption of a device such as the power used byan electric motor, heater, etc.

load, lagging A load in which the voltagereaches its maximum value before the current.A load that has a higher inductive than capacitivevalue.

load, leading A load in which the currentreaches its maximum value before the voltage.A load that has a higher capacitive value thaninductive value.

load-line A line representing the current inthe load vs. voltage drop across a device in se-ries with the load for a given input voltage. Itsintersection with the device current vs. voltagecurve is the operating current and voltage forthe given device-load combination at the giveninput voltage.

loadstone Also known asmagnetite,it wasdiscovered to be a natural magnet several cen-

Load, lagging.

Load, leading.

The operating point determined from the load line and

the device I-V.

turies BC. It is basically Fe3O4 and was foundoriginally in Magnesia of Asia Minor.

lobe In the directivity pattern of an antennaor array, an area of the pattern bounded by di-rections of minimum radiation.


lobe length In a Token Ring network, thelength of cable between themulti-station accessunit, i.e., the central hub and the workstation.

lobe switching A method used for obtainingoptimum signal from downcoming waves. Thisis done by switching into circuit networks thataccept a chosen direction for optimum signal.

logic gates Electronic devices in which thelow or high voltage states of inputs and outputsdetermine a binary value of 0 or 1. They areused for logic operations, mathematical compu-tations, storage, etc. (e.g., AND, OR, NANDgates).

logic operations The operations NOT, AND,OR, NAND, NOR, XOR that act on Booleanarguments to produce a Boolean result.

longitudinal waves A wave in which the di-rection of some vector characteristic of the wave— for example, the displacement of the particles— is along the direction of propagation. Alsocalled compressionalor dilatational waves.Longitudinal waves always have the highest

propagation speed,c1 =[

E(1−µ)ρ(1+µ)(1−2µ)

] 12, with

ρ as the density,µ the Poisson’s ratio, andE asthe elastic modulus of the medium.

long sight (hypermetropia, hyperopia) A vi-sion defect of the eye. The shortness of the eye-ball makes it difficult for the lens of the eye toaccommodate to project the image of near ob-jects onto the retina. The correction for longsight is a converging lens that focuses parallelrays to the far point. This correction is treatedby using a combination of thin lenses.

Correction for long sight.

loop gain The product of the amplifier gainand the feedback fraction in a series voltagefeedback loop. The input voltage to the am-plifier will be given byVi = Ve +λVi whereVe

is the externally supplied input voltage andλ isthe loop gain.

loss, acoustic energy in liquids and solidsLoss of acoustic energy that occurs during thepropagation of acoustic waves through a liquidor solid medium. The sources of these lossesare either associated with losses at the boundaryof the medium (viscous shear, heat conductionbetween medium and walls) or by those due todissipation within the medium (viscous and ther-mal losses, losses due to molecular energy ex-change conversion of the compressional energyof the medium into internal energy of molecu-lar vibration). For small damping, such lossesalong thex direction are described by the atten-uation constantα, and the pressure amplitudepA at any point in the medium ispA = p0e

−αx.

loudness The magnitude of the subjectivephysiological sensation produced by a sound.The measurement of loudness involves relat-ing the loudness of a sequence of sounds to thesound pressure level, as well as determining itsvariation as a function of a single parameter,such as frequency content or duration (to de-termine loudness functions and equal loudnesscontours), or differentiating between sounds thatvary only in level (loudness discrimination).Loudness functions are influenced by both phys-ical characteristics of the sound (frequency, tem-poral properties, duration, sound pressure levels,background noise levels) and the auditory sys-tem of the listener. Loudness functions describeloudness (expressed in sones) as a function ofthe sound pressure level (in decibels) for binau-ral single frequency tones. Equal loudness con-tours for a set of loudness levels (expressed inphons) are plotted in the frequency-sound pres-sure level diagram.

loudspeakers A system of interdependentelectromechanical components with a trans-ducer that converts electrical signal energy intoacoustical energy that it radiates into a boundedspace, such as a room, or into open space. Alsoknown as aspeaker.A loud speaker system con-


sists of drive element(s), radiation aid(s) (suchas baffle, enclosure, horn), and crossover. Thedrive element or driver is the electro-mechano-acoustic transducer.Electrodynamic driversarethe actuator counterpart of electrodynamic mi-crophones. Theenclosureis the cabinet inwhich the loud speaker is mounted and thebaf-fle is the support structure for the driver. Theelectronic dividing network that filters and dis-tributes the electrical signal to the drive ele-ments of the loud speaker system is called thecrossover.

love waves Horizontally polarized shear(transversal) waves in plates. A horizontal dis-persive surface wave in geophysics, multire-flected between internal boundaries of an elas-tic body, applied chiefly in the study of seismicwaves in the earth’s crust.

lumen The SI unit of luminous flux (withdimensions of energy/time). It is equal to theamount of light (measured with respect to thevisual response of the eye) emitted per second ina unit solid angle of one steradian from a uniformsource of one candela. This is approximatelyequivalent to 0.00146 W of light energy at awavelength of 555 nm.

luminance A measure of the brightness ofthe surface of a light source as measured withrespect to the visual response of the eye. Moreprecisely, it is the luminous intensity of the sur-face of a light source divided by the projectionof the area of the surface perpendicular to theline between the surface and the observer. Lu-minance is measured in SI units of candela permeter2. Seeluminous intensity.

luminescence The emission of light by a ma-terial substance other than as a result of thermalenergy (or incandescence). Luminescent emis-sion in general follows a process in which a sub-stance either absorbs radiation or is bombardedby charged particles, causing excitation of elec-trons into higher energy states from which theydecay by the emission of radiation in the formof light.

luminosity The magnitude of the visual sen-sation produced by a source of light, measured

in terms of luminous flux emitted or reflectedfrom the source. In astronomy, the term refersto the total output of radiation from a celestialobject.Seeluminous flux.

luminous efficiency A dimensionless ratiodefined by the luminous flux emitted by a sourceof light divided by the total rate of electromag-netic energy emitted by the source; it is mea-sured in SI units of lumens per watt. Luminousefficiency measures the relative effectiveness ofa source of electromagnetic energy in evokingthe visual sensation of brightness. Quantity isalternatively defined as the luminous flux emit-ted by a light source divided by the total powersupplied to the source.

luminous emittance Luminous flux emittedfrom a light source per unit area, measured in SIunits of lumens per meter2 (lm/m2). This con-trasts withradiant emittance,defined to equalthe total radiant energy per second emitted froma source per unit area.Seeluminous flux.

luminous exitance Luminous flux emittedfrom a light source per unit area per unit wave-length interval, and measured in SI units of lu-mens per meter3 (lm/m3). This contrasts withradiant (or spectral) exitance, defined to equalthe total radiant energy per second emitted froma source per unit area per unit wavelength inter-val. Seeluminous flux.

luminous flux The amount of light passing agiven point per second measured in terms of thecapacity of light to evoke the visual sensation ofbrightness. It is measured in SI units of lumen(lm), equal to 1 candela times 1 steradian. (Seeluminous intensity and accompanying definitionof candela.) The capacity of light to evoke thesensation of brightness in the visual response ofthe eye depends on frequency or wavelength ofradiation as determined by therelative luminos-ity curve shown below.

For a given quantity of radiant energy, thevisual response of the eye is a maximum forradiation of wavelength 555 nm. The defini-tion of candelaequates 1 watt of light of wave-length 555 nm to luminous flux of 685 lm; as aconsequence, 1 watt of radiation, with a wave-


Luminous flux.

length corresponding to a relative luminosityRλ, equates to a luminous flux of 685× Rλ lm.

luminous intensity The rate of light emissionfrom a (point) source of light per unit solid an-gle centered on the direction between the sourceand the observer, as measured with respect to thevisual response of the eye. It is equivalent to theluminous flux per steradian, and measured in SIunits of candela (cd), equivalent to 1 lumen persteradian. The candela (in practice) defines theSI unit of the lumen, and is approximately equiv-alent to the luminous intensity of one candle.More precisely, one candela is defined to equal1/60-th of the luminous intensity of a square cen-timeter of the surface of a blackbody at the melt-ing temperature of platinum, 2042 K, measuredin the direction perpendicular to the surface.

Lummer-Gehrcke plate Instrument usedfor the study of spectral lines. It consistsof a few-millimeter thick plate of accuratelyplane-parallel glass or quartz with an attachedprism at one end, through which light entersthe plate in a manner such that its angle ofincidence at the inner surfaces of the plateis slightly less than the critical angle fortotal internal reflection. Transmitted raysresulting from internal reflections then leavethe surface at nearly grazing angles andare brought to a focus on a viewing screen

by a lens so as to produce identical sets of fringeson either side of the plate.Seeaccompanyingdiagram.

Multiple reflections at the interior surfaces of Lummer-

Gehrcke plate.

This design has the advantage of a high re-flection coefficient near the critical angle evenfor ultraviolet radiation.

lumped parameter In circuit analysis, anycomponent, such as inductance, capacitance, orresistance, that can be treated as a single param-eter concentrated at a point in an electric cir-cuit. This treatment is only valid for a certainfrequency range where the wavelength of the al-ternating current in the conductors is larger thanthe dimensions of the component.

Lyman series A series of lines in the emis-sion or absorption spectrum of hydrogen (or hy-drogenic) atoms corresponding to wavelengthsλ(= 1/f) in the ultraviolet part of the electro-magnetic spectrum. Wavelengths of successivelines in series are given by the formula

1λ

= R

(1− 1

n2

), n = 2, 3, 4, . . . ,

whereR is the Rydberg constant with the value

R = 1.0973732 · 107m−1 .

Lines in the series correspond to radiationemitted or absorbed in transitions of single elec-trons in an atom between the (allowed) outer or-bits in the atom and theinnermostorbit in theatom.


Mmacromolecules, biological Biologicalmacromolecules are molecules formed by hun-dreds and even thousands of individual atomsthat work together and react as a unit. Theirfunction is prescribed by their particular com-ponents.

Biomolecules do not behave as static struc-tures but as dynamic ensembles of charge andshape. Macromolecular assembly and interac-tions are dictated largely by the electrostatic po-tential surfaces around the macromolecules andby the innate dynamic behavior and flexibilityof specific components. Understanding thesefeatures is central to the study of protein andRNA folding, in which the pathways and macro-molecular forces that drive folding are beingstudied.

Examples of macromolecules are polymersbuilt from monomers by condensation reactions.The reverse effect, i.e., polymers broken downinto monomers, can be achieved by hydrolysisreactions. In condensation reactions water isremoved as monomers are joined, while in hy-drolysis reactions water is added to break poly-mers down into monomer units. Among themost important polymers are the carbohydrateswhose monomers are monosaccharides. Thegeneral molecular formula for carbohydrates isCn(H2O)m. The most common ones have 5 or6 carbons.

Other examples of macromolecules found inbiology include lipids (such as phospholipids,fatty acids, and triacylglycerols), proteins, andamino acids that form DNA and RNA.

macroscopic transport parameters (cell)Transport through the cell membrane is deter-mined by the diffusion constant of moleculesand ions that diffuse across the membrane(passive diffusion), the reaction time betweenmolecules and transmembrane proteins (facili-tated diffusion), and the rate of energy conver-sion (active transport).

In passive diffusion,molecules directly crossthe phospholipid bilayer dissolving in the aque-ous solution at the other side of the membrane.The direction of transport is from the high tothe low concentration side of the membrane.Only small, relatively hydrophobic moleculesare capable of this (e.g., O2, CO2, benzene,H2O, and ethanol). On the other hand,facil-itated diffusionis carried out by the assistedpassage of molecules through the membrane byproteins that enable the crossing without allow-ing the molecule to interact with the hydropho-bic interior of the bilayer. Molecules participat-ing in this diffusion include polar and chargedmolecules (e.g., carbohydrates, amino acids, nu-cleosides, and ions). Becauseactive transport(ATP) is usually against the concentration gradi-ent of the molecule, it requires the use of energyas provided by the hydrolysis of ATP.

magnetic amplifier This comprises an iron-core transformer with an extra winding to whicha control signal can be applied. The amplifiermodulates the voltage across the load in an ACcircuit.

magnetic analysis The determination of themagnetic characteristics, under either direct oralternating fields, of ferromagnetic alloys canthrow light on their phase-structure. Magneticanalysis has been used in identifying compo-nents in alloy systems, and in studying the ef-fects of heat-treatment and other physical andmechanical variations.

magnetic axis A line that passes through theeffective poles of a magnet.

magnetic balance A type of fluxmeter inwhich the force required to prevent the move-ment of a current-carrying coil in a magneticfield is measured.

magnetic blowout A coil used in circuitbreakers to deflect any electrical arc formed tolengthen the arc or apply it to a cool surface andthus extinguish it.

magnetic bottle A configuration of magneticfields used to confine a plasma for long enoughfor the plasma to react.


magnetic braking A type of brake in whichthe brake is activated and deactivated by the ac-tion of an electromagnet.

magnetic circuits These constitute the com-plete paths for magnetic flux lines. A magneticcircuit is analogous to an electric circuit withmagnetomotive forces as the equivalent of volt-age, flux density as the equivalent of current,and reluctance as the equivalent of resistance.

magnetic damping The slowing down of themotion of a conductor when it passes througha magnetic field due to the production of eddycurrents.

magnetic disk Information storage devicewhich encodes information in magnetic bits ina thin layer of magnetic material on the surfaceof the disk. Data can be read from and writ-ten to this storage device. It has faster accesstime than tape storage since the read-write headcan move directly to the position on the diskwhere the data is stored. Magnetic disks come intwo categories: floppy disks that can be insertedinto and removed from a disk drive attached tothe computer, and hard disks that are perma-nently installed in a computer. Floppy disksare usually 3.5 inches in diameter and can hold1.44 megabytes of information after formatting.Hard disks are usually installed in a computerand have capacities in the gigabyte range. Zipdisks, which are removable and hold approxi-mately 100 megabytes (or more), of informa-tion, are also used for information storage.

magnetic drum Information storage devicein the shape of a drum that is coated with a thinlayer of magnetic material and is rapidly rotat-ing. Information is encoded on the outer surfaceof the drum. Fixed read-write heads which sensethe stray field of magnetic bits are used to writeand retrieve data.Seemagnetic recording.

magnetic effect of current Hans C. Oersteddiscovered in 1820 that electric currents producemagnetic fields. The exact mathematical ex-pression for the magnetic field produced by aline element of an arbitrary current distribution

is known as theBio-Savart law:

dB =µo

4πids sin θr2

.

magnetic element (1) Small part of a largermagnetic circuit or magnetic material.

(2) A small section of a magnetic circuit bro-ken into elements in a computer model so thatmagnetic properties can be modeled.

(3) A part of an instrument that a magneticfield acts on.

(4) The three magnetic elements that describethe earth’s magnetic field: thehorizontal com-ponent,the angle of dip(inclination), and theangle of declination. Seeinclination, magnetic;declination, magnetic.

magnetic equator A circle on the earth’ssurface oriented perpendicular to a line joiningthe magnetic north and magnetic south poles;also referred to as theaclinic line. The mag-netic equator lies approximately halfway be-tween these two poles. The magnetic equatormarks the points on the earth’s surface wherethe vertical component of the earth’s magneticfield is zero. The position of this line changesslightly each year as a result of the slow drift ofthe earth’s north and south magnetic poles.

magnetic field The field of a magnet (or cur-rent carrying wire) is the region surrounding themagnet (or current carrying wire) where mag-netic forces occur.

magnetic field, given by Faraday’s inductionlaw

∮E. dl = −dΦ/dt, whereΦ is the mag-

netic flux linking the closed circuit. Alterna-tively, ∇ × E = −∂B/∂t by Stoke’s theorem.This relationship forms the basis of electricalgenerators.

magnetic field lines A set of lines that de-scribe the strength and direction of a magneticfield; also calledlines of magnetic force.Arrowson the lines point in the direction of the magneticfield. The spacing of lines is inversely propor-tional to the magnetic field strength. Closelyspaced lines indicate a strong magnetic field.Lines never cross. For permanent magnets, thelines of magnetic field emerge from the north


pole and enter the south pole of the magnet. Fora straight current-carrying wire, the lines formcircles about the wire perpendicular to the lengthof the wire. Seemagnetic field of long straightconductor.

magnetic field of a conductor The magneticfield of a conductor is determined by the electriccurrent flowing in it. It can be calculated us-ing Biot-Savart’s law or Ampere’s law in casesof high symmetry. SeeAmpere’s law; Biot-Savart’s law; magnetic field of a long straightconductor.

magnetic field of circular loop For a circularloop of wire of radiusa, carrying currenti, lyingin thex-y plane with its center at the origin, themagnitude of the magnetic field along thez-axisis given by

µoia2/2

(a2 + z2

)3/2.

µ0 is 4π × 10−7T.m/A. If the electric currentcirculates anticlockwise as seen from above, themagnetic field on thez-axis points in the+zdirection. Seeright-hand screw rule; magneticfield of conductor.

magnetic field of displacement currentDisplacement current arises from a changingelectric field and gives rise to a magnetic field.The value of the displacement current througha certain area is given byεodΦE/dt whereε0 is8.85 × 10−12 farad/meter and the second termis the rate of change of electric flux throughthe area of interest. This magnetic field maybe calculated using Biot-Savart’s law or Am-pere’s law (cases of high symmetry) once thevalue of the displacement current is known.SeeBiot-Savart’s law; Ampere’s law; displacementcurrent.

magnetic field of long straight conductorThe magnetic field strength a distancer from along straight wire carrying currenti is

µoi/2πr .

Its direction is given by theright-hand screwrule with the magnetic field lines circulating an-ticlockwise and forming circles when the wireis viewed end-on with the current coming out of

the page.Seeright-hand screw rule; magneticfield of conductor.

magnetic field, vector nature The magneticfield is a vector. It has a direction as well asa magnitude. The magnetic field vector pointsaway from any nearby north magnetic poles andtoward any nearby south magnetic poles. Thevector is also along magnetic field lines at anypoint in space.Seemagnetic field lines.

magnetic field, work done by The workdone by a magnetic field on a magnetic mate-rial in a complete cycle is given by the area ofthe hysteresis loop. This energy is dissipated inthe magnet and is referred to ashysteresis loss.Mathematically it is the integral of HdM. Thisquantity tends to be small for soft magnets suchas those used in transformers and large for per-manent magnets such as those used in electricmotors. These losses come about due mostly topinning of domain walls which hinder reversalof the magnetization.Seehysteresis loss.

magnetic force between parallel conductorsThe magnitude of the magnetic force per unit

length (F/L) between two parallel wires sepa-rated by a distanced and carrying currentsi1andi2 is µ0i1i2/2πd. The force is attractive ifthe currents are parallel and repulsive if the cur-rents are antiparallel. The source of this forcecan be thought of in the following way: One ofthe currents creates a magnetic field at the posi-tion of the second current. This magnetic fieldthen exerts a force on the second current withthe result that the first current exerts a force onthe second current.

magnetic force on a conductor The mag-netic force on a conductor-carrying currenti canbe determined from

dF = idLxB

wheredL is a small length of conductor whosedirection is along the current, andB is the ap-plied magnetic field. For a long straight wire oflengthL in magnetic fieldB the force is

iLxB .


magnetic force on complete circuit A mag-netic field exerts a force on parts of a circuitcarrying a current. For a complete circuit inthe presence of a uniform magnetic field, theseforces add up vectorially to give zero. If a partof the circuit is movable, the magnetic field maycause that part of the circuit to move.Seemag-netic force on a conductor.

magnetic force on moving charge The mag-netic force on a moving chargeq is perpendicu-lar to both the magnetic fieldB and the particlevelocityv and is given by

qvxB .

Thus if the magnetic field is parallel or antipar-allel to the velocity no force is exerted. If themagnetic field is uniform and perpendicular tothe particle velocity, the particle will execute cir-cular motion. If the magnetic field is not perpen-dicular to the particle velocity, then the particlewill trace out a spiral pattern. Note that the forcewill reverse direction if the sign of the chargeqis reversed.

magnetic induction Also called magneticflux density. Commonly denoted byB, hasCGS units of Gauss and MKS units Tesla orWebers/meter2, and is a vector quantity. OneTesla is equal to 104 Gauss. It is determinedby the applied magnetic field (or magnetic in-tensity)H and the magnetic moment per unitvolumeM of the medium. In CGS units

B = H + 4πM

and in MKS units

B = µo(H +M) .

SeeTesla.

magnetic intensity Also known as mag-netic field strength. Commonly denoted byH, has CGS units of Oersteds and MKS unitsamps/meter and is a vector quantity. It is deter-mined by source of the magnetic field only, anddoes not depend on the medium. One Oerstedis equal to1000/4πamps/meter.SeeOersted.

magnetic leakage Phenomenon by whichmagnetic field lines leak out of a magnetic mate-rial. Can occur at cracks or imperfections in the

surface of a magnetic material. Also refers to amagnetic field in the airgap in a magnet wheremagnetic field lines bow outward weakening themagnetic field in the airgap. Magnetic leakagegenerally reduces the efficiency of operation ofa device. It also can be used to detect flawssuch as cracks in materials that are magnetized,since imperfection changes the distribution ofmagnetic flux.Seemagnetic field lines.

magnetic lens A magnetic field arrangementdesigned to focus or guide moving charged par-ticles. This magnetic field arrangement usuallyhas axial symmetry. The magnetic field can begenerated by current-carrying coils and perma-nent magnets. The path followed by the chargedparticles depends on their velocity and charge aswell as on the magnetic field configuration of themagnetic lens. Thus only charged particles ofa pre-selected charge and energy are guided orfocused by a selected magnetic field configura-tion. Seemagnetic force on moving charge.

magnetic meridian An imaginary line thatpasses overhead from the magnetic south pole tothe magnetic north pole of the earth. This linefollows the direction of the horizontal compo-nent of the earth’s magnetic field.

magnetic mirror A configuration of mag-netic fields that reflects a moving charged par-ticle impinging on it. It can be used to confinea plasma and is used to confine the plasma incontrolled fusion experiments. A simple mir-ror would consist of a magnetic field along thez-axis that got stronger with increasing valuesof z. A particle moving with a component ofits velocity along the+z direction, will spiralalong the field line due to the magnetic forceon it and can be reflected. This is less effec-tive for particles whose direction of travel liesclose to the+z direction and some of these maynot be reflected. More complex magnetic fieldconfigurations are possible and are used to moreefficiently reflect and confine charged particles.Seemagnetic lens.See alsomagnetic force ona moving charge.

magnetic moment A property of a perma-nent magnet, a current-carrying circuit, or ma-terial that has an induced magnetic moment in


the presence of a magnetic field (a diamagneticor paramagnetic material). Commonly denotedby m, has CGS unit of EMU and MKS unitsof amp.m2. One EMU is equal to 103 amp.m2.It is the sum of the moments of the atoms ormolecules making up the material in the caseof permanent magnets where all the atomic mo-ments are aligned. Equal to (current× area) fora current-carrying circular coil.

magnetic monopole Isolated north or southmagnetic pole analogous to isolated positiveand negative electric charge. Predicted to existby some symmetry elementary particle theoriesbut so far have not been found to exist in na-ture despite a number of experimental searches.If these particles exist the equations describingelectric phenomena would have the same struc-ture as those describing magnetic phenomena.Maxwell’s equations, which describe electro-magnetism, would be symmetric with respectto electric and magnetic fields.

magnetic needle A needle made up of a mag-netic material that can be thought of as a simplebar magnet. This needle is pivoted at its cen-ter so that it is free to rotate and line up with amagnetic field. Commonly used in a compasswhich is used to determine the direction of mag-netic north on earth.

magnetic poles North and south poles of amagnet. Always found to exist in pairs. Mag-netic field lines emerge from the north pole andenter the south pole of the magnet. Like polesare repelled and unlike poles are attracted to eachother. If a bar magnet is cut in half, a new northand south pole appear on either side of the breakwith the two new magnets each having one northand one south pole.Seemagnetic field lines.

magnetic potential A scalar magnetic po-tential useful in the area of magnetostatics. Thedifference in magnetostatic potential between aninitial (i) and final(f) position for a magneticpole is defined as the work done in moving a unitmagnetic pole fromi to f . Mathematically, itis given by the integral of−Hdl between thesetwo points.H is the magnetic field intensity anddl is an infinitesimal path length.

magnetic quantization In the presence ofa magnetic field a moving charged particle hasonly certain allowable energies, referred to asLandau levels.Effects of quantization are onlyobservable at low temperatures. It can lead tode Haas-van Alphen oscillations of diamagneticmoment of a material, and Shubnikov-de Haasoscillations of the electrical resistance of a mate-rial in the presence of a changing magnetic field.These oscillations are related to electronic struc-ture — in particular, the size and shape of theFermi surface — and are a useful probe of thisstructure.

magnetic recording Recording of informa-tion in the magnetization of a material. Themagnetic material is usually made up of gamma-Fe2O3 particles in a binder that is coated onto asupport structure such as a flexible plastic tapeor a solid plastic disk. The recorded magnetiza-tion is usually in the plane of the material. Thismagnetic information is read using a read headwhose primary component is a small sensingcoil that senses the stray field from the recordedmagnetization. It can also be read using a laserbeam of polarized light with the polarization ofthe reflected light being rotated depending on themagnetization of the medium. The magnetic in-formation can be written using a small appliedfield that can be created by the sensing coil. In-formation can be in analog form such as is usedin a simple tape recorder where the magnitudeof the recorded magnetization is proportional tothe signal generating it. It can also be digitalsuch as with computer magnetic disks and com-puter tapes. In this latter case, the information isstored as magnetic bits, corresponding to onesand zeros.Seemagnetic disk; magnetic tape;magneto-optic.

magnetic resistance Reluctance of a mag-netic circuit.Seereluctance.

magnetic saturation State of a magneticallyordered material in which all the atomic mag-netic moments are aligned. No domain wallsare present and the material can be thought of asone large domain. An external magnetic field isoften applied to get complete alignment. Appli-cation of a larger magnetic field than is requiredfor saturation may only increase the magnetic


moment by a small amount due to induced mag-netic contributions.

magnetic screening The screening of a re-gion of space so that no magnetic field enters thisregion from outside. Materials with a high mag-netic permeability are used to screen the region.The most widely used is mumetal, an alloy con-sisting mostly of Ni and Fe.Seepermeability,magnetic.

magnetic shell A thin sheet of magnetic ma-terial in the shape of a sphere whose inner andouter surfaces have equal pole densities of op-posite sign. It can also refer to the partially filledelectron shell of an atom or ion where the spinand/or orbital angular momenta of electrons maynot be zero so that a net atomic magnetic mo-ment exists. In the rare-earths, the 4f sub-shellis the magnetic shell. It is partially filled and theelectrons of this shell have a net spin and orbitalmagnetic moment that add to give the total mo-ment of the rare-earth atom. In the special caseof Gd there is only a spin magnetic moment.This idea does not apply to transition metal el-ements since the 3d electrons giving rise to themagnetism are not localized in an atomic shellbut rather are distributed in the conduction band.

magnetic susceptibility, measurement belowm1 For paramagnetic salts, the magneticsusceptibility is temperature sensitive and pro-vides a thermometric parameter. However, dis-crepancies between the thermodynamic temper-ature and the “magnetic” temperature occur attemperatures below 1 K. For a widely used ther-mometer salt such as CMN, the thermodynamicand magnetic temperature differ by less than 1%down to 6 mK for single crystals and powderspecimen. For salts such as CPA, CMA, andFAA the differences in the two temperatures be-come significant at temperatures of 0.4 to 1 K.

Magnetic susceptibility is the ratio of theintensity of magnetization produced in a ma-terial to the intensity of the magnetic field towhich the material is subjected; it measures theamount of magnetization of a substance by anapplied magnetic field. Diamagnetic materialshave small negative susceptibilities while para-magnetic materials have small positive suscepti-bilities, and ferromagnetic materials have large

positive susceptibilities. The magnetic suscep-tibility varies linearly with1/T in accordancewith Curie’s law above 1 K; however, it be-comes almost temperature independent at verylow temperatures since it behaves like a Fermigas.

magnetic tape A flexible plastic tape (oftenmylar) coated with a magnetic material, usu-ally particles ofα−Fe2O3, on which informa-tion may be stored in the form of magnetic bits.Each magnetic bit contains manyγ−Fe2O3 par-ticles and the magnetization of the magnetic bitlies along the length of the tape.Seemagneticrecording.

magnetic torque Torqueτ exerted by a mag-netic fieldB on a magnetic momentµ. Givenby

τ = µxB .

Tends to align magnetic moments along an ap-plied magnetic field. The earth’s magnetic fieldexerts a torque on a compass needle since thecompass needle is magnetized. This aligns thecompass needle along a magnetic north-southdirection.

magnetic viscosity Phenomenon by whichmagnetization of a material changes with time.This usually occurs when the applied magneticfield is changed to a new value and held at thatvalue. In some materials — for example, Tb-Coamorphous alloys — energy barriers are hinder-ing magnetic reversal and a magnetic viscos-ity results when the magnetic system overcomesthese barriers by thermal activation. At very lowtemperatures (below 10 K), mesoscopic quan-tum tunneling may also lead to a magnetic vis-cosity in this and similar systems. This type ofmagnetic viscosity is always accompanied bymagnetic hysteresis. Magnetic viscosity mayalso be observed in materials where the struc-ture is changing with time, such as in Fe with asmall amount of carbon.

magnetic well A configuration of magneticfields designed to contain a plasma. Two mag-netic mirrors designed to reflect moving chargedparticles would serve as a simple magnetic well.A magnetic well is used in fusion experiments


to contain a plasma that has a very high temper-ature.Seemagnetic lens; magnetic mirror.

magnetism, molecular theory of In themolecular theory of magnetism (also called theWeiss theory of magnetismor mean field the-ory) it is assumed that the exchange interactionusually described by−Jµi ·µj can be replacedby its average value. (J is the exchange con-stant and theµs represent neighboring magneticmoments.) When summed over nearest neigh-bors(µ′is), this average value is proportional tothe magnetizationM of the material, and theterm is written asλMµi. This theory qualita-tively describes many aspects of magnetism inthree dimensions but ignores the effects of ther-mal fluctuations of magnetic moment below themagnetic ordering temperature. It leads to in-correct predictions for details of the magnetictransition, such as the critical exponents associ-ated with magnetization as a function of appliedfield and magnetic susceptibility as a function oftemperature. In lower dimensions where ther-mal effects on magnetization are stronger, thefailures of this theory are more apparent.

magnetization Magnetic moment per unitvolume within a material as a result of the mag-netic polarization of the material. This resultsfrom the alignment of permanent atomic mag-netic moments in the case of Fe, Co and Ni.Contributions from induced magnetization suchas diamagnetism are also present and are usuallymuch smaller than the atomic moment contribu-tion. Seemagnetic moment; diamagnetism.

magnetization curve A plot of magnetiza-tion (or magnetic induction) as a function ofapplied magnetic field. Used to find importantmagnetic parameters describing a magnetic ma-terial. These include the saturation magnetiza-tion and the coercive force. Differences in thesecurves when the magnetic field is increased andthen decreased indicate the presence of magnetichysteresis.Seecoercivity; magnetic saturation;hysteresis.

magnetization, intensity of Given byµoMwhereM is the magnetization per unit volumeand has a unit of Tesla. Only defined for MKSsystem of units.

magnetizing current The electric currentthat flows through a coil surrounding a core,usually made of a soft Fe alloy, and which es-tablishes an applied magnetic field in the core.This applied magnetic field magnetizes the core.The coil and core make up the main part of anelectromagnet.Seeelectromagnets.

magneto-accoustic emission This involvesthe study of the propagation of sound wavesin metals in the presence of a magnetic field.At low temperatures, the interaction of soundwaves with electrons is the primary source ofattenuation of the sound wave. This attenuationis modified by the presence of a magnetic fieldand this effect can be used to probe electronicstructure. It can also refer to accoustic energygenerated by changes in magnetization, and canbe associated with strains and magnetostriction.Seemagnetostriction.

magnetohydrodynamic wave Electromag-netic waves in a plasma coupled to an oscillationin the plasma density in the presence of a mag-netic field. Frequency is usually low — less thanthe cyclotron frequency of the charged ions inthe plasma. Important in the earth’s ionosphereand in the various layers of the sun.

magnetometer A device used to measure amagnetic field such as the earth’s field, or themagnetic field created by the magnetic momentof a magnetic material. Various devices exist toperform this function and operate using a num-ber of different principles. It can measure themagnetic moment of a material in a vibratingsample magnetometer. In this device, a piece ofthe material is mechanically vibrated in a coiland the induced voltage from the stray field ofthe material is measured and is proportional tothe magnetic moment. It must be calibrated us-ing a known magnetic standard and can sensemagnetic moments as small as 10−5 EMU. Itcan also measure a magnetic field by an induc-tion method through the movement of a coil, byrotating it, for example in a magnetic field. Itcan also monitor a property that changes with themagnetic field such as the Hall voltage across amaterial induced by the magnetic field.


magnetometer, impedance The ratio of in-duced voltage to current in pick-up coils of avibrating sample magnetometer. In general, itis a complex quantity. If the capacitive and in-ductive parts of the impedance cancel, then thetotal impedance is real and the induced voltageand current are in phase. It is an important quan-tity since detection electronics must be matchedin impedance to the pick-up coils in the magne-tometer.Seemagnetometer.

magnetometer, Q A measure of losses.Larger Q corresponds to smaller losses.Seemagnetometer.

magnetomotive force The magnetomotiveforce around a complete magnetic circuit is de-fined as the work required to move a magneticpole of unit strength once around the magneticcircuit. Seereluctance, magnetic.

magneto-optic Interaction of light with amagnetic material. In theFaraday effect,thepolarization direction of polarized light is ro-tated when the light passes through a transpar-ent material in the presence of a magnetic fieldalong the direction of propagation. In theKerreffect,polarized light has its direction of polar-ization rotated on reflection from a ferromag-netic material. This latter effect forms the ba-sis for magneto-optical recording. A number ofother magneto-optic effects exist including theZeeman(normal and anomalous)effect,wherespectral lines are split by a magnetic field, theVoight effect,where an anisotropic substanceplaced in a magnetic field becomes birefringent,and theCotton-Mouton effectin which doublerefraction of light in a liquid in the presence of amagnetic field occurs.SeePaschen-Back effect;magnetic recording.

magnetoresistors Material whose resistancechanges when subjected to a magnetic field. Ob-served with the magnetic field parallel or per-pendicular to the electric current. Observed inmany magnetic materials. The increase in re-sistance is a few percent or less in most alloysstudied when fields up to 5 Tesla are applied.It can be large in semiconductors and in metals.High field magnetoresistance yields informationon the electronic structure of metals, in partic-

ular the fermi surface shape. At low temper-atures quantum oscillations of the magnetore-sistance, calledShubnikov-de Haas oscillations,are observable in single crystal metals and yieldmore detailed information on the fermi surface.More recently nanostructured magnetic materi-als (particles or layers with dimensions of tensof nanometers) have been discovered that showa giant magnetoresistance — the resistance canchange by more than 100% when the material ismagnetized. This forms the physical basis fora read head based on a magnetoresistance formagnetic recording.

magnetostatic energy Energy stored in amagnetic field or energy required to create amagnetic field. Energy density (energy per unitvolume) is

B2/2µo .

To get the magnetostatic energy, this energy den-sity must be summed (integrated) over the vol-ume containing the magnetic field.

magnetostriction Compressive or extensivestress in a magnetic material when its magne-tization is changed by, for example, placing itin a magnetic field. This leads to a change in adimension of a material when its magnetizationis changed. The strain (the fractional change ina dimension of a material) is typically small —of the order of 10−5 or less in going from theunmagnetized state to the saturated state. Mag-netostriction significantly influences the type ofdomain pattern in a material.Seemagnetostric-tion oscillator.

magnetostriction oscillator A device inwhich one or more dimensions of a magne-tostrictive material oscillate. An oscillating ap-plied magnetic field leads to oscillations of themagnetic moment of the material. One or moredimensions of the material oscillate in responseto the oscillating magnetic moment of the ma-terial converting magnetic energy to mechan-ical energy. This device is easily realized inprinciple by placing a coil with an alternatingcurrent around a soft magnetostrictive magnet.The alternating current provides the oscillat-ing magnetic field. It can be used to generatea sound wave in the audible to ultrasonic fre-quency range in any medium that is physically


in contact with the magnetic material.Seemag-netostriction.

magnetron A device that converts DC elec-trical energy to microwave energy with high ef-ficiency. Electrons emitted by a cathode to-ward an anode interact with crossed electric andmagnetic fields in a cavity to produce these mi-crowaves. The shape and size of the cavity deter-mines the frequency of operation. It can operatein a pulsed mode generating microwave powerfor radar applications, or continuous mode formicrowave cooking.

magnification The magnification of an op-tical system indicates the effectiveness of en-larging or reducing an image. There are severalkinds of magnification: lateral magnificationof an image,axial magnificationof an image,or magnification of the magnifying powerof anoptical instrument. It is important which mag-nification should be considered for use to treatoptical magnification. The termmagnificationis sometimes used simply to mean lateral mag-nification or the power of a lens without quali-fication.

magnification, angular The symbol used isM or γ. Angular magnification of an opticalsystem is the ratio of the angles subtended atthe eye by the imagetheta′ and objecttheta.It can be obtained approximately by the ratioof the tangent function value of the angles forsmaller angles. The angular magnification of alensMθ and power of the lensP have a relation-shipMθ = 1

4P . This equation is known as thequarter-power equation.It applies to the lensthat is used without accommodation; an imageis seen at infinity and the lens is set close to theeye. With accommodation,Mθ = 1

4P +1. It isalso known as theconverging ratio.

magnification, axial Also called longi-tudinal magnification. For an object withdepth (e.g., three-dimensional), the magnifica-tion along the optical axis should be considered.The axial magnification is the ratio of lengthalong the optical axis to the conjugate length inthe object. The axial magnificationMx is de-fined as the ratio of a short length in the imagemeasured along the optical axis to the conjugate

Angular magnification.

length in the object:

Mx =dxi

dxo,

wherexi andxo are the distance between the im-age and the focal point and the distance betweenthe object from the focal point, respectively.

Axial magnification.

The axial magnificationMx for small dis-tances from the focal plane is equal to the squareof the lateral magnificationMl : Mx = M2

l .

magnification, lateral The ratio of the sizeof an image perpendicular to the optical axisy′

to the size of the object perpendicular to the axisy. It is often simply calledmagnification.

Lateral magnification.

The lateral magnification can be obtained asthe ratio of object distances′ to the image dis-tances.

magnification, longitudinal Seemagnifica-tion, axial.


magnification, normal The multiplicationof the primary magnification and the magnifica-tion of ocular. The normal magnification multi-plied by the tube factor is the general magnifi-cation of the instrument.

magnification of lens For the refraction of athin lens, the relationship of the image distances, object distances′, and the focal lengthf , canbe written as

1s

+1s′

=1f,

1f

= (n′ − n)/n(

1R

+1R′

),

whereRandR′ indicate the radii of the surfaces.For a concave lens,f is less than zero. Thelateral magnificationm can be calculated as

m = −s′

s.

The magnifying powerP of a thin lens is thealgebraic sum of the pair of surfaces of the lensP1,P2;P = P1+P2. The angular magnificationof a lensMθ and power of the lensP have arelationshipMθ = 1

4P . This equation is knownas thequarter-power equation.It applies to thelens used without accommodation; the image isseen at infinity and the lens is set close to theeye. With accommodation,Mθ = 1

4P + 1.

magnification of mirror For the reflection ofa mirror, the relationship of the image distances,the object distances′, the radius of the surface ofmirrorR, and the focal lengthf , can be writtenas

1s

+1s′

=1f,

f = −R2.

For a concave mirror,f is greater than zero. Thelateral magnificationm can be calculated as

m = −s′

s.

Where the refractive index of a given medium isn, the refractive power of a mirrorP is definedas

P =−2R

.

magnification of optical instruments Ameasure that indicates how much an optical in-strument enlarges or reduces the image of anobject.Seemagnifying power.

magnification, primary The transversemagnification provided by the objective lens.The magnified real image formed by the ob-jective lens is called the primary image. In acompound microscope, the image formed by theobjective is an inverted image of the object. Theprimary magnificationmp is a lateral magnifi-cation:

Mp =−Lf

,

where the distance of the image and the focalpoint of the objective lens isL, and the focallength of the objective lens isf .

magnifications in vibrations The magnitudeof the transfer function of a periodically forcedvibrating system. It describes the magnitude ofa measured output signal, such as displacement,velocity or force, transmitted to the base for thesystem excited with a sinusoidal direct-force ex-citation of magnitudeM and frequencyω. Fora damped, second order, one degree of freedomsystem, for example, the peak response of thetransfer function is usually at the resonant fre-quencyωr = ωn

√1− 2ζ2 with ωr as the reso-

nant frequency in rad/sec,ωn as the undampednatural frequency in rad/sec, andζ as the non-dimensional damping ratio.

magnification, transverse Seemagnifica-tion, lateral.

magnifier Seemicroscope, simple.

magnifying power Seemagnification, angu-lar. It is also known as instrument magnifica-tion. This is the ratio of the size of the retinalimage of an object formed by an optical sys-tem to the size of the retinal image of the objectseen with the unaided eye at the least distance ofdistinct vision (normal viewing distance). Thesize of the retinal image can be measured as theangle subtended at the eye by the image. Theimage of the object seen with the object meansthe in situ image for telescopes, and the imageat the conventional distance of distinct vision,


i.e., 25 cm from the eye, for a microscope. In acompound microscope, the magnification poweris the ratio of the angle subtended at the eye tothe real image that is formed in space on theplane of the field stop of the eyepiece within themicroscope tube. For projection with a micro-scope, the magnification by the eyepiece shouldbe treated as a lateral magnification. For visualobservation, it should be an angular magnifica-tion, in a telescope, the real image formed aheadof the first focal length of the eyepiece.Seemag-nification of optical instruments.

Malus’ law A law (first published in 1809by Malus) determining the intensity of radiationtransmitted through a polarizer in terms of theincident intensity. Malus’ law equates the trans-mitted intensity,I, to the product of the incidentintensity,I0 , and the square of the cosine of theangleθ between the polarization vector of theincident radiation and the direction of the planeof transmission of the polarizer, as expressed bythe equation

I = I0 cos2 θ .

mammography A type of X-ray examina-tion of the breasts specifically tailored for thedetection of tumors. Amammogramis an X-ray image of the breast that comes out of theexam.

manostats Pressure-control devices used forcontrolling the vapor pressure of a boiling liquidto within certain limits. A stable manostat usesa capacitance pressure sensor and operationalamplifiers that activate solenoid valves to con-trol the vapor flow. Pressure stability of betterthan 0.1% is achieved with4HE from 2 to 1000Torr, corresponding to variations of< 1 mK.

maser Acronym for microwave amplifica-tion by stimulated emission of radiation.Moreprecisely, a device (invented in 1955) that pro-duces a narrowly directed beam of coherentmonochromatic radiation with a (central) fre-quency in the microwave region of the electro-magnetic spectrum between109 and1011 Hz.Analogous to laser (invented later) but operat-ing at microwave rather than optical frequen-cies. The first maser operated at the “inversion

transition” frequency of the ammonia molecule,2.387× 1010 Hz.

maser in communication A maser designedto amplify microwave signals (as from artificialsatellites) used in communications. It amplifiesFM signals from a satellite by the stimulatedemission of tunable Zeeman separated lines inions of paramagnetic crystals (such as ruby).The maser has the advantage of amplificationwith extremely low noise.

masking, acoustic A number of decibels bywhich the listener’s threshold of audibility for acontinuous sound is changed by the presence ofanother sound, called themasking sound.Whenthe threshold of audibility of the original tonealone isadecibels, and the threshold of the sametone in the presence of the masking tone isbdecibels, the masking of the original tone by thesecond one (characterized through its level indecibels) isb−a. Increasing the intensity of themasking sound to a level at which the originalsound ceases to be audible results in the maskingby the other sound. The masking effect is morepronounced above the frequency of the originaltone than below. Noise can also cause masking.Masking will be perceived as beats when thefrequencies of the two tones are close.

mass attenuation coefficient In general, anattenuation coefficient is a measure of the rateof decrease of an average power with respect toa distance along the transmission path.

Given a material of thicknessd, densityρ, anarrow beam of monoenergetic photons incidenton one side with intensityI0, and emerging onthe other side with intensityI, and defining themass thicknessl = ρ · d, the decay in intensityis given by the exponential attenuation law

I/I0 = exp[−(µ/ρ) · l] .

Rewriting the expression, we get the attenuationcoefficient

µ/ρ = l−1 · ln (I0/I) .

Then the mass attenuation coefficient can be ob-tained from experimental measurements ofI0andI.

Some values ofµ/ρ rely on the theoreticalvalues for the total cross section per atom,σtot,


which is related toµ/ρ by

µ/ρ = σtot/(u ·A) ,

whereu is the atomic mass unit,A is the relativeatomic mass of the target element, andσtot is thetotal cross section for an interaction by the pho-ton. In general, the attenuation coefficient, pho-ton interaction cross sections, and related quan-tities are functions of the photon energy.

An experiment that relies on the mass attenu-ation coefficient is X–ray computed tomography(CT), where the reconstructed image representsthe distribution of X-ray photon attenuation co-efficients from the body under examination. Aninteresting problem arises because the X–raysused are not monochromatic. In this case theattenuation coefficient distribution has to be re-constructed at a certain effective energy of thebeam. However, the highly non–linear depen-dence of the attenuation coefficient on photonenergy results in systematic inaccuracy in thereconstructed image, known as thebeam hard-ening artifact.

mass energy absorption coefficient In thecontext of the scattering event described in themass energy transfer coefficient,the mass en-ergy absorption coefficient involves the furtheremission of radiation produced by the chargedparticles in traveling through the medium, andis defined as

µen/ρ = (1− g)µtr/ρ ,

whereµtr/ρ is the mass energy transfer coeffi-cient. The factorg represents the average frac-tion of the kinetic energy of secondary chargedparticles (produced in all the types of inter-actions) that is subsequently lost in radiativeenergy–loss processes as the particles slow torest in the medium.Seemass energy transfercoefficient.

mass energy transfer coefficient In the con-text of the interaction of photons with matter,the mass energy transfer coefficientµtr/ρ, whenmultiplied by the photon energy, is proportionalto the sum of the kinetic energies of all the pri-mary charged particles released by unchargedparticles (here photons) per unit mass. In otherwords,µtr/ρ takes into account the transfer of

radiation coming from secondary photon radia-tions initially produced at the photon–atom in-teraction site, plus the quanta of radiation fromthe annihilation of positrons originating in theinitial pair– and triplet–production interactions.

Henceµtr/ρ is defined as the sum of all thecontributions coming from the total cross sec-tions from photoelectric absorption, incoherentscattering, pair and triplet production. Then

µtr/ρ =(fpeσpe + fincohσincoh

+ fpairσpair + ftripσtrip) /(u ·A) ,

where the factorsf refer to the energy–transferfractions, andσ to the individual cross sections.The factorsf represent the average fractions ofthe photon energyE that is transferred to ki-netic energy of charged particles in the remain-ing types of interactions. In particular,

fpe = 1− (X/E) ,

whereX is the average energy of fluorescenceradiation emitted per absorbed photon;

fincoh = 1−(〈E

′〉+X

)/E ,

where 〈E′〉 is the average energy of theCompton–scattered photon;

fpair = 1− 2mc2/E ,

wheremc2 is the rest energy of the electron; and

ftrip = 1−(2mc2 +X

)/E .

The fluorescence energyX depends on thedistribution of atomic–electron vacancies pro-duced in the process under consideration and isin general evaluated differently for photoelec-tric absorption, incoherent scattering, and tripletproduction.X should include the emission of“cascade” fluorescence X–rays associated withthe complete atomic relaxation process initiatedby the primary vacancy.

Because in calculatingµtr/ρ, only the char-acteristics of the target atom are involved, forhomogeneous mixtures and compounds, the to-tal µtr/ρ can be obtained by

µtr/ρ =∑

i

wi (µtr/ρ)i


wherewi is the fraction by weight of theithatomic constituent.

mass fragmentography Analysis and iden-tification of chemical substances by the study ofparts or fragments of the whole compound.Seemass spectrometry, medical applications.

mass spectrometer A device used to sepa-rate out particles, usually atoms or molecules,according to mass. Particles are acceleratedand enter a region where a magnetic fieldBperpendicular to the velocity of the particle ispresent. The particles follow a curved path dueto the magnetic force on them. The radius of thecurved path depends on the massm, chargeq,and velocityv of the particle according to

mv/qB .

Particles with larger mass follow paths with alarger radius allowing different masses to be se-lected.Seemagnetic force on a moving charge.

mass spectrometry, medical applicationsBy mass spectrometry, chemical substances canbe identified by sorting gaseous ions in electricand magnetic fields. Amass spectrometeris adevice that performs this type of sorting by us-ing electrical means to detect the sorted ions.Devices that use photographic or other nonelec-trical means are calledmass spectrographs.

Mass spectrometry allows precise measure-ment of the mass of ions, to show the presenceof different isotopes, and to measure the rela-tive abundance of ions and isotopes in a mixture.Analysis may reveal that organic chemicals haveproduced a spectrum of ions from the fragment-ing of the parent molecule. Then, by identifyingthe fragments according to their masses and rel-ative abundances, the structure of the originalmolecule can be established.

Compounds such asH2, C13, N15, O17,andO18 have an enhanced proportion of iso-topes that make them ideal to label substancesinvolved in biological processes, and thus ap-propriate for mass spectrometry measurements.This tagging allows for precise chemical studiesof such complex reactions as metabolism, pho-tosynthesis, plant respiration, enzymatic reac-tions, phosphate-transfer reactions, and the di-rect application of oxygen in physiological ox-

idation. The details of the metabolic pathwaysinvolved in these reactions can be understood byanalyzing the products from such processes bymass spectrometry.

matter waves Waves associated with a parti-cle of matter, as described by quantum mechan-ics. Also known asde Broglie’s waves.It hasbeen shown that particles with momentump actlike waves with a de Broglie wavelengthλ, givenby λ = h/p, whereh = 6.626 × 10−34 J ·s isthe Planck constant. According to de Broglie’stheory, particles of matter have wavelike proper-ties that can give rise to interference effects, andelectrons in an atom are associated with standingwaves on a Bohr orbit.

maximum ratings The maximum value of aninput that a device can accept with no damage,or the maximum value of an output that a devicewill provide.

Maxwell bridge An electric network de-signed for accurate inductance measurements.A schematic diagram is shown. TypicallyR2

andC3 are adjusted to achieve balance. Anequation of balance is given as

Lx = R1R3C3

Rx = R1R3/R2 .

Maxwell bridge.


mean free path of sound The average dis-tance sound travels between successive reflec-tions in an enclosure. Property that quantifiesthe propagation of sound in an enclosure.

mechanical properties, bone Bones are adynamic system where there is a continuous re-placing of old cells with new ones. Bones reg-ulate themselves so as to remove cells that areno longer functional and at the same time serveas the regulator of the amount of calcium in theblood. On the other hand, there are other parts ofthe bone that do not get renewed, like the bone’sshaft. The cyclic discarding and renewal of cellsis performed by two types of bone cells: theos-teoblasts,which manufacture new bone tissue,and theosteoclasts,which dispose of old andworn cells.

The bones get their full strength and flexi-bility from the connective tissue that surroundsand intermingles with it, serving also as the con-nection between different bones. Vitamin Dis known to be essential to the deposition ofminerals in the bone structure. These miner-als are chiefly calcium and phosphorus. Also,vitamin C, probably most effective as calcium,magnesium, manganese, and zinc ascorbates, isabsolutely essential to the connective tissue’sstrength, flexibility, and endurance.

On a per weight basis, healthy bones areas strong as steel. The bone’s interior is con-structed somewhat like a bridge. Tiny strandsof connective tissue, each strand capable of sup-porting a weight of 25 lbs, act like crossed wiresto give great strength and flexibility. Thus,healthy bones will usually bend rather thanbreak, as they so commonly do.

Of course, the total amount of force that abone can sustain is dependent on the geometri-cal properties of the bone in consideration. Forexample, compression force properties of thefemur indicate that human femur subjected tocompression splinters at 1600 pounds/in2. Thisis about twice the force exerted on the leg bonesof a 160 pound runner and far more than currentweightlifters can heft.

mechanical waves Within the scope ofacoustics, mechanical waves are defined as vi-brations of rigid or elastic solid bodies, with thespectrum of vibration frequencies in the acous-

tic range. Typical examples are high-frequencyelastic waves in delay lines or in nondestruc-tive testing equipment, ground vibrations nearfactories, forge hammers, sound transmissionthrough walls, ceilings, and enclosures, etc.

mechano-caloric effect Refers to the transferof heat and thereby the presence of a temperaturedifference from an imposed pressure difference.It is the inverse process of thethermomechanicaleffectwhere an imposed temperature differencecauses a pressure difference.

medium, acoustic Acoustic disturbances canbe treated as small amplitude perturbations tothe ambient state. The ambient state for a fluid,characterized by the pressure, density and veloc-ity of the fluid when the perturbation is absent,defines the medium through which sound prop-agates. An acoustic medium is homogeneouswhen all ambient quantities are independent ofposition.

medium, homogeneous A switching net-work is considered homogeneous if every con-nection between an inlet and an outlet uses thesame number of crosspoints.

megaphone A rectangular or conical hornused for amplifying or directing the sound of thespeaker’s voice.See alsohorns, sound from.

megger (1) A test for measuring the resis-tance of the insulation in an electric motor. It isusually performed by passing a high voltage atlow current through a motor’s windings.

(2) The type of moving coil galvanometerused to measure high resistances. Part of thecoil is in series with the unknown resistance,while the other part, which carries current di-rectly from the source, is independent of it. Thereading depends on the relative currents of thetwo parts of the coil, and is thus independent ofthe source voltage.

(3) The trade name of an instrument that isspecifically designed to measure high electricalresistance. For example, it is used in testingthe insulation resistance of power and commu-nication lines, high tension insulators, wiring inbuildings and moving craft.


Meissner effect (1) Named after Germanphysicist Walther Meissner (1882–1974). Theeffect by which magnetic flux is excluded from asuperconductor material when the temperatureof the material is reduced below its supercon-ducting transition temperature. It is often re-ferred to asperfect diamagnetismsince the ap-plied magnetic field induces a magnetization inthe superconductor that is opposite the appliedfield direction and exactly cancels the appliedmagnetic flux within the superconductor. Al-lows superconductors to be magnetically levi-tated. Flux is completely excluded in a typeI superconductor and is partially excluded in atype II superconductor.Seesusceptibility, dia-magnetic.

(2) A superconductor is highly diamagnetic.It is strongly repelled by and tends to expel amagnetic field regardless of whether the fieldwas applied above or below the transition tem-perature. This effect shows that superconduc-tivity is more complicated than simply being astate of zero resistance since that by itself wouldcause the flux to be trapped in a sample whichwas cooled through its transition temperature ina field less than its critical value.

membranes, vibration in Many sound gen-erators in practical use take advantage of vibra-tions of membranes or diaphragms. Only a fewdiaphragms in practical applications are mem-branes in the strict sense; more often they areclassified as plates. Vibrations in a membraneare described by the differential equation for thenormal free vibration displacementξ as∇2ξ =1c2

∂2ξ∂t2 , where the velocityc is c =

√T/ρ (T is

the tension in the surface of the membrane andρ the surface density). The solution of the gov-erning equation for the motion of the membraneshows that on the nodal lines of the membraneno motion takes place. For a circular membrane,they can take the form of nodal lines and nodalcircles. The number of natural frequencies of acircular membrane is large in a frequency rangeabove the fundamental; thus membranes are of-ten driven below their fundamental.

memory An electronic device or area inwhich information can be stored. Commontypes are random access memory (RAM), dy-namic random access memory (DRAM), read

only memory (ROM), and programmable readonly memory (PROM).

meridian plane Also known asmeridionalplane,this is a plane that includes or contains theaxis of an optical system and the chief ray. Thesagittal planeis the plane that contains the chiefray and is perpendicular to the median plane.The median plane and the sagittal plane are usedto consider the astigmatism of a lens. The raysof light, which pass in the meridian plane, forma focal line. The focal line is called themeridianfocal line. It is also known astangential plane.

Meridian plane.

message Groups of characters or symbolsprocessed and transmitted from one point to an-other and relayed over a communication system.For successful communication, the message atthe destination should be identical to that em-anated from the source.

metallic glasses Rapidly quenched combina-tion of metals and semimetals that do not havelong range structural (crystalline) order but dohave some short range structural order. Manycombinations of elements have been prepared asmetallic glasses. Some combinations of Fe, Coand semimetals have extremely soft magneticproperties and are useful as cores in transformerswhere rapid reversal of the magnetization withlow hysteresis losses is required. Combinationsof rare-earths and transition metals in metallicglass form such as Tb-Co have extremely largecoercivity. Seehysteresis.

meteorological acoustics A branch of at-mospheric physics or physical meteorology, inwhich the physical processes occurring in the at-mosphere are described, modeled and explained.


meter, electric Any electrical measuring in-strument. Chiefly, the term refers to an integrat-ing meter such as the watt-hour meter used tomeasure the total energy consumed in an elec-trical circuit. See alsometer, integrating.

meter, integrating Any instrument that mea-sures the time integral of an electrical quantity.As an example, the domestic watt-hour meter isused to measure the total amount of electricalenergy consumed.

Michelson-Morley experiment An experi-ment, first performed in the late 1880s by A.A.Michelson and E.W. Morley, designed to searchfor the effect on the speed of light produced bythe motion of the earth through the proposedether,conjectured to fill all of space. The ex-periment divided a light beam from a singlesource into two beams, which were directedalong perpendicular paths and subsequently re-flected back to the division point, as indicated inthe diagram below.

Michelson-Morley experiment.

Assumption that light traveled with a fixedspeed in the ether (and adjustment of the lengthsof the perpendicular paths to be equal) led tothe expectation that the earth’s motion with re-spect to the ether would cause relative speed oflight and its consequent travel time along thetwo paths to differ. It was thought that the re-combined light from the two paths would pro-duce an interference pattern that could be alteredby a change in the orientation of the source-mirror system relative to the earth’s velocity vec-tor through the ether. In contrast, the experimentfound the orientation of the source-mirror sys-tem to have no effect on the interference pat-

tern of the recombined light beams, and led tothe eventual abandonment of the notion of ether.An explanation for the null result of the exper-iment was subsequently provided by Einstein’sspecial theory of relativityin 1905, which rec-ognized the speed of light in free space to be afundamental constant of nature independent ofthe state of motion of the observer.Seeinterfer-ometer, Michelson-Morley.

micro-densitometry In micro–densitometrymeasurements, the optical density of materialscan be measured at the microscopic level. Fromthe output of such measurements the concentra-tion of a substance can be attained.

In clinical uses, a densitometer measures con-centrations of substances on surfaces of film orother supporting media by either a photocellmeasurement of the transmitted light through themedium, or by measurement of the distributionof a specific radioactive element on a radiochro-matogram, as in a radiochromatogram scanner.

micro-dosimetry A micro–dosimetry appa-ratus yields accurate measurements of doses,down to the microscopic level. The techniqueacquires importance when microscopic doses ofradiation have to be achieved in, for example, thetreatment of cancer.

micro-electrophoresis By micro–electro-phoresis the migration of charged colloidal par-ticles or molecules through a solution under theinfluence of an applied electric field is studied.The applied electric field is usually provided byimmersed electrodes. By examining the posi-tions of the particles at different times duringtheir migration toward one of the electrodes,their mobilities are calculated. Properties, likethe mass and charge, of the moving particles canbe determined from inputs such as the viscosityof the suspending media.

A related topic iscataphoresis,by which sub-stances, especially proteins, are separated andmolecular structures analyzed by measuring therate of movement of each component in a col-loidal suspension while under the influence ofan electric field.

microfluorimetry By microfluorimetry, flu-orescence radiation emitted from a sample is


measured. The emitted radiation is in theultraviolet-visible range. The apparatus used tomake fluorescent measurements, the fluorome-ter, is similar to that used to make measurementsof scattered radiation. The detector is usuallyplaced perpendicular to the path of the incidentradiation in order to avoid detecting the incidentradiation. Because fluorescent intensity is, atlow concentrations, linear in concentration, out-puts from a microfluorometer can be callibratedto directly measure the concentration of the flu-orescent substance under investigation.Seemi-croscopy, fluorescence.

microphone, carbon A resistive sensor inthat sound waves incident on a diaphragm applya force on carbon particles in a container througha plunger attached to the diaphragm. The resis-tance of the carbon granules is proportional tothe force applied to the diaphragm. Carbon mi-crophones are characterized by high sensitivity,poor linearity and dynamic range, and are quiterugged. They are used in speech communica-tions when high fidelity is not a requirement. Inthe past they were commonly used as telephonemicrophones.

microphone, hot wire Consists of a fine plat-inum resistance wire grid mounted on a glassrod and attached to a container (that acts as aHelmholtz resonator) by means of a holder onthe neck of the resonator. Electric current ispassed through the grid causing resistive heat-ing, so that the grid behaves as a hot wire. Thesound wave passing across the opening of thecontainer leads to an air surge, thus cooling thewire and causing its resistance to change. Theresistance of the hot wire is sensed in a Wheat-stone bridge arrangement. The grid is calibratedby varying the velocity of the air stream it isexposed to. High sensitivity and small inertiacharacterize hot wire microphones, so that theirresponse is practically instantaneous.

microphone, moving coil A sensor in thatthe pressure of sound waves incident on a di-aphragm is converted into electrical signalsthrough the motion of the coil attached rigidlyto the diaphragm. Also referred to as anelec-trodynamic microphone.Under the influence ofpressure oscillations the diaphragm is exposed

to, the coil moves in the magnetic field gener-ated by a permanent magnet. In this way electriccurrent is induced in the coil by means of elec-tromagnetic induction. The sensitivity of themicrophone scales directly with the intensity ofthe magnetic field and the length of the magneticpath and inversely with the specific acoustic im-pedance. Moving coil microphones are char-acterized by low self-noise, relatively low im-pedance (suitable in applications requiring longcables); they are omnidirectional, insensitive tovariations of ambient temperature and humidity,and are also quite rugged.

microphone, ribbon An electrodynamic mi-crophone, similar in operation to a movingcoil microphone, with a corrugated ribbon sus-pended in the magnetic field replacing the coiland the diaphragm. Both sides of the ribbonare exposed to the sound field, and the ribbondeforms under the influence of the pressure dif-ference. The motion of the ribbon in the mag-netic field induces electric potential in the rib-bon. Since it is a pressure-gradient sensor, itsresponse is bidirectional, described by the di-rectivity factor. The sensitivity of the ribbonmicrophone is similar to moving coil devices, itis less rugged, and the lower moving mass re-sults in better frequency response.

microphones Acoustic transducers that gen-erate electrical signals proportional to the acous-tic pressure or pressure gradient in the ambi-ent in the vicinity of the transducer face. Theproperties of microphones that determine theirfeasibility in a specific application are theelec-troacoustic performance(sensitivity, directiv-ity, frequency response, linearity. . .), electricalcharacteristics(output impedance),sensitivityto external influences,andcost. Microphonesare used as communication devices (telephones,hearing aids), sound recording and broadcastingdevices, and general-purpose and measurementdevices. Microphones can be classified accord-ing to the mechanism that is employed to con-vert acoustic into electric energy:moving coilandribbon (electrodynamic) microphones, con-denser, piezoelectricandelectric microphones,as well ashot wireandcarbon microphones.


microscope, compound A microscope thatutilizes two lenses or lens systems. One lensforms an enlarged, real, inverted image of theobject and is called anobjective lens.The otherlens magnifies the image formed by the objec-tive and is called anocular or eyepiece.Themagnification of the objective lens is the trans-verse magnification. For visual observation, themagnification by the ocular is angular magnifi-cation. For projection with a microscope, themagnification by the ocular should be treated asa lateral magnification.

Compound microscope.

microscope, electron An instrument thatmagnifies objects, it uses beams of electronsinstead of rays of light. In an electron micro-scope, electrons that are emitted from a heatedfilament source are accelerated by electrostaticlens and have a very high energy. The electronsare focused to a very small point on the surfaceof a specimen by an electromagnetic lens. Theimage is projected onto a fluorescent screen orX-ray CCD plane. The non-destructive analy-sis of a sample can be made by measuring theback-scattered electrons and so on.

The resolution of an electron microscope de-pends on the wavelength of the electron whichis related to the energy of the electron by deBroglie’s equation.

Transmission electron microscope.

microscope, field emission A type of elec-tron microscope. High positive voltage is ap-plied to the metal tip surrounded by low-pressuregas (usually helium). The image is formed byfield ionization at the surface of the specimenmounted at a very sharp and cooled (20 to 100 K)tip in an ultra-high vacuum micro chamber. Astrong electric field creates positive ions by elec-tron transfer from surrounding atoms or molecules.To produce a field ion image, carefully con-trolled amounts of image gas are introduced intothe vacuum system. The type of image gas de-pends on the material to be investigated. Usu-ally, neon, helium, hydrogen and argon are usedas an image gas. The images are caused by im-age gas ions striking the fluorescent screen. In-dividual atoms on the surface of the tip can beresolved. The first observation of the atoms isrealized by using FEM.

microscope, flying spot A microscope inwhich a spot of light, produced in the lens sys-tem is scanned through an object. The spot oflight passing through the object is detected by aphoto cell. Use of a photo cell makes the quan-tum efficiency higher than a usual photograph.The image is produced on a cathode-ray tube.

microscope, simple A diverting lens systemthat forms the enlarged image of a small object.The lens of a simple microscope is corrected forspherical and chromatic aberrations. Usuallyit is used to magnify the object at its focus to


form an enlarged image at infinity. For a simplemicroscope, the angular magnificationM is de-scribed by the least distance of distinct visionLand the object distances

M =L

l

(=

25l

),

where the distances are measured in cm. Usu-ally the image is viewed at infinity, therefore,

M =L

f,

wheref is the focus length. It is also known as amagnifier,or magnifying glass. Seemagnifyingpower; least distance of distinct vision.

microscope, stereoscopic A microscopecontains a pair of microscope systems. The twomagnified images of the same field at differ-ent angles (a stereo pair) are observed simul-taneously with a stereo viewer. Some kind ofelectron microscopes use the same technique toobtain the stereoscopic magnified image of thespecimen.

microscope, traveling A low magnifying-power (about 10× ) microscope equipped with agraticule in a plane of the ocular, it also has a rail.Therefore, it is possible to travel horizontally orvertically in order to make very accurate lengthdeterminations.

microscopy, fluorescence Fluorescence mi-croscopy detects and analyzes light emitted af-ter a previously induced absorption event hap-pened. At the moment of absorption of electro-magnetic radiation (ultraviolet to visible range),excitation of the atoms being irradiated occurs.This results in one or more vacant orbitals nearerto the nucleus. Emission of radiation occurswhen the excited electron returns to a lower en-ergy electron orbital.

In general, after a photon is absorbed to an ex-cited state, the decay of the electron can resultin a release of heat, excitation of neighboringmolecules, driving of a chemical reaction, or theemission of photons of lower energy. If the lastcase happens the emitted radiation is also termedluminescence. Luminescence is observed at en-ergies that are equal to or less than the energycorresponding to the absorbed radiation.

Emission can occur by either of two mech-anisms: fluorescence or phosphorescence. Influorescence,the excited electron returns to thelower electron orbital immediately after absorp-tion. When absorption ceases, fluorescence alsoimmediately ceases. Inphosphorescence,theexcited electron decays to an intermediate or-bital with an intermediate spin–flip, and then re-turns to the original orbital with a spin–flip thatreturns the electron to the original spin state.Phosphorescence occurs with low probability.Because inversion of the spinning electron dur-ing the last transition can require a relativelylong time, the emission does not immediatelycease when the absorption ceases. Therefore,fluorescence can be distinguished from phos-phorescence by the time delay in the emission.

A standard method in the study of morphol-ogy is the use of fluorescence microscopy. Be-cause the excitation and emission of light in thefluorescence process is typically done with spe-cific radiation of determined wavelengths, veryspecial fluorescent molecules are used to “tag”the tissue under consideration. In essence, theimages obtained from fluorescence microscopyare produced by the emission from molecules offluorescent dyes added to cells that attach to spe-cific cellular components. Fluorescent antibod-ies are used to locate specific kinds of proteinsand other materials in certain cells of a tissueor in certain regions of a cell. The antibod-ies can be prepared by, for example, injectinginto a rabbit an antigen (myosin), which stimu-lates white blood cells to synthesize antibodiesthat react specifically with the antigen. Afterthe antibodies are isolated and purified, the flu-orescent dye, fluorescein, becomes attached tothem by a chemical reaction. Once the fluo-rescent antibodies are spread over a tissue, theyattach to the molecules that stimulated their for-mation (myosin). In this way, the image comingfrom the fluorescence microscope reveals thesites containing the antigen-antibody complexas bright areas in a dark background.

microscopy, ion The field ion microscope isa development of the field emission microscope.A distinction between the field emission mi-croscope and other electron microscopes is thatthe field emission microscope has a wire with asharpened tip that is mounted in a cathode-ray


tube. Electrons are then drawn from the tip to-ward a screen showing the image by the use of ahigh intensity electrical field. Because the highfield at the tip exerts a large mechanical stress,only strong metals, such as tungsten, platinum,and molybdenum, can be examined in this way.The magnification of the field emission micro-scope is proportional to the ratio of the radius ofcurvature of the screen to the radius of the metaltip. A typical implementation may reach up toone million magnitudes in magnification.

In the field ion microscope,the tip is sur-rounded by helium gas at low pressure. Thegas close to the atom planes on the tip is ionizedand produces an image that can have a magni-fication of up to 10 million magnitudes. Thefield ion microscope has examined metals andsemiconductors, as well as biological systems.

A further development of the field ion micro-scope is the atom probe. In this instrument, in-dividual atoms can be removed from the tip andthen passed through a time–of–flight spectrom-eter, which measures their energy and charge–to–mass ratio. In this way, the chemical natureof each atom in the field ion image may be de-termined.

microwave generator A device that pro-duces waveforms with a high frequency (usu-ally from 1 GHz to 1 THz). Microwave ovensuse these waves to heat the water molecules indifferent substances.

midband frequency The central frequencyof an amplifier’s operating range. It is in thefrequency region where the amplifier responseis nearly independent of frequency. It is com-monly taken asω0 =

√ω1ω2 whereω1 andω2

are the low and high frequency 3 dB points.

A plot of amplifier gain vs frequency showing the low

and high frequency 3 dB points ω1 and ω2 and the

midband frequency ω0.

midband gain The gain of an amplifier atthe midband frequency. The gain is equal to theoutput signal divided by the input signal.Seealso loop gain.

midband loop gain The loop gain of a se-ries voltage feedback loop circuit at the midbandfrequency of the amplifier.See alsoloop gain.

miller coding Seemodulation, delay.

Miller effect The increase in the effectivevalue of the base-collector capacitance in tran-sistors due to the gain in a bipolar transistor. It isimportant for purposes such as rolloff frequencycalculations.

mirage An optical phenomenon caused by abending of light rays in the atmosphere duringabnormal vertical air density distribution.

mirror An optical device for producing re-flection, generally studied under plane, spher-ical, and various surfaces of revolution (e.g.,paraboloid, ellipsoid, aspheric). A plane mir-ror reflects the light without either convergingit or diverging it. The virtual image of the ob-ject is formed and the image is located behindthe mirror at the same distance as the object islocated in front of the mirror.

Virtual image formed by a plane mirror.


A triple mirror, which consists of three planemirrors mounted at right angles to each other,is a kind of multiple plane mirror (seemirror,triple). Fresnel’s mirror and Lloyd’s mirror arealso multiple plane mirrors. A concave mirroris a curved mirror. The curvature of this kindof mirror is concave to the direction of the ob-ject or light source. It forms a real image of theobject and acts as a converging lens. The spher-ical mirror equation describes the relationshipbetween the focal lengthf and the radius of thecurvature of a spherical mirrorR:

f = −R2.

For a concave mirror,R < 0, f > 0. The lateralmagnificationm can be calculated as

m = −s′

s,

where the image distance iss and the object dis-tance iss′. The radius of its curvature is convexto the direction of the object and acts like a di-verging lens.

Real image formed by a concave mirror.

One of the applications of a convex mirroris thekeratometer.The keratometer is a clinicaldevice used for the measurement of the astigma-tism. A spherical mirror can form a point imageof a point object only when the object is put in thecenter of the curvature of the surface of the mir-ror. A mirror is free from chromatic aberration.However, a usual mirror has a spherical aber-ration. An aspherical mirror is used to reducethe spherical aberration. A paraboloidal mirror

and an ellipsoidal mirror are well known as as-pherical mirrors. A paraboloidal mirror is usedfor applications in which an image or an objectis at infinity. A paraboloidal radio antenna is akind of parabolical mirror. Mirrors are used notonly with visible light but with various kind ofelectromagnetic waves (e.g., microwave, X-ray,infrared, and ultraviolet).

mirror, aspherical A mirror of which thesurfaces differ from a spherical surface. Itis used to reduce spherical aberrations. Aparaboloidal mirror and an ellipsoidal mirror arewell known as aspherical mirrors.See alsomir-ror.

Aspherical mirrors.

mirror, concave A curved surface mirror thathas a concavely curved surface in the form of apart of a sphere. It can form either inverted realimages or erect virtual images. For a sphericalconcave mirror, from the spherical mirror equa-tion, focal lengthf is calculated from the radiusof the curvature of a spherical mirrorR(< 0):

f = −R2> 0 .

mirror, convex A mirror of which the surfaceis formed from the exterior surface of a sphereor paraboloid. It forms erect virtual images andgives a diminished wide image. For a sphericalconcave mirror, from the spherical mirror equa-tion, focal lengthf is calculated from the radiusof the curvature of a spherical mirrorR(> 0):

f = −R2< 0 .

mirrors, acoustic A surface with a dif-ferent specific acoustic impedanceZS fromthe medium; acoustic waves propagating in a


medium are reflected from this surface. The re-flected plane wave is formed according to thelaw of mirrors, such that the angle of incidenceθ1 equals the angle of reflection (measured fromthe normal to the surface). The reflection coeffi-cient is determined asR(θ1, ω) = ξ(ω) cos θ1−1

ξ(ω) cos θ1+1 ,where the ratio of the specific acoustic imped-ance of the surface to the medium isξ(ω) =ZS/ρc. See alsoimpedance, acoustic.

mirror, spherical A spherical mirror ofwhich the surface forms a portion of a sphere;it forms the images of real objects. The spher-ical mirror equation describes the relationshipbetween the focal lengthf and the radius of thecurvature of a spherical mirrorR

f = −R2.

The lateral magnificationm can be calculatedas:

m = −s′

s,

where the image distance iss and the object dis-tance iss′. A spherical mirror can form a pointimage of a point object only when the object isput in the center of the curvature of the surfaceof the mirror.

mirror, triple Three plane mirrors aremounted at right angles to each other. A to-tal reflecting prism cube, of which the corner iscut off by a plane crossing each cube face withequal angles. Any ray entering into a triple mir-ror will be returned parallel to the direction thelight comes from. It is also known as acube-corner retro-reflectoror athree plane mirror.Itis also used in some types of interferometers.

mixer A device in a receiver system that car-ries out frequency conversion.Seefrequencyconversion.

mobility Mobility is the carrier drift velocityper unit electric field. It is related to the conduc-tivity by σ = neµ whereσ is the conductivity,n is the carrier concentration,e is the electroncharge, andµ is the mobility.

mobility coefficient (biophysical) Considera system of charged particles immersed in a liq-

Triple mirror.

uid, subject to an external fieldE. The mobilityµ is given by

µ = D(e/kBT ) ,

whereD is the diffusion constant,e the elec-tronic charge,kB the Boltzmann constant, andT the temperature. This relation is known as theEinstein relation.

The mobility of molecules under the influ-ence of an external electric field forms the basisof the electrophoresis technique.In this tech-nique the mobility of particles suspended in anelectrolytic solution is determined for differ-ent particle types, because the mobilityµ de-pends on the diffusion constant of each con-stituent. In essence, particle-type separationcan be achieved from the dissimilar resultingspeeds.Seemicro-electrophoresis; Raman scat-tering; electrophoretic.

modem A single unit consisting of the twodevicesmodulator anddemodulator used fortransmission of digital data that is converted toan analog signal for transmission over a net-work, such as between terminals and a centralcomputer over telephone lines. That is, it is adevice used to convert one form of signal to an-other form for facility compatibility.

modes of vibration Possible patterns of vi-bration of a system. For standing waves, in a vi-brating string, for example, the lowest frequencyf1 is known as the fundamental frequency, andλ1(= 2L) is the fundamental wavelength. Thepossible vibration patterns are characterized by


the frequenciesfn = n2L

√Tµ and wavelengths

λn = (2/n)L, whereL is the length of thestring, T , the tension andµ, the mass densityof the string. The value labels modes or possi-ble vibration patterns. The frequencyfn = nf1is thenth harmonic of the string.See alsohar-monics; frequency, fundamental.

modulation The variation in a reference sig-nal’s amplitude, frequency or phase used to en-code a second signal.

modulation, acoustic Superposition of twotraveling waves of equal amplitude and differ-ent frequenciesω1 andω2. The resulting wave isp = cos(ω1t−γ1x)+cos(ω2t−γ2x), which canbe rewritten asp = 2 cos

[ω1+ω2

2 t− γ1+γ22 x

]cos[

ω1−ω22 t− γ1−γ2

2 x]. The second equation

describes the modulated wave, with the first termrepresenting the high frequency carrier wave andthe second one, the low frequency envelope.The velocity of the first termc = (ω1 +ω2)/(γ1

+γ2) is called the phase velocity, and the veloc-ity of the second termvg = (ω1−ω2)/(γ1−γ2),the group velocity.See alsointerference, acous-tic.

modulation, amplitude A method of encod-ing a signal by changing a reference signal’s am-plitude. (Used to transmit certain radio signals:AM.)

modulation, delay Also known asmillercoding. Signaling scheme used for magnetictape recording and phase shift keyed signaling,since it utilizes a relatively narrow spectral band-width. The majority of the signaling energy liesin frequencies less than one half the symbol rate.

modulation, digital Modulation blends adata signal into a carrier for transmission overa network. Amplitude, duration, and positioncan be modulated. Carrier signals can be var-ied in the following manner. The most commonmethods are: (1) amplitude modulation (AM),which modulates the height of the carrier wave,(2) frequency modulation (FM), which modu-lates the frequency of the wave, and (3) phasemodulation (PM), which modulates the polarityof the wave. The signals are produced by modu-lating a baseband digital carrier or a pulse train.

Such communication methods have been usedfor speech transmission, sampled data systems,and telemetry.

modulation, frequency A method of encod-ing a signal by changing a reference signal’s fre-quency. (Used to transmit certain radio signals:FM.)

modulation index It is common in FM anal-ysis to denote the transmission bandwidth onthe relative magnitudes of the frequency devia-tion by this parameter, which is the ratio of themaximum allowable frequency deviation to thebaseband bandwidth or actual modulation fre-quency.

modulation, phase A method of encodinga signal by changing the phase of a referencesignal.

modulation, pulse A method of transmit-ting an analog signal using pulses. The ana-log signal may be represented by the ampli-tude (pulse-amplitude modulation, PAM), width(pulse-width modulation, PWM), or position(pulse-position modulation, PPM) of the pulses.Alternatively, the signal may be encoded in a se-quence of binary numbers by pulse-code modu-lation, PCM.

modulation, pulse code (PCM) Modulationof a pulse train by coded representation of sig-nal samples. It provides digital transmissionsystems that offer improved solutions to noiseimmunity and noise accumulation problems as-sociated with analog transmission.

modulation, pulse frequency (PFM) In thistype of modulation, the parameter varied is thefrequency of the pulses with time.

modulation, pulse height Also known aspulse amplitude modulation(PAM). The blend-ing of a signal into a carrier wave by varying theamplitude of the carrier. In this type of modu-lation, the parameter varied is the amplitude ofthe pulses with time. Broadcasting systems usethis kind of modulation.


Modulation, pulse position (PPM).

PAM.

modulation, pulse position (PPM) In thistype of modulation, the parameter varied is theposition of the pulses with time. It is delayed byan amount depending on the amplitude of themodulating signal.

modulation, pulse rate (PRM) In this typeof modulation, the parameter varied is the rateat which the pulses are generated with time.

modulation, start-stop This refers to a tele-graph modulation technique that is isochronousfor each character or block. However, it pos-

sesses an undefined interval between consecu-tive blocks or characters.

modulation, suppressed carrier In the re-ception of an amplitude-modulated signal, anapparent reduction in the depth of modulationof a wanted signal caused by the presence at thedetector of a stronger unwanted signal.

modulator A device or circuit used to changea reference signal in order to encode an inputsignal.

Moiré fringes Set of dark fringes createdwhen two ruled gratings or periodic patterns aresuperimposed on one another with the angle ofintersection between the patterns,θ, less than45. Fringes represent the loci of points of inter-section between the superimposed patterns, andhave in general a spatial separation,D, which in-creases as the angleθ decreases in accord withthe equationD = d/θ, whered represents thespatial period of the two patterns. Example ofMoiré fringes produced by superposition of twogratings is shown in figure below. In general,the orientation of the fringes with respect to thelines of the original patterns approaches 90 asthe angleθ goes to 0.

Moiré fringes allow the fidelity of the repli-cation of a diffraction grating or ruled patternto be checked to a high degree of precision, and


combine with measurements ofD andd to makepossible a determination of extremely small an-gles of intersection,θ, on the order of 1 secondof arc.

molar conductivity The conductivity of anelectrolyte solution divided by the concentrationof the electrolyte present.

molecular weight The sum of the atomicweights of all the constituent atoms present inone molecule.

Mollier diagram This is a plot of enthalpy(kJ/kg)versus entropy (kJ/(kg· K)). In this dia-gram, constant-pressure, constant-volume, andconstant-temperature lines are indicated.

monochord An instrument that uses thetransverse vibrations of a string; an ancientmethod to generate musical sound. The mono-chord used by Pythagoras consisted of a sound-ing board and a box with a scale that allowedstretching one or more strings. Nowadays themonochord is used to compare the pitch of tones.The modern monochord consists of a thin metal-lic wire spanning two bridges. The wire isstretched by a weight hanging over a pulley or bya spring tensioning device, and a movable bridgeallows changing the vibrating length of the wire.Also calledsonometer.Sonometers are usuallyequipped with a second wire of fixed length andtension that generates a reference frequency ascomparison.

monochromatic wave Wave with a harmonicvariation in time (and/or space) characterized bya single frequency.Seelight, monochromatic.

monochromator An instrument that is usedto isolate a narrow band of wavelengths radi-ation (monochromatic radiation) from a wideband spectral source.

monopole, electric A single point chargehaving either positive or negative polarity.

morphometrics By morphometry, the mea-surement of the external form or shape (topol-ogy) of an object is understood.

Modern microscopy, e.g., confocal mi-croscopy, allows for the study of the mor-phometry of objects. In some cases, a three–dimensional surface or tissue reconstruction canbe achieved from two–dimensional cross sec-tional data.

Morse code The Morse code, named afterSamuel Finley Breese Morse, represents a bi-nary valued variable-length memoryless codewherein each character is signified by a distinctgroup of dots and dashes. Each character groupof dots and dashes are separated from each otherby a space.

The Morse code may also be signaled by flag-ging — raising both arms vertically above thehead signifies a dot; outstretching both arms hor-izontally to the two sides refers to a dash; low-ering both arms to 45 vertical angles meansspace; circular arm motion overhead signifieserase or repeat; rapid vertical arm motion infront of the torso symbolizes end of message.The Morse code combines source encoding withchannel coding. The Morse code needs 9.296signaling time units to represent the averagegiven information symbol, 24% over the opti-mum minimum. While a more efficient alter-nate but more complex coding scheme may bederived, the Morse code represents a sensiblecompromise between coding efficiency and easefor manual use.

MOS integrated circuit SeeMOS logic cir-cuit.

MOS logic circuit (metal oxide semicon-ductor) One of two major categories of chip de-sign, the other being known as bipolar. It usesmetal, oxide and semiconductor layers. Thereare several varieties of MOS technologies, in-


Morse CodeMorse Occurr. Signal.

Character Code Prob. Time Units

Space 0.1859 6A .- 0.0642 9B -... 0.0127 13C -.-. 0.0218 15D -.. 0.0317 11E . 0.1031 5F ..-. 0.0208 13G –. 0.0152 13H .... 0.0467 11I .. 0.0575 7J .— 0.0008 17K -.- 0.0049 13L .-.. 0.0321 13M – 0.0198 11N -. 0.0574 9O — 0.0632 15P .–. 0.0152 15Q –.- 0.0008 17R .-. 0.0484 11S ... 0.0514 9T - 0.0796 7U ..- 0.0228 11V ...- 0.0083 13W .– 0.0175 13X -..- 0.0013 15Y -.– 0.0164 17Z –.. 0.0005 15

cluding PMOS, NMOS and CMOS. Metal ox-ide semiconductor is a dominant main storagetechnology.

motor, asynchronous An alternating-currentmotor in which the rotating magnetic field pro-duced by the stator causes or aids the rotorin reaching and maintaining the operating ro-tational frequency, e.g., induction motors, ACcommutator motors.See alsomotor, induction.

motor, electric General term used for a widevariety of machines that convert electrical en-ergy into mechanical energy.

motor, induction An alternating-current mo-tor in which the current in the rotor windings isinduced by the alternating magnetic fields setup by currents in the stator windings. The in-teraction of the stator magnetic fields and thoseinduced in the rotor sets the rotation frequency.Induction motors are classed as asynchronousmotors.See alsomotor, asynchronous.

motor, repulsion An alternating-currentcommutator motor in which the rotor is placedin an alternating magnetic field produced by asingle phase stator winding, and in which the ar-mature remains short-circuited in a line at a pre-determined angle with respect to the stator fieldflux. The short-circuit is accomplished throughbrushes which rest on the commutator and arejoined by a low resistance connector. It belongsto the class of asynchronous motors.See alsomotor, asynchronous.

Repulsion motor.

motor, synchronous An alternating-currentmotor, like induction motors, that operates onthe principle of the revolving magnetic field,usually produced by the stator. However, the ro-tor of the synchronous motor is designed to pro-duce a constant magnetic field. Consequently,it requires some means to spin the rotor at a ratethat is in step with the revolving stator field sothat the two fields will lock together and the rotorwill be pulled around by the revolving field.

multi-channel analyzer multiplexing (MUX)In such systems, many signals with overlap-ping frequency spectra are multiplexed, with thecombined signal being used to phase-modulatethe radio frequency (RF) carrier.

multiple access, code division Seemulti-plex.

multiplex Combining two or more signalsinto a single bit stream that can be individu-ally recovered. It is a way of combining twoor more signals into a single signal for trans-mission via a telephone wire, television broad-cast, microwave, or another medium. At the


receiving end, the signals are separated againby a demultiplexer. Some examples of differ-ent multiplexing technologies aretime divisionmultiplexing(TDM), frequency division multi-plexing(FDM), andcode division multiple ac-cess(CDMA). CDMA multiplexing refers to theprocess that occurs in wireless communicationwith satellites in contrast to multiplexing utiliz-ing cables.

multiplexing, code division (CDM) Differ-ent users employ signals that have very smallcross correlation. Correlators can be used to ex-tract individual signals from a mixture of signalseven though they are transmitted simultaneouslyand in the same frequency band. Two ways ofdoing this arefrequency hoppinganddirect se-quence.

multiplexing, color division In optical com-munication systems, multiplexing of channelson a single transmission medium, since eachcolor corresponds to a different frequency anda different wavelength. Each color in a trans-mitted polychromatic light beam on a channelis transmitted in one optical fiber or bundle offibers. This occurs in the visible region of theelectromagnetic radiation frequency spectrumand is the same process asfrequency divisionmultiplexingin the non-visible region of spec-trum.

multiplexing, frequency division (FDM) Amultiplex in which the multiplexed signals, forsimultaneous transmission, occupy separate fre-quency ranges. It refers to the process of usingseveral frequencies on the same channel to trans-mit several different streams of data simultane-ously. This method is used for cable TV trans-mission. Filtering is the key operation used inthe insertion or dropping of a signal or groupsof signals that are multiplexed.

multiplexing, pulse mode Switching, for si-multaneous transmission of signals over a sin-gle channel, in which connections between in-lets and outlets of one or more switching stagesare provided by a plurality of separate metallicpaths.

multiplexing, space division (SDM) Thecombining of several independent and isolatedfibers or wires in a single bundle or cable in or-der to be able to use each fiber, or bundle, asa separate communication channel over whichseveral signals can be transmitted. Each spaceddivision multiplexed channel may be time divi-sion or frequency division multiplexed.

SDM.

multiplexing, time division (TDM) A multi-plexing technique, in which two or more signalsare transmitted at the same time over the samecommunications channel. The individual sig-nals are combined by interleaving bits. In thismethod of multiplexing, a channel is shared ona time basis rather than frequency. This form ofmultiplexing is a cheaper and simpler form thanfrequency division multiplexing.

multiplexing, trunk group A set of trunkstreated as a unit from traffic point of view inwhich telephone companies multiplex manyconversations over a single physical trunk. Eco-nomically, it costs about the same to install andmaintain high and low bandwidth trunks, com-pared to permanent connection between any twoswitching stages in an exchange.

multiplier A resistor used with a voltmeterto allow the measurements of voltages not in therange of the voltmeter. An operational amplifiercircuit whose output is the product of the twoinputs.

multivibrator, astable A multivibrator withtwo quasi-stable states. It oscillates between thestates spontaneously without reaching a steadystate. Often used as a square wave or clockingwaveform generator.

multivibrator, bistable A multivibrator thathas two stable states that it switches between


when triggered. It is very often used as a mem-ory device in digital flip-flops.

multivibrator, monostable A multivibratorthat has one stable and one quasi-stable state. Itchanges to the quasi-stable state on being trig-gered and then returns spontaneously to the sta-ble state. It can be used to gate other circuits.

multivibrators Oscillating circuits, typicallybased on the saturation of transistors, whichhave two possible states used in applicationssuch as timers and logic circuits.

multivibrator, stable states Voltage levelsin a multivibrator that will remain unchanged inthe absence of a triggering signal.

muscle contraction, force-velocity relationThe force–velocity relationship during musclecontraction is experimentally determined by al-lowing a muscle to contract while doing force ona constant load. The resistive force of the mus-cle while shortening is then plotted as a functionof its velocity (seefigure below). The horizon-tal axis is plotted relative to a maximum velocityVmax, while the vertical axis is plotted relativeto a maximum isometric forceP0. Point (A) inthe figure illustrates where the sarcomeres aregetting longer; negative relative velocity meansthat the actin and myosin are moving in oppositedirections. Point (B) illustrates the shortening ofthe sarcomeres.

Muscle contraction.

muscle contraction, length-tension relationThe length–tension relationship of the muscle

is determined experimentally by measuring theforce a muscle can generate while being ex-tended to a certain length (isometric conditions).

From these studies it was determined that theforce generated by a muscle at a given lengthis a function of the magnitude of the overlapbetween the actin and myosin filaments (seemuscle, mechanics). By the simple argumentthat the force performed by the muscle has tobe linearly proportional to the number of crossbridges, then the length–tension curve has apeaked shape. The argument is the following:at short muscle lengths the thin and thick fila-ments in the sarcomeres overlap so much thatthe number of cross bridges is very small, thusa small force. As the muscle extends, there is apeak in the force right at the point that the thickand thin filaments do not overlap anymore (max-imum number of cross bridges). With longerlengths, the number of cross bridges goes to zeroas the overlap diminishes.

Muscle contraction.

In the figure above, the initial rise of the curve(A) indicates the overlap between the actin andmyosin reaching a plateau (B) where the overlapis minimum. The curve then decreases (C) dueto loss of cross bridges and would continue downto zero if it were not for the passive stiffnesscontribution of the muscle that then makes thecurve increase rapidly (D).

There are additional contributions to thelength–tension curve due to the spring–like re-sponse of the muscle that is linear in the dis-placement (seemuscle, mechanics).

muscle, mechanics Muscle fibers in skele-tal muscles consist of bundles of myofibrils that


contract in response to neural or electrical stim-uli. The myofibrils, in turn, consist of repeatedcylindrical units calledsarcomereswhich are thesmallest contractile unit in the muscle. The my-ofibrils are surrounded by a bag–like structurecalled thesarcoplasmic reticulum.

The sarcomeres are composed of thin andthick protein filaments that, upon movement ofone relative to the other, achieve the contrac-tion. The thin filament is composed of pairs ofpolymerized actin monomers arranged in a he-lix. The thick filaments are formed by myosinmolecules, each one having two entwined tails(≈ 150 nm long) and a double globular head.

Contraction of the muscle is basically a resultof the cyclical interactions between the thin andthick filaments. During contraction the glob-ular heads of the myosin molecules attach toreceptor sites on the actin molecules. These at-tachments form what are calledcross bridges.Shortly after, the myosin undergoes a confor-mational change that exerts a pulling force onthe actin filaments after which they detach fromone another and the cycle starts again.

The whole process is triggered by a signalfrom a motor neuron that depolarizes the sar-coplasmic reticulum making it releaseCa2+

into the sarcomeres. This constitutes the firstpart of the cycle where theCa2+ then bindsto the actin monomers producing a conforma-tional change that exposes a receptor site forthe myosin head to attach. After attachment ofthe myosin and actin, the actin molecule rotates,provoking the pulling that contracts the space inthe sarcomeres and later detachment of the twofilaments, at which point everything relaxes toits original position. The cycle continues as longasCa2+ and ATP are present. When the depo-larizing signal stops,Ca2+ is pumped back intothe sarcoplasmic reticulum causing relaxation.

With respect to the mechanical response ofmuscles, muscles are like a spring. Muscles gen-erate a restoring force when they are stretchedbeyond their resting lengthL0 with a force ofthe form

F = k (L− L0) ,

wherek is the spring constant. When the muscleis stimulated, the contractile elements shortenand the muscle starts to develop tension that can

be understood as ifL0 got shorter, i.e.,L0 getssmaller as contraction occurs.

If a weight were attached directly to the mus-cle, the weight would be pulled up progressivelyas the rate of stimulation increased until themuscle’s restoring force precisely matched theweight.

musical instruments Instruments that pro-duce musical sound. They are classified accord-ing to the nature of the primary vibrator generat-ing the sound asstring instruments,wind instru-ments andpercussioninstruments. Acousticsstudies the laws governing the action, design,and construction of musical instruments.

musical sound A combination of soundsin rhythm, harmony and counterpoint, gener-ated by musical instruments or human voice,as a medium of artistic expression. Musicalacoustics considers the physical characteristicsof sounds that might be perceived as music, theproduction of musical sound, and its transmis-sion to the listener. The perceived qualitiesof musical sound, such as loudness, pitch andtimbre, depend on physical parameters, such assound pressure, frequency and spectrum.

mutual capacitance Mutual capacitance,also known as thecoefficient of induction,is de-fined as the ratio of the induced charge on theother conductor to the potential of a giving con-ductor when no other conductors are nearby.

mutual conductance The derivative of thedrain current with respect to the gate-sourcevoltage in a common source FET circuit.

mutual intensity Quantity defined to mea-sure the coherence between fields at two sepa-rated space points. Equal to the average over thetime t of a product of the “normalized” fieldsat the two separated space points evaluated ata common timet. The quantitative definitionof mutual intensity requires introduction of a(scalar) field function of position and time,V (r, t), defined to determine the time averagedintensity,I, of the field at a space pointr via theequation

I(r) = 〈V ∗(r, t)V (r, t)〉 ,


where the angular brackets denote an averageover t in a time interval large compared to oneperiod of oscillation of the field. Latter def-inition allows mutual intensity of the fields atpositionsr1 andr2 to be defined by the quantity

Γ (r1, r2) ≡ 〈V ∗ (r1, t)V (r2, t)〉 .

Quantity has the significance of determining theinterference term in an expression for the fieldintensity resulting from the superposition of twofields derived from spatially separated pointsources.

myoelectric activity Myoelectric activityrefers to the electrical properties and responseof muscle tissue. Electrical impulses result-ing from these myoelectric properties may beamplified and used in diverse applications suchas the monitoring of the response of specific

muscles to stimuli as well as the control or op-eration of prosthetic devices.

myopia (short-sightedness) This results fromthe lenses of the eye refracting the parallel raysof light focused not on a retina. It is usuallycaused by an abnormally long eyeball. For cor-rection, diverting spectacle lenses are used tomove the image back to the retina.Seeeye,near-sighted.

Myopia.


NNAND A Boolean operation in which theoutput is 0 if both inputs are 1 and the output is1 if either of the two inputs is 0.

near point The nearest point at which a hu-man can focus on an object with accommoda-tion. It is different from the least distance ofdistinct vision. The lenses of the human eye be-come harder with age which causes the nearestpoint to recede with age.See alsoleast distanceof distinct vision.

negative logic Logic devices in which a highvoltage determines a binary level 0 and a lowvoltage a binary level 1.

negative resistance A resistance with a neg-ative value. This occurs when the current in anelectronic device decreases as the voltage acrossit increases, e.g., Esaki (tunnel) diodes, resonanttunneling diodes.

negentropy In information theory, negen-tropy is related to the amount of informationcontained in a given system. It is the analog ininformation to what entropy is in thermodynam-ics. The analogy stems from the formal equiva-lence between the mathematical expression forentropy (S = k logW ) and Shannon–Wiener’sinformation (H = − log2 pa). The motivationfor the negative sign in front is to yield the pos-itive quantityH since probabilitiespa are lessthan1.

neon liquefier Any one of several machinesthat liquefy neon by causing it to undergo adia-batic expansion and to do external work.

neon tube A tube in which neon gas at lowpressure is ionized and heated to produce light.

nephelometer An instrument that measuresthe size and concentration of particles suspendedin liquids by analyzing the scattered light from

the liquid. The nephelometer finds applicationsin the quantification of the turbidity in watercaused by colloidally dispersed particles.

Nernst equilibrium potential The Nernstpotential defines the potential of the cell mem-brane due to a particular ion that is distributedboth outside and inside the cell. The Nernstequilibrium potential for an ion is given by

V =RT

zF· ln Co

Ci,

whereR is the gas constant,T the absolute tem-perature,z the charge of the ion in considera-tion, F Faraday’s constant, andCo andCi arethe concentrations of the ion outside and insidethe cell, respectively.

Nernst potentials exist separately for eachion. The total membrane potential is determinedby the flow of all the ions that cross the plasmamembrane.

nerve conduction velocity Nerve conduc-tion velocity refers to the velocity of propaga-tion of an electrical impulse (action potential)through nerve fibers. The velocity of conduc-tion along the nerve fiber is dependent uponseveral factors. The first factor is the outsidediameter of the nerve fiber. The fastest conduc-tion velocity occurs in the largest diameter nervefibers. Another factor is the temperature of thenerve fiber. Conduction velocity increases athigh temperature and decreases at low. Con-duction velocity is also affected by myelinationof the nerve fiber. Since ions cannot cross thelipid content of the myelin sheath, they spreadpassively down the nerve fiber until reaching theunmyelinated nodes of Ranvier. The nodes ofRanvier are packed with a high concentrationof ion channels, which upon stimulation, prop-agate the nerve impulse to the next node. In thismanner the action potential jumps quickly fromnode to node along the fiber in a process calledsaltatory conduction(from the Latinsaltare,“tojump”).

Because there is a definite relationship be-tween the diameter of the nerve and the conduc-tion velocity, the diameters of the nerves forma basis for classifying mammalian nerve fibersinto groups in order of decreasing diameter anddecreasing conduction velocity.


Seenode of Ranvier, electrical characteris-tics.

nerve impulses, propagation of A nerve im-pulse has its origins when an external stimulusis applied to a neuron. The stimulus induces achange in the membrane potential of the neuronat the point of stimulation. In order for this nerveimpulse, calledaction potential,to travel alongaxons or dendrites, the membrane needs to besequentially depolarized all along the length ofthe path that the signal will follow. These mem-brane depolarizations are very localized in spaceand time and typically consist of a change in themembrane potential from−60 mV to approxi-mately+30 mV, as measured in squid axons, inless than a millisecond. The depolarization is aresult of a series of rapid and sequential openingand closing of voltage-gated Na+ and K+ chan-nels. After the signal traverses a particular pointin the membrane, the depolarization ceases soonafter and returns to its resting value.

There are several steps in the depolarizationprocess. This process starts with a relativelysmall initial change in the membrane potential(−60 to−40 mV) that leads to the rapid openingof Na+ channels. The Na+ then flows into thecell by diffusion due to the concentration gradi-ent and also driven by the membrane potential.The large influx of Na+ subsequently changesthe membrane potential to+30 mV, approach-ing the Na+ equilibrium potential of approxi-mately+50 mV. At this time the Na+ channelsare inactivated and voltage–gated K+ channelsare opened, substantially increasing the perme-ability of the membrane to K+. Similarly to theNa+, the K+ flows rapidly out of the cell lead-ing to a−75 mV in the membrane potential. Atthis membrane potential, approximately equalto the K+ equilibrium potential, the K+ chan-nels are inactivated and the membrane returnsto its resting level of−60 mV.

This process is repeated for every elementof length that is adjacent to the initial stimulusthereby allowing the action potentials to traveldown the length of nerve cell axons as electricsignals.

network An interconnection of basic com-ponents such as resistors, capacitors, and induc-tors in series, parallel, delta,π, etc, groupings to

form a system that performs a specific function.The response of this system to an electrical inputis dependent on the component type, value, andthe manner of connection. The name given to anetwork may be described by:

• the types of components, such as re-sistive, resistance-capacitance (R-C),inductance-capacitance (L-C), induc-tance (L) networks, etc.

• the method of interconnection, such as se-ries and parallel networks,

• the response, such as linear networks thathave linear relationships between the volt-ages and the currents.

Networks are termed as active if they have anenergy source or sink other than normal ohmiclosses. Passive networks, however, do not havean energy source.

The term network may alternately be used torefer to the interconnection of communicationfacilities, e.g., computer terminals, telephones,etc.

network, distributed parameter A circuitthat behaves as if parameters such as resis-tances, capacitances, and inductances exist con-tinuously over a physical length. For example,in a two wire transmission line, the distributedparameters are series resistance, series induc-tance, shunt conductance, and shunt capacitanceper unit length of line.

network, ladder A network that consistsof H, L, T, or π networks connected in tan-dem. Ladder networks have been used as nar-row bandpass filters and digital-to-analog con-verters.

Ladder network.


network, linear A network in which the cur-rents and voltages of the circuit elements havea linear relationship. This usually implies thatthe elements, such as capacitances, inductancesor resistances, are constant in magnitude withvarying currents.

network, lumped parameter In circuit anal-ysis, any network in which distributed parame-ters such as inductances, capacitances, or resis-tances can be treated as single parameters con-centrated at a point. This approach is usuallyvalid for a specific frequency range.

network, nonlinear A network in which thecurrents and voltages of the circuit elements donot all have a linear relationship. For exam-ple, the voltage across a semiconductor diode isnot directly proportional to the current throughit. In signal transmission systems, a nonlinearnetwork is one in which the signal transmissioncharacteristics depend on the magnitude of theinput signal.

neuron The neuron (nerve cells) forms thebuilding block of the nervous system in the body.Neurons are highly specialized cells that trans-mit electrochemical signals throughout the bodyin response to internal and external stimulus. Inhumans, neurons may extend to more than a me-ter long, while in some invertebrates (e.g., thesquid) the neuronal projections (axons) diame-ters can be as large as 1mm.

The neurons are composed of a cell bodywith nucleus, an axon (main signaling projec-tion) and one or more smaller projections calleddendrites. The axons carry signals over longdistances and the dendrites, having a branch–like structure, serve to receive incoming signalsfrom other neurons. The basic mechanism ofthe neuron is to respond (or not) by an actionpotential (seenerve impulses, propagation of)to several electrical inputs from the dendrites.If the neuron “fires” an action potential, then itpropagates the signal to other neurons by releas-ing neuro–transmitters across the synapse withother neurons (seeneuro–transmitters).

Major types of neurons include theassocia-tive neurons,found within the central nervoussystem (CNS), which link sensory and motorneurons, andmotor neurons,which take im-

pulses from the CNS to muscles, glands, or othereffector tissues.

neuro-transmitters Neuro–transmitters aresmall hydrophilic molecules stored in the axon’sbulbous end. There are more than 300 knownneuro–transmitters of which the endorphins andacetylcholine are the most common examples.

The function of neuro–transmitters is to al-low different neurons to communicate by elec-trical signals even when their membranes arenot in direct contact. The way they work isthe following: If an electrical signal is travelingthrough the membrane of a neuron, upon reach-ing the end of the axon the signal is carried fromone neuron to the other via neuro–transmittersthat are released in the space (synapse) in be-tween the two neurons. The neuro–transmittersthen travel from one (presynaptic) membrane tothe other (postsynaptic) membrane, ultimatelybinding to the postsynapse neuron receptors inquantity that is in proportion to the strengthof the electrical signal. The receptors, usu-ally ligand–gated ion channels, then are opened(e.g., acetylcholine receptors of muscle cells).

In muscles, acetylcholine opens channels thatare permeable to both Na+ and K+. The en-trance of Na+ to the muscle cell depolarizes thecell membrane and triggers an action potential.The action potential in turn opens the voltage–gated Ca2+ channels, leading to the increase inintracellular Ca2+ that results in contraction (seemuscle, mechanics).

neutral point The common point of a Y-connection in polyphase circuits. Also refers tothe point at zero voltage in a system consistingof a number of identical parts. Although bothof these definitions generally refer to power anddistribution transformers, they can also be usedfor systems where the circuit elements are non-reactive resistances. In this case, the number ofresistances is two for direct-current or single-phase alternating-current, four for two-phase,and three for three-, six- or twelve-phase sys-tems.

neutron therapy Neutron therapy falls intothe larger category ofradiation therapiesbywhich energy is transported from a source toa target. The source can be ionizing radiation


Neutral point for a Y-connection 3-phase circuit.

(e.g., X–rays, gamma rays) or particles (e.g.,electrons, protons, neutrons). When the source,electromagnetic or particle, interacts with tis-sue, a production of free radicals and oxidantsoccur that subsequently damage or break cellu-lar DNA, impairing cells from the capacity todivide and may even lead to cell death.

The therapy is widely used in cancer treat-ment. Damage to normal cells is diminishedby the careful shielding of adjacent areas to thetreated ones. When used properly, radiationmay cause less damage than surgery and canoften preserve organ structure and function.

Atomic elements used to some extent in can-cer therapy as a source for neutrons in neutrontherapy are the heavier actinides — those be-yond plutonium in the periodic table.

Newton’s rings Interference pattern pro-duced when light is reflected from the upper andlower surfaces of an air film of variable thick-ness formed in the space between a convex sur-face and a flat surface in which it is in contact.Typical geometry used to show Newton’s ringsinterference pattern involves light incident onair gap between glass surfaces of different cur-vature. Diagram below shows example of inter-fering light rays in this geometry.

Newton’s rings.

The maximum in the interference pattern forlight of a particular wavelength occurs at posi-tions where the film thickness matches an in-teger multiple of that wavelength. Resultingfringes follow lines of equal thickness corre-sponding to concentric rings centered at the con-tact point between the curved and flat surfaces.

A dark spot occurs in the center of the fringepattern as a consequence of a phase change ofπbetween rays reflected from glass to air and airto glass surfaces, respectively. Where incidentradiation corresponds to white light, the inter-ference pattern consists of colored rings witha central dark spot. The first observation ofrings, credited to Isaac Newton (1642–1727),may have been made by Robert Hooke in thesame era.

nitrogen, liquid A single stage air liquefac-tion process using the Stirling cycle is usuallythe basis for Philips nitrogen liquefiers. Liquidnitrogen is usually preferred over liquid air ongrounds of safety and constant temperature. Itis obtained at a temperature of 77 K liquid, andnitrogen is usually used in the precooling stagewhen cooling with4He. The quality of vacua inthe laboratory is improved by using liquid nitro-gen cold traps. Liquid nitrogen is relatively in-expensive compared to liquid helium for coolingand is therefore preferred for cooling equipmentusing high temperature superconductors.

NMR nuclei in biological materials A non-invasive diagnostic technique drawn from an ap-plication ofnuclear magnetic resonance(NMR)ismagnetic resonance imaging(MRI) where ra-dio waves are beamed into a person who is underthe influence of an external powerful magneticfield. Because different atoms in the body ab-sorb radio waves at different frequencies of theradio waves, the absorption can be measuredand specific data from specific atomic speciescan be reconstructed by a computer to renderthree–dimensional images of internal structuresin the body. Molecules containing hydrogen(e.g., water molecules in body tissue) are espe-cially suited to align magnetically and resonate,giving out enhanced absorbance.

Unhampered by bone and capable of produc-ing images in a variety of planes, MRI is usedin the diagnosis of brain tumors and disorders,


spinal disorders, multiple sclerosis, and cardio-vascular disease. The procedure is consideredto be without risk to the patient.

NMR probes Probes that can be imagedby nuclear magnetic resonance (NMR) includeatoms of fluorine-19 and phosphorus-31, amongothers (not including hydrogen). These atomscan serve as probes for various tracer studies.

NMR zeugmatography In typical magneticresonance imaging (MRI) studies, the nuclearsignal, coming from the coupling of the exter-nal magnetic field and radio waves to the atomicnuclear spin, has a large wavelength (tens of me-ters). This means that any directional informa-tion is lost or very hard to determine.

One way to get spatial information from MRIstudies is by using the dependence of the reso-nance of the nucleus to the external driving fre-quency of the radio waves. In this way spatialinformation is coded in the frequency part of thesignal. This is accomplished by using a mag-netic gradient instead of a constant field. Then,by changing the magnetic gradient spatial infor-mation is put into the frequency domain. ByFourier transformations the signal can be con-verted fromk–space to real space. This gradientfield method is calledzeugmatography.

nodal planes/points A pair of conjugatepoints or planes on an axis of an optical sys-tem. An incident ray passing through one ofthem causes an emergent ray to pass throughthe other.

nodal slides The nodal slides are used fordetermination of nodal points or the nodal planelocation for a lens system. The nodal slidesare equipped with one collimating telescope thatcontains a distance marker. The collimator num-ber coincides with the focal lengths of the lenscombination.

node Locations in a standing wave at whichthe displacement is zero at all times. Thedisplacementyn for a vibration correspond-ing to the nth harmonic frequencyωn in astanding wave, as a function of the longitu-dinal coordinatex and timet, is representedbyyn(An cosωnt+Bn sinωnt) sin ωnx

c , where

An andBn are constants andc the speed ofsound. The frequencyfn = nc/2λ is n timesthe fundamental frequency, andλ is the wave-length. By considering the termsin(ωnx/c) =(nπx/λ), it becomes obvious that the displace-ment yn is zero at all times forx such that(nπx)/λ = mπ, with m = 0, 1, 2, 3, . . . , n.These positions are called thenodal pointsornodes of the standing wave pattern.

node of Ranvier, electrical characteristicsThe nodes of Ranvier are regularly spaced gapsin the myelin sheath along the length of a nervefiber. Because these gaps are the only placeswhere the membrane of the neuron is in contactwith the extracellular plasma, this region has ahigh concentration of ion channels that permitthe flow of ions in and out of the membrane.

The myelinated spacing is one of the keycomponents so that the action potential does notdecay rapidly with distance (high capacitance),and the high concentration of ion channels per-mit the repeat of the signal from node to node(rapid depolarization), thus giving rise to the“saltatory” transport of the action potential.

node of Ranvier, equivalent electricalnetwork Under the assumptions of the Ca-ble model, the circuit fragment models an axon,where the vertical RC components represent theaxon membranes at the nodes of Ranvier, the toprow of resistors corresponds to the resistance ofthe intracellular fluid, and the bottom row resis-tors correspond to the intracellular domain. Thevertical resistors in the RC components repre-sent the fact that the nodes of Ranvier act asleaky capacitors.

Because of the resting potential of the mem-brane, all the capacitors are charged. The actionpotential then corresponds to a discharge of acapacitor at a particular node.

Following further the assumptions of the Ca-ble model, all of the horizontal resistances arethe same. Using the “infinite axon” approxima-tion, where both ends of the circuit are assumedthe same, and the minimal current is flowing ex-cept in the node where the action potential isoccurring, we can simplify the above circuit.

Typically R, the resistance of the fluid (≈12kΩ), is an order of magnitude smaller than


the membrane resistanceRm (≈ 200kΩ). Rt isthe equivalent resistance of the rest of the axon.

Ranvier.

noise Sound in the acoustic spectrum thatis unwanted, either because of its effect on hu-mans, interference with the perception or de-tection of other sounds, its effect on fatigue,or malfunction of physical equipment. Sourcesof noise include industrial sources (machinery),transportation (moving sources, such as aircraftand road vehicles as well as airport or roadnoise), community noise, or noise generated byair conditioning systems.

noise, ambient, level The surrounding back-ground noise level caused by natural and man-made sources. The noise at radio frequenciesthat is generated by natural causes is referred toas static noise. Static noise normally originatesfrom the atmospheric conditions caused by thepresence of static electricity in the air throughwhich radio waves are propagated. Manmadenoise arises from human sources, usually elec-trical devices. At radio frequencies, ambientnoise is usually heard as an unpleasant cracklingsound at the output loudspeaker. Ambient noiseshould not be confused with noise generated byelectrical circuits in receivers and transmitters.

noise, biological effects of The effect ofnoise on humans depends on the intensity ofnoise and its duration, and ranges from annoy-ance, interference with speech, communicationsand sleep, to hearing damage (for example byrupturing the eardrum), and hearing loss in moresevere cases. In addition to the acoustical fac-tors nonacoustical factors such as attitude andenvironmental factors, influence the level of an-noyance noise poses. Descriptors and measures,involving factors such as loudness and loudnesslevel, noisiness and perceived noise level, soundlevels, articulation index, speech interferencelevel, indoor noise criteria, equivalent and per-centile sound levels, day-night sound levels, etc.are available to find criteria regarding accept-able noise levels. Means to combat noise andits negative effects on humans include hearingprotection devices (earplugs, earmuffs or hel-mets), passive measures of noise control (soundabsorbing or damping materials, vibration iso-lation, sound barriers, enclosures), as well asactive noise control (sound sensed by a micro-phone is processed and fed back through a loud-speaker to destructively interfere with the soundemitted by the primary source).

noise, density in space The RMS noise-voltage density,vN , is given by

VN (rms) = vNB12 ,

whereVN is the RMS noise voltage measure ina bandwidthB. In general, detected noise de-pends on the measurement bandwidth. A white-noise source has avN that does not depend onfrequency. The squared noise densityv2

N is alsocommonly encountered.

noise factor The ratio of the product of thesignal outputSo and input noise powerNi to theproduct of the input signalSi and output noisepowerNo, = SoNi/SiNo.

noise, Johnson Noise arising from fluctu-ations of carrier velocities in a resistor at a fi-nite temperature. This is also calledthermalorNyquist noise.These fluctuations give rise toa mean square noise voltageV 2

noise = 4kTRBwherek is Boltzmann’s constant,T is the tem-perature,R is the value of the resistance wherethe noise is generated, andB is the bandwidth.


noiseless coding A source coding scheme isnoiseless if all the encoder’s input symbol val-ues are uniquely related to the encoder’s outputsymbol values. That is, no distortion or noisehas been introduced in the coding process. Oth-erwise, the source code embodies a corruptedrepresentation of the original data. The aver-age number of bits necessary to encode a dis-crete memoryless source must equal or exceedthe source’s entropy.

noise level The value, usually given in deci-bels, of a noise signal measured at a particularconnection.

noise meter The process of quantitativelydetermining one or more acoustical properties(such as duration, sound pressure level, varia-tion as function of time, frequency content, pres-ence of pure tones or background noise level,impulsive character) of acoustic noise. No sin-gle descriptor is available to uniquely character-ize noise. Noise generated by moving sources iscommonly characterized in terms of sound pres-sure levels at a certain defined location. Station-ary sources are usually described by their soundpower output that is measured either by mov-ing a small source into a reverberation room orby measuring it on a hemispherical or rectangu-lar measurement surface and applying a correc-tion to these data. Data are also obtained in thenormal environment in case of larger sourcesof noise and corrected appropriately. The re-sponse of the equipment used to measure noiseis often weighted to match the way the humanauditory system would respond to it. The soundlevel of noise is defined through the sound-levelmeter built and operated according to AmericanStandard requirements.See alsonoise; noise,biological effects of.

noise, pseudo-noise (1) Random noise isnoise that arises from any randomly occurringtransient disturbance. If the rate of occurrenceof the disturbance is sufficiently high, it resultsin white noise, similar to thermal noise. If therate of occurrence is low, random noise con-tributes to impulse noise. All electronic cir-cuits and devices suffer from thermal and ran-dom noise.

(2) A very accessible and attractive techniquefor generating a source of digital noise is to gen-erate a pseudo-random sequence. A pseudo-random sequence of binary digits has similarprobability and correlation properties to an idealstring of random digits. The pseudo-randomsequence is, however, totally predictable andrepeatable although any portion of such a se-quence looks for all intents and purposes justlike a truly random sequence. It is easy to gener-ate sequences of bits that have good randomnessproperties using standard deterministic logic el-ements such as shift registers.

noise, random Random noise describes anacoustical quantity, such as acoustic pressure,whose variation as a function of time is de-scribed through the Gaussian (normal) distri-bution. Examples of such noise are the ran-dom noise caused by the random motion ofair molecules. Electrical quantities can also becharacterized by random noise (motion of elec-trons).

noise, shot (Also known as Schottky noise,flicker noise.) Shot noise results from the sta-tistical fluctuations of charge carriers across ajunction and is given by

i2N = 2eIB ,

where iN is the RMS noise current,e is thecharge of an electron,I is the DC current, andB is the bandwidth of the measuring instrument.Shot noise is independent of frequency. The dif-ference between thermal noise and shot noise isthat the latter is related to the DC current throughthe junction.

no-load (electric circuit) Operation of anyelectric circuit under rated operating conditionsbut in the absence of an impedance at the output.

non-inverter A logic device that does notchange the state of the input voltage.

nonlinear circuit A nonlinear circuit is acircuit in which the amplitude of the current isno longer proportional to the amplitude of thevoltage. A circuit containing a nonlinear devicesuch as a diode is an example of nonlinear cir-cuit.


non-linearity The type of relationship of twoquantitiesxandy that cannot be expressed in theform y = ax+ bwith a andb constants. A con-dition in an amplifier circuit in which the outputis not proportional to the input. A property ofcircuits that cannot be analyzed with just seriesand parallel branches of linear elements.

nonlinear susceptibilities Tensor quanti-ties that relate the electric polarization vec-tor in a material medium,P(r, t), to productsof the components of the electric field vectorE(r, t), which induce the polarization. Moreprecisely, functionsχ(N)

jkl...(r, t−t1, t−t2, . . . t−tN ), termedelectric susceptibilities,are definedthrough an expansion of thej-th component ofthe electric polarization vector in products of theelectric field components in a given medium inthe form:

Pj(r, t) =

∞∫−∞

dt1 χ(1)jk (r, t− t1)Ek(r, t1)

+

∞∫−∞

dt1

∞∫−∞

dt2 χ(2)jkl(r, t− t1, t− t2)

Ek(r, t1)El(r, t2)

+

∞∫−∞

dt1

∞∫−∞

dt2

∞∫−∞

dt3 χ(3)jklm

(r, t− t1, t− t2, t− t3)Ek(r, t1)El(r, t2)Em(r, t3) + · · · ,

where use is made of the summation convention(implying a sum over repeated indices from 1 to3).

The quantities χ(N)jkl...(r, t − t1, t − t2,

. . . t−tN ), forN > 1, are then defined to be thenonlinear susceptibilities, andχ(1)

jk (r, t− t1) isdefined to be thelinear susceptibility. As de-fined, the magnitudes of the susceptibility func-tions are determined strictly by the propertiesof the material medium, while the time depen-dence of the functions is restricted by causality

through the equation

χ(N)jkl...(r, t− t1, t− t2, . . . t− tN ) = 0 ,

ts > t (s = 1, 2, 3, . . . ) .

Fourier transformation of the functionsPj(r, t) andEj(r, t), and replacement of theintegration over the transform variableω by asummation over discrete values ofω, make pos-sible a definition of Fourier-transformed suscep-tibility functions via an equation of the form:

Pj(r, t) =∑N=1

∑±ωn

e−iωnt∑

±ω1,...±ωN∑N

sωs=ωn

χ(N)jkl... (ωn;ω1, ω2, . . . ωN )×Ek (ω1)El (ω2) . . . Em (ωN ) ,

where the dependencies on the space coordinater are suppressed on the right of the equality sign.In general, the magnitudes of the nonlinear sus-ceptibilities are small compared to the magni-tude of the linear susceptibility, and the terms inthe expansion forPj(r, t) beyond the first termare therefore small compared to the first term,except where the magnitude of the electric fieldE is large.

non-reciprocal device A device for whichthe reciprocity theorem does not apply. In thesedevices,Io/Vi measured with the output termi-nals shorted is not equal toIi/Vo with the in-put terminals shorted. Most such devices areactive devices, although some passive devicesinvolving gyromagnetic material may be non-reciprocal.

non-saturating circuit A circuit in which theoutput does not asymptotically approach a limit-ing value as the input is increased or decreased.Transistor-based switching circuits that do notoperate in the saturation region of the transistor.

NOR A logic operation that gives 0 if eitherof the two inputs is 1 and gives 1 if both inputsare 0.

Norton equivalent A current source in par-allel with a resistor that is equivalent to a circuitcontaining voltage sources and resistors. It is


often used to simplify the analysis of complexcircuits.

note A sign, defined by convention, that indi-cates the pitch of a musical sound by its positionon the staff, and its duration by the shape of thesign.

n-type silicon Silicon (Si) that is doped witha donor impurity (pentavalent atoms such as ar-senic (As), phosphorus (P), etc.) and in whichelectrons, which are negatively charged, are themajority carriers (electron concentration ismuch higher than hole concentration).

nuclear angiography Magnetic resonanceimaging (MRI), of the same nature as nuclearmagnetic resonance (NMR) phenomena, can betailored to visualize flowing blood using mag-netic resonance angiography, ornuclear angiog-raphy.

Because MRI uses the same principles asNMR, using radio waves coupled with highmagnetic fields to detect structural and biochem-ical information about tissue in the body, it is anoninvasive procedure that is safer than imagingwith X–rays or gamma rays.

Nuclear angiography takes advantage of thefact that MRI needs a longer scanning time than,for example, CT, which makes MRI more sensi-tive to motion studies. A direct application is theimaging of flowing blood by visualizing arter-ies and veins. Other areas that make use of thistechnique are in the examination of the bladderand a blood flow to the brain. Abnormal flowmay indicate obstructions or other pathologicalconditions.

nuclear magnetic resonance (NMR) Res-onant absorption of radio frequency radiationby the nucleus. A nucleus has discrete closelyspaced energy levels corresponding to the orien-tation of the nuclear magnetic moment in an ap-plied field. The nucleus is able to resonantly ab-sorb radiation composed of photons whose en-ergy corresponds to the difference between theseenergy levels. Can be useful in chemical iden-tification since small changes in local magneticfield at the nucleus due to chemical bonding canbe accurately measured in this technique. Formsthe physical basis for magnetic resonance imag-ing (MRI) in medicine.Seephoton.

Nusselt equation Gives the relationship be-tween the heat transfer coefficient to the thermalconductivity of the gas, the effective diameter ofthe tube, its dimensions, and the viscosity andheat capacity for the gas. Nusselt’s equation forheat transfer by convective flow is given by

Nu = const(Re)x(Pr)y (De/L)z,

where Nu represents the Nusselt number, Re,Reynold’s number and Pr, Prandtl’s number.Values for the indices and the constant are foundexperimentally. The Nusselt number is obtainedby the ratio of the density of the heat flux in thepresence of natural convection to the density ofthe heat flux with non-moving interstitial fluid.

Nyquist criterion A condition for an ampli-fier to be stable, it states that if a polar plot ofthe loop gain as a function of the complex fre-quency encloses the point (1,0), the amplifier isunstable.


Oobject distance The distance from the ver-tex to the object. From the thin-lens equation,the image distances for a thin-lens, whose fo-cal length isf , can be calculated from objectdistances′ as

1s

=1f

+1s′,

1f

=(n′ − n)

n

(1R

+1R′

)where the radius surface of the lens isR andR′.The object distance of a thick lens should bemeasured from the principal planes of the lens.Seeimage distance.

objective, immersion A high-power micro-scope objective in which the space between thelens and the cover glass is filled with an oil. Therefractive index of the oil should be the same asthe objective and the cover glass in order to re-duce reflection losses and spherical aberration.The numerical aperture of a microscope objec-tive indicates the brightness of the image (sim-ilar to the f-number), and it is proportional tothe refractive index of the immersing medium.Because the refractive index of the immersingmedium is higher than that of the air, the nu-merical aperture becomes higher. This causesan increase in the resolving power of the micro-scope.

Immersion objective.

object, real An object that actually emits raysof light in an optical system. Sometimes usedsynonymously with real image.

object, virtual Sometimes used synony-mously withvirtual image. An object that ap-pears to emit rays of light, but actually does not.

octave A band in the frequency scale forwhich the higher frequencyfmax is twice thelower frequencyfmin, fmax = 2fmin. The in-terval between any two frequencies having a ra-tio of 2 to 1. The concept of fixed frequency ra-tios, which define proportional frequency bands,is used in the theory of musical temperament.In the equal temperament with the 12 note peroctave scale, the successive notes are 1/12 oc-tave apart. An interval with the frequency ratio21/12 = 1.0595 is called a half step.

ocular An ocular is a kind of magnifier. Itenlarges the intermediate image of the object asformed by preceding lenses in the optical systemand is not used to view an actual object. Anocular forms a virtual image. It is also knownas aneyepiece.

Ocular: Huygen’s, Ramsden, and Kellner.

There are several kinds of oculars. AHuy-gen’s ocularconsists of a pair of plano-convexlenses of which the convex sides of both lensesare facing the object. ARamsden ocularcon-sists of two plano-convex lenses. The two lensesare almost the same as their convex sides arefacing each other. AKeller ocularhas a plano-convex lens and an achromatic doublet as aplano-convex lens for the eye-lens. It is anachromatic Ramsden ocular. The usage of anachromatic lens causes an increase in imagequality, and is used for wide field telescopes.

An orthotropic ocular and a symmetrical ocu-lar have long eye relief with wide-field and highmagnification. AErfle ocularconsists of threeachromatic doublets. It has well-corrected aber-rations and is used for wide-field application.


Ocular: orthotropic, symmetrical, Erfle.

Oersted The CGS unit of magnetic fieldintensity abbreviated Oe. Named in honor ofDanish physicist and chemist Hans C. Oersted(1777–1851) who is famous for pioneering workin electricity and magnetism.Seemagnetic in-tensity.

OFF The state in which no power or inputis given to a circuit or in which the circuit isproducing no output signal. The condition inwhich the output of a digital flip-flop is low.

Offset The deviation of a signal from zero.Often used for the DC portion of a combinedAC/DC signal (a zero signal offset).

ohmic loss The power dissipated by the re-sistance of an electrical circuit as a result of cur-rent flowing through it. The power dissipated,P , current,I, and resistanceR are related by

P = I2R .

The total ohmic loss for a circuit in whichthere is a distribution of resistances with differ-ent currents flowing through each is obtained bysumming the ohmic loss in each resistance.

Ohmic loss can occur when a current flowsthrough an ionized gas as electrons dissipatetheir energy by collisions with ions, atoms, andmolecules.

ohm, international Symbol:Ω. The SI unitof electrical resistance. A current of 1 amperethrough a resistance of 1 ohm requires a potential

difference of 1 volt across it. Its definition isthe resistance offered to a steady current by acolumn of mercury, 14.4521 gms in mass and106.300 cm long, at 0 C.

ohmmeter An instrument used for the di-rect measurement of electrical resistance. Thecomponent with an unknown resistance is usu-ally connected across two terminals that have apotential difference between them. The magni-tude of the current that flows is proportional tothe resistance. A part of this current also passesthrough a galvanometer, and the value of the re-sistance is indicated by the deflection of the gal-vanometer, measured against a calibrated scale.However, the galvanometer, in many ohmme-ters of this type have been replaced with a dig-ital display. These use a variety of analog todigital conversion circuitry before the value ofthe resistance is displayed. Ohmmeters that relyon this type of two-terminal configuration havelimited accuracy. Four-terminal or bridge typearrangements (e.g., Wheatstone bridge) must beused for greater accuracy.

Ohm’s law States that the electrical cur-rent, I, flowing through a metal conductor isdirectly proportional to the potential difference,V , across it. This relationship is expressed as

V = IR ,

where the constant of proportionalityR is theresistance of the conductor. Ohm’s law is onlyapplicable to circuits carrying direct currentthrough conductors with a constant resistance.Consequently, the temperature of the conductormust be held constant since resistance is usuallytemperature dependent.

Ohm’s law cannot be applied to alternating-current circuits, since the current no longer sim-ply depends on the resistance and potential dif-ference, but also on the frequency of the sourceand the inductance and capacitance that may becontained in the circuit. However, the law hasbeen modified to include the effects of these fac-tors by substituting the impedance,Z, in placeof the resistance,R, such that

V = IZ .

It is sometimes convenient to rewrite Ohm’s lawin terms of the current densityJ and the electric


field strengthE across the conductor as

J = σE ,

where the constant of proportionalityσ is theconductivity.

Ohm’s law in acoustics Describes the wayindividual partials of a musical sound are per-ceived by the human ear. The ear can separate acomplex tone into its spectral components andperform a kind of Fourier spectral analysis.

ON The state in which a circuit is receivingpower and input and is producing an output. Thestate in which a digital flip-flop output is high.

online An electrical device, such as a pe-ripheral device of a computer, is online if it isdirectly connected to the computer and is readyto perform its function. On the other hand, off-line denotes a peripheral device, possibly an on-line device, which is switched off, broken, ordisconnected from a computer.

Onsager theory Deals with the flow of botha heat current and an electric current in a wire si-multaneously. Entropy is generated in the wiredue to both processes. In the absence of a po-tential difference, a heat current depends onlyon the temperature difference but when there isa potential difference as well, the heat currentdepends on both the temperature difference andthe potential difference. When both temperatureand potential differences exist across a wire, theelectric current depends on both of these differ-ences. The heat flow, the entropy flow, and theelectricity flow are irreversible, coupled flowsthat exist in a wire as a result of a departurefrom equilibrium conditions. If the departurefrom equilibrium is not too great, the heat andelectric currentIs and I, respectively are lin-ear functions of the temperature difference,∆T ,and potential differences,∆E, and are given bythe following Onsager equations:

Is = L11∆T/T + L12∆E/TI = L21∆T/T + L22∆E/T .

This expresses the linearity between the flows(or currents) and the generalized forces∆T/Tand∆E/T . They can also be represented by

Onsager’s reciprocal relationL12 = L21 whereLs are coefficients connected with electric resis-tance, thermal conductivity and the thermoelec-tric properties of the wire.

opacity This measure indicates how amedium is opaque to electromagnetic radiationand is reciprocal to the transmittance.

opaque object This is an object that does nottransmit electromagnetic radiation, especiallylight.

open circuit Term applied to part or all ofan electrical circuit in which the impedance isinfinite. In practice, this may be done by phys-ically disconnecting a conductor or componentnecessary to complete that part of the circuit.

open circuit voltage The voltage measuredat a terminal when there is no load connected tothe terminal (i.e., no current).

open-loop An amplifier circuit in which thereis no feedback connection.

opera glass A very simple compact binocularGalilean telescope. Its magnification is low andits field of view not very wide.

operating point The point that correspondsto the current and voltage values of a device un-der load. It is the intersection of the load-lineand the device I-V curve.See alsoload-line.

operational amplifier A very high gain DCdifferential amplifier with two inputs and a sin-gle output. It produces positive output whenthe noninverting input is higher than the invert-ing input and negative output when the invertinginput is higher. It was originally used for math-ematical purposes.

ophthalmometer An instrument to measurecurvature of the anterior corneal surface andastigmatism of the eye. It is also known as aKeratometer.

ophthalmoscope An optical device to ob-serve the inside of the eye (the retina, the fun-


dus, and so on) through the pupil. It illuminatesthe eyeground through the pupil.

optical activity Phenomenon in which theplane of polarization of a linearly polarized lightwave is rotated in passage through a materialmedium. The effect occurs in materials in whichthe crystalline layers of the material or its con-stituent molecules exhibit a systematic twist,causing right- and left-handed circularly polar-ized light to propagate in the material with dif-ferent speeds. The phenomenon occurs in quartzcrystals and in certain sugar crystals as well as insugar solutions. Rotation of the plane of linearpolarization is explained by the fact that a lin-early polarized light wave can be decomposedinto two circularly polarized light waves, withoppositely directed polarizations, that propagateat (slightly) different speeds through opticallyactive material, so as to produce a rotation ofthe plane of linear polarization at the exit sur-face of the material. The amount and directionof rotation of the polarization vector is in gen-eral dependent on both the wavelength of thelight and the properties of the given material.

optical axial angle The size of the angle be-tween the two optic axes of a biaxial crystal.The angleθ can be calculated from the refrac-tive indicesα, β, andγ. α, β, andγ are theindex of the fastest light, intermediate light, andthe slowest light:

tan2

(θ

2

)=

1α2 − 1

β2

1β2 − 1

γ2

.

Whenθ > π/2, the crystal is obtuse bisectrixand negative. Whenθ < π/2, the crystal isacute bisectrix and positive. The optical axialangle is constant for each particular substance ata given temperature and pressure. The opticalaxes of the wave normals is obtained from thepoints of the two wave normal surfaces.Seeoptical axial plane.

optical axial plane Also called theopticalplane. The plane that is defined by two opticalaxes of a biaxial crystal. The normal of the op-tical axial plane is called theoptic normal.Theoptical axial angle is constant for each particularsubstance at a given temperature and pressure.

Optical axial angle.

The axis that bisects the angle between the op-tic axes is called thebisectrix. See alsoopticalaxis; optical axial angle.

Optical plane.

optical axis A straight line passing throughthe optical center, it is also known as theprinci-pal axis. See alsomeridian plane.

optical bench A rigid but movable rod ortrack equipped with mountings for optical ex-periments. When an optical bench is used, it ispossible to slide optical components along thebench and to determine the position preciselywith attached scales. Some optical benches haveinternally damped honeycomb structures.


Optical axis and optical center of a lens.

optical bistability Phenomenon in whichthe intensity of the light output from a materialmedium switches discontinuously between highand low values as the intensity of the input lightis continuously varied. Effect in general occursonly where the interaction between the light andthe material medium exhibits a nonlinear de-pendence on the electric field of the light wave,which causes the intensity of the light outputfrom the medium,Iout, to be a multiple-valuedfunction of the input intensity,Iin , correspond-ing to a graph ofIout vs. Iin of the form shownin the diagram.

Optical bistability.

optical center A point on the axis of opticalcomponents, in which each ray passing throughthe optical center does not deviate. It is alsoknown as thepole.

optical channel An optical channel may befree space, the atmosphere, or silica glass fiberoptics, depending on the material medium of theoptical communication system. The free spaceoptical channel embodies an ideal communica-tions channel because free space neither distorts

nor attenuates the transmitted signal. An atmo-spheric optical channel, with even minute tem-perature variations, may significantly broadenthe spatial directivity or bend the path of op-tical signals. Fog or snow would sufficientlyattenuate the optical signals to render outdooratmospheric optical communication unfeasiblefor distances much farther than that betweenadjacent buildings. Indoor optical communica-tion channels would not suffer from this fog andsnow problem, but temperature variation wouldstill affect the channel. Silica glass rods pro-vide unparalleled transparency to optical sig-nals. Signal distortion, however, occurs in long-distance transmission as the light wave reflectsoff the boundary between the two glass layersin the coaxial cylindrical step-index fiber opti-cal fiber, wherein the core layer is made of glasswith a slightly higher refraction index than thecoaxial outer layer.

optical communication Optical communi-cation refers to the transmission and receptionof information using electromagnetic waves inthe visible and infrared parts of the electromag-netic frequency spectrum. Optical communica-tion may also be through free space (in inter-satellite communications), through the earth’satmosphere, or through silica glass optic fibers.Free space or atmospheric optical communi-cations differ from the more traditional radiocommunications primarily in the use of elec-tromagnetic waves at much higher frequencies(on the order of 300,000 GHz), such that thesignal wavelengths are much shorter than thedimension of the hardware devices themselves.This exceedingly high frequency provides verywide bandwidths for individual channels, facil-itates the miniaturization of hardware compo-nents, exploits the unparalleled transparency ofsilica glass as a transmission wave guide viafiber optic communications. Lasers, with theirsuperior spatial directivity relative to incoher-ent radiation, are often desirable optical lightsources in free space, through the atmosphere,or in optical fibers. However, fog or snow wouldsufficiently attenuate the optical signals to ren-der outdoor atmospheric optical communica-tion unfeasible for distances much farther thanthat between adjacent buildings. Optical signalsmay be modulated using the conventional am-


plitude, frequency or phase techniques, but in-tensity modulation and polarization modulationare most common.

optical density A measure of the light-stopping power of a transparent material, de-fined to equal the logarithm of the ratio of theintensity of the field in the incident light,I0 , tothe intensity of the field in the transmitted light,I.

optical glass Glass used in the manufac-ture of optical parts, lenses, prisms, and so on.It should be free from defects such as bubblesand strain. In choosing a proper glass, refrac-tion and dispersion of the glass are important.Practically, more than three refractive indicesare used to characterize a glass. Usually, threeFraunhofer lines are used to specify the refrac-tive index of the glass: the “Hydrogen C-line”(λ = 656.2816 nm, red), the “Helium D-line”(λ = 587.5618 nm, yellow), the “Hydrogen F-line” (λ = 486.1327 nm, blue). Usually, theindex of refraction for yellow light (“Sodium D-lines”; wavelength 589.3 nm) is used as the mainrefractive index. There are more than 200 kindsof optical glass, which is divided into seven cate-gories: crown, borosilicate crown, dense crown,light flint, flint, dense flint, and others.

Typical Value ofRefractive Index ofOptical Glass

Name nd

Borosilicate crown 1.51Crown 1.52Dense crown 1.6Light flint 1.58Flint 1.62Dense flint 1.7

Ordinary crown glass has a refractive indexwithin the range 1.51 to 1.54. Flint glass con-tains a refractive index between 1.58 and 1.72.Lanthanoid oxide is added to optical glass tomake the refractive index higher.

optical path/path length The optical path issimply the product of length/distance and the re-fractive index. More precisely, the optical pathtis the integral of the refractive index (n(s)) overelements of length along the path (P) which rayspass through:

t =∫

Pn(s)ds .

It is also known asoptical distanceor opticallength. Fremat’s principle says that light tra-verses the paths of which the optical pass-lengthhas the smallest value.

optical pumping Process whereby selectedquantum energy levels in atomic or molecularsystems are excited by optical radiation (calledpumping radiation) so as to produce an inver-sion in the thermal distribution of the selectedlevel(s) with respect to the lower (or ground)levels. Process is necessary for production oflaser or maser radiation via enhanced stimulatedemission.

optical switching Process in which a non-linear interaction between light and a materialmedium causes an optical signal to switch be-tween two or more output modes as a function ofthe input intensity. Contrasts withelectro-opticswitchingin which an optical signal is switchedbetween output modes via application of an elec-tric field to the medium. The phenomenon (inprinciple) makes possible signal routing pro-cesses used in optical communications and logi-cal operations required in all-optical computing.High field intensity for enhancement of nonlin-ear interaction needed for switching is in generalthe result of either partial confinement of lightinside a (nonlinear) etalon or concentration oflight in a waveguide or directional coupler. Op-tical switching devices divide into types basedon a single input beam and on both an input sig-nal and a control beam.

optics, collecting Seeoptics, detection.

optics, detection That part of a microscopeof any kind, spectrometer, or other instrumentthat deals with the analysis of visible light, thatgathers, collects, or detects the relevant light thatis scattered or transmitted in any way from the


body in observation. An example is given inconfocal detection optics.

optometer Any of a large class of optical andmechanical instruments used to measure refrac-tive errors of the eye, generally combining bothsubjective and objective elements in measure-ments; predecessors of current refractometersand automated refractors.

OR A logic operation that gives 1 if either ofthe two inputs is 1 and gives 0 if both inputs are0.

organ pipes, vibration in Aerodynamicallygenerated vibrations of air columns in pipes arethe source of sound in the organ (flue pipe). Con-stant air flow enters the pipe through an openingand a lip strikes a knife edge above the mouth.The resonator, an open pipe, is connected tothis lower structure. The air stream emergingthrough the lip undergoes unstable oscillation,and it can, at the appropriate stream velocity, ex-cite the resonator. The fundamental frequencyof the resonator determines the pitch of the pipefor self-excitation. The larger the cross sectionof the pipe (resonator) the more the fundamentaldominates the tone produced by the pipe.Seealsopipes, sound from.

orthogonal code An orthogonal code refersto a code wherein various message bits are rep-resented by distinct sequences of digits and anytwo such sequences are orthogonal to each other.That is, if each sequence is considered a vector,then the inner product of any two sequences inthe code has an inner vector product equal tozero.

oscillation, parasitic Spontaneous oscil-lations in a circuit generated by lead induc-tances and inter-lead capacitances. They canbe sources of noise in oscillator circuits.

oscillations, energy of Energy (E) associ-ated with wave motion caused by the action ofspringlike forces and involving kinetic (K) andpotential (u) energies. Waves can transport en-ergy without the transport of mass. For exam-ple, the average energy density of transversetraveling waves on a stretched string with mass

Organ pipe.

per unit lengthµ under tensionT is⟨

dEdx

⟩=⟨

dKdx

⟩+ 〈u〉 = 1

2µω2y2

0 , with ω as the angularfrequency andy0 the wave amplitude. The en-ergy density is itself a traveling wave transport-ing energy at speedv = ω/k =

√T/µ and it

is proportional to the squares of oscillation am-plitude and frequency.See alsopower in wavemotion.

oscillations, spontaneous Oscillations oc-curring naturally in unstable circuits or devicesin the absence of a triggering signal, as opposedto those that occur only when triggered by anoutside signal.

oscillator A device or circuit used to set upand maintain oscillations at a desired frequency.The most common form is an LC circuit givingan oscillator frequency ofω = 1√

LCwhereL is

the inductance andC is the capacitance.


oscillator, beat frequency An oscillator thatproduces a reference signal to combine with theincoming signal to produce an output. It is usedin heterodyne reception.

oscillator, blocking A tube circuit oscillatorin which the tube is highly conducting for a shortperiod followed by a long period in which it isnon-conducting.

oscillator, Colpitts An oscillator in whichtwo capacitors are used in series in the tank cir-cuit with feedback taken from between the ca-pacitors.

A Colpitts oscillator, L, C1 and C2 form the tank cir-

cuit, while the remainder of the components provide

the feedback to sustain the oscillations.

oscillator, crystal An oscillator in which thepiezoelectric properties of a quartz crystal areused to produce the oscillations. The high Qvalue of the quartz leads to a very stable oscil-lator.

oscillator, Hartley An oscillator in which thefeedback connection to the tank circuit is madeto the inductor.

oscillator, Hertzian An oscillator consistingof two capacitors connected to a conducting rodwith a spark gap. It produces highly damped os-cillations at the frequencyω = 1√

LCwhereL is

the mutual inductance andC is the capacitance.

oscillator, local An oscillator used in a su-perheterodyne receiver to give the reference fre-

A Hartley oscillator, L and C are the tank circuit while

the rest of the circuit provides the feedback to sustain

the oscillations.

quency to the mixer used to convert the input fre-quency to the output frequency. Changing thelocal oscillator frequency changes the input fre-quency that will be converted to output, tuningthe receiver.

oscillator, master An oscillator used in am-plifiers to establish the carrier frequency of theoutput.

oscillator, relaxation An oscillator based onthe charging and discharging of a capacitor. Arelaxation oscillator can be quite stable, and nor-mally produces a triangular or sawtooth output.

oscillator, squegging A tube-based oscilla-tor that reaches a high amplitude, and then isbrought to zero by the blocking effects of leak-age current on the grid.

oscillator, Wien bridge A low distortion os-cillator based on a feedback amplifier with a180 phase shift at the desired frequency. AWien bridge oscillator can operate at frequen-cies ranging from 20 Hz to 200 kHz with a verysmall distortion as long as the amplifier is keptin the linear regime.

osmocomformer Refers to the way that somemarine invertebrates adjust to changes to thesalt concentration of their surroundings. At anygiven instant, these animals have the same os-motic pressure as the sea water that surroundsthem in order to diminish osmotic flow fromtheir bodies. When there occurs a change inthe concentration of the surroundings, osmo-


comformers change their osmotic concentrationto match that of the external environment andthus keep the osmotic balance. This behavioris in contrast toosmoregulators,animals thatkeep constant their own osmotic concentrationin spite of external changes in salinity.

osmometer An osmometer is a devicefor measuring osmotic pressure. The generalscheme is given by the accompanying figure,whereM is a membrane,I is the solvent withchemical potentialµ1, and II is the solventand dissolved polymer with chemical potentialµ2. Both chemical potentials are at atmosphericpressurePo. The difference in pressures,P−Pogives the osmotic pressureπ.

In this example, the membrane is permeableto molecules of the solvent but not to moleculesof the solute (polymer in this case). The equi-librium condition requires the equality of thechemical potentials,µ1 = µ3, where

µ3 = µ2 +∫ Po+π

Po

(∂µ2

∂P

)T

dP .

Because(∂µ2/∂P )T = V2, V2 is the partialmolar volume of the solvent which in practice isnot pressure dependent. Henceµ3 = µ2+V 2π.

In the limit when the concentration of the so-lutec2 in II tends to zero, the osmotic pressureis given byπ/c2 = RT/M2, which expressesvan’t Hoff’s law. M2 is the molecular weightof a polydisperse polymer that in principle isthe mean molecular weight from a mixture withMi,

M2 =∑

i niMi∑i ni

.

osmoreceptors In general, osmoreceptors re-act to minute changes in the osmolarity (concen-tration of particles) of some fluid in their vicinityby releasing some substance to counteract thechange.

As an example, in the hypothalamus thereare osmoreceptors (central receptors) that sensethe osmolarity of extracellular fluid. When theosmolarity is high, they send a signal to the pitu-itary gland to release the hormone ADH (vaso-pressin, primary regulator of body water). ADHthen acts on the distal tubules of the kidneys to

Osmometer.

increase reabsorption of water. This has the ef-fect of increasing blood volume and blood pres-sure. Inhibition of production of ADH is con-versely signaled when the osmolarity is low.

Another example of osmoreceptors are pe-ripheral receptors. Peripheral receptors are os-moreceptors in the mouth and throat that signalthe brain stem in the case of thirst.

osmoregulators Osmoregulators and os-moregulatory mechanisms form one of the mostimportant evolutionary innovations that enabledmulticellular animals to carry around their owninternal sea and stable internal fluid environ-ment out of primordial waters. Osmoregulatorsmaintain an internal environment that is constantin osmotic pressure and salt balance. Differ-ent species require specialized osmoregulatorymechanisms, which vary widely according tothe nature of an organism’s habitat.

In amphibians, the majority of the input ofwater occurs through the skin and across the wallof the bladder. This is realized by the produc-tion of a hormone that causes water to enter thebody through the skin triggered from the brainwhen the animal is on a moist surface or im-mersed in water. In addition, before dehydra-tion, hormonal stimulus can cause water to be“reclaimed” from the bladder and returned to theextracellular fluid. This is in contrast with other


land vertebrates in which the bladder is largelya receptacle for urine that will be excreted.

In many terrestrial vertebrates, to counteractthe loss of water from the lungs, the nasal cavi-ties act as a “countercurrent exchange system”.During inhalation, air passing through the nasalcavities is warmed by heat from adjacent tissues,but in the process, the temperature of these tis-sues falls. One result of this is a cold nose – adog’s nose is a good example. The inhaled airis further warmed and humidified in the lungs.Then, as it passes back out during the next exha-lation, the warm, moist air flows over the coolernasal surfaces, and the air gives up some of itsheat. As the air cools, much of the water vaporcondenses out on the nasal surfaces and so is notbreathed out of the body. This process can saveup to 20% of an animal’s total need for water.

Also, in most terrestrial vertebrates, by con-trolling the amount of water and salt lost in theurine, the kidneys form the primary regulatoryorgans to keep a constant osmotic balance in thebody.

osmosis, negative, anomalous Negative os-mosis, orreverse osmosiscan be achieved byapplying pressure to the side of a solution sep-arated by a membrane in order to achieve thereverse flow of particles that would otherwisehappen in normal osmosis.

It can be used as a separation technique inwhich a semipermeable membrane is placed be-tween two solutions containing the same sol-vent. The membrane allows passage of thesolution while preventing passage of largermolecules. Reverse osmosis occurs when pres-sure is applied to the solution on the side of themembrane that contains the lower solvent con-centration. The pressure forces the solvent toflow from a region of low concentration to oneof high concentration.

Negative osmosis is often used for waterpurification, concentrating impurities, recover-ing contaminated solvents, cleaning up pollutedstreams, and desalinizing sea water.

osmotic equilibrium (cell) Cellular cyto-plasm contains a high concentration of organicmolecules including macromolecules, aminoacids, sugars and nucleotides. In the ab-sence of a counterbalance, this concentration of

molecules would drive water inward by osmosis,which if let undisturbed would result in swellingand eventual bursting of the cell.

Osmotic equilibrium across the cell mem-brane is then established by the proper diffu-sion of molecules through the membrane me-diated by mechanisms such as active, passive,and facilitated diffusion. In passive diffusionsmall uncharged molecules diffuse across thecell membrane equilibrating the concentrationsbetween out and in. Active transport requiresATP hydrolysis and is the main transport car-ried out by ion pumps. The ion pumps providethe required counterbalance to the osmotic gra-dient by providing counter gradients of Na+ andK+. In particular, the pumps establish a higherconcentration of Na+ outside the cell. Flow ofK+ through open channels further establishes anelectric potential across the plasma membrane.This potential in turn drives Cl− out of the cell.The differences in ion concentration balance thehigh concentration of organic molecules insidecells, equalizing the osmotic pressure and pre-venting the net influx of water.

In some simpler organisms (protists),membrane–bound contractile vacuoles pumpfluid in a cyclical manner from within the cellto the outside by alternately filling and thencontracting to release its contents at variouspoints on the surface of the cell. This cyclicpumping functions in maintaining osmoticequilibrium.

osmotic pressure, Donnan Such equilib-rium as that found between a charged, immo-bile colloid (such as clay, ion exchange resin,or cytoplasm) and a solution of electrolyte iscalledDonnan equilibrium.In this system ionsof like charge to the colloid tend to be expelled,and ions of opposite charge tend to be attractedby the colloid. The result is that the colloidcompartment is electrically polarized relative tothe solution in the same direction as the colloidcharges, creating aDonnan potential,and theosmotic pressure is higher in the colloid com-partment.

osmotic responses, kinetic theory, (cell) Anosmotic response is the result of the unbalancedpressures on both sides of a membrane that sep-arates a solvent with different solute concentra-


tions on both sides. The number of solvent par-ticles hitting the side of the membrane with themost solute per unit time per unit area is lessthan the side with less solute. This means thatthe flow of particles from each side will be un-equal, the side with the more solute gaining moresolvent per unit time. The unequilibrated rate ofexchange of solvent particles establishes then aflow from the solvent–rich to the solution–richcompartment creating an osmotic pressure thattends to equalize on both sides of the membrane.Seeosmotic transport.

osmotic transport Osmotic transport refersto the transfer of a liquid solvent through asemipermeable membrane from a region with alow concentration of solute (high concentrationof solvent) to one with a higher concentration ofsolute (low concentration of solvent). The mem-brane is only permeable to the solvent. Osmo-sis can be realized in practice by the followingexperiment: given a vessel separated into twocompartments by a semipermeable membrane,if both compartments are filled to the same levelwith a solvent, and if a solute is added to oneside, osmosis will occur, increasing the level ofsolvent at the side with the solute. The mini-mum pressure applied to the side with solute tostop the solvent transfer is called the osmoticpressure (seeosmometer).

Dialysis is called thetransfer of solute(ratherthan the solvent). The direction of transfer isfrom the area of higher to the area of lower con-centration of the material transferred. In dialy-sis, dissolved salts are removed from solutionsof proteins or other large molecules.

output A signal (current or voltage) pro-duced by a circuit and either measured directlyor used as the input to a separate circuit. Theterminal from which the output is taken.

output capacitor A capacitor used to trans-mit the AC component of an output signal, whileblocking the DC component of the signal.

output characteristics The electrical prop-erties (impedance, capacitance, etc.) associatedwith the output channel of a device. The de-pendence of the output current on the input andoutput voltages for a transistor circuit.

overload A condition in which the desiredinput to or output from a circuit is larger thanthat which can be accommodated.

overtones Components of the complex soundwhose frequencies are integer multiples, greaterthan 1, of the fundamental frequency. Harmon-ics other than the fundamental component.Seealsofrequency, fundamental; harmonics.

Owen’s bridge Owen’s bridge is an AC vari-ation of the Wheatstone bridge. Owen’s bridgeis used to measure inductance of an unknown in-ductor in terms of other known resistances andcapacitances. A circuit diagram is shown be-low. Only R3 andC3 should be adjustable ifyou want the resistance and inductance balancesto be independent of each other. The equationsof balance are shown below:

Rx = R1C2

C3

Lx = R1R3C2 .

Owen’s bridge.

oxygen, liquid Oxygen liquefies at 90 K andcan be produced in air liquefaction and oxygenseparation plants. In liquid form it is very usefulas a propellant for guided missiles and rockets.One of its advantages is that for a given mass,the volume of liquid is much less than in thegaseous state and it is therefore preferred fortransportation and storage.


Ppacemakers, cardiac Cardiac pacemakersare any of several electronic devices used tostimulate or regulate contractions of the heartmuscle in people with cardiac problems. Thedevices are usually miniaturized and surgicallyimplanted in the patient.

packet switching A form of switching usingpackets to carry information. The packets are acombination of information (data) and a descrip-tion of this information (metadata or header).The packet switch is a switch that examines apacket header to determine the packet’s destina-tion.

paleomagnetism Study of the intensity anddirection of the magnetic field of the earth in thepresent and the geological past, its origin and itschange with time. This involves study of mag-netized rocks in the earth’s crust. When theserocks form from magma, a remanent magneticmoment is frozen in them along the direction ofthe earth’s field. These rocks carry informationabout the intensity and orientation of the earth’smagnetic field from that time.

panoramic An optical instrument or a lensthat takes a wide field of view.

pantograph A sliding current collector barused on top of electric trains to make contactwith an overhead electric wire. The bar is sup-ported by a four-bar parallel linkage, with nolinks fixed. The current collector bar is thrustupwards by powerful springs with sufficientpressure to follow variations in the height ofthe overhead wire and therefore maintain a low-resistance contact. The oscillations in the over-head wire and the pantograph, which may oc-cur during high speeds, can lead to a break inthe connection between them or cause excessivearcing.

parallax Apparent change in the direction orposition of an object when viewed from a differ-ent position or direction, often used to recognizethe relative positions of the objects. In astron-omy, the angle of parallax is used to determinedistances of nearby stars.

parallel resonance A condition in which themagnitude of the parallel inductance and capac-itance in a load are equal.

parallel voltage feedback A feedback condi-tion in an operational amplifier circuit in whichthe feedback current is parallel to the input cur-rent and proportional to the output voltage of theamplifier.

paramagnetic materials Materials whoseatoms carry a permanent magnetic moment butare not magnetically ordered so that they do nothave a spontaneous magnetic moment. Theyhave a permeability smaller than a ferromag-net. Examples include all magnetic materialsthat show magnetic order when they are abovetheir ordering temperature.Seeparamagnetism;permeability, magnetic.

paramagnetic probes Probe used inelec-tron spin resonance(ESR) studies, also calledelectron paramagnetic resonance(EPR). ESRis in general used to detect and measure un-paired electrons in the sample under study thatultimately leads to structural and dynamic infor-mation. Because the probe is paramagnetic (nonet magnetic moment) it will not influence thesample or the measurements.

Ideally suited to study the effect of exter-nal magnetic fields within the surface of a sam-ple, specifically over superconductors in the de-tection of induced electric currents when sub-jected to changing magnetic fields. The para-magnetic probes are usually molecules that inprinciple could also be attached, for example, inthe myosin head for the study of muscle fiberactivity. In addition, they could serve as probesof molecular motion of biomolecules and duringexperiments on phase transitions.

paramagnetic resonance Also calledelec-tron spin resonanceor electron paramagneticresonance. Refers to resonant absorption of


electromagnetic radiation, usually in the mi-crowave range, by paramagnetic ions in a mag-netic field. A magnetic atom has discrete en-ergy levels corresponding to the orientation ofthe magnetic moment of the atom in an appliedfield. The atom is able to resonantly absorb ra-diation composed of photons whose energy cor-responds to the difference between these energylevels. Gives information on the gyromagneticratio of magnetic atoms and ions.

paramagnetism Describes a system of per-manent magnetic moments, usually in a solid,that are not ordered but have a positive mag-netic susceptibility. This susceptibility is inde-pendent of the applied field but is a strong func-tion of temperature, thus:

χ = Np2/3k (T − T0)

whereN is the number of magnetic atoms perunit volume,p is the effective magnet momentper ion,k is Boltzmann’s constant,T is tem-perature, andTo is a constant called theCurie-Weiss temperature.For materials that do not or-der magnetically at low temperatures,T0 is verysmall and may be zero. In the paramagnetic statethe magnetic system is above its magnetic tran-sition temperature and so magnetic momentsare thermally agitated so that they point in ran-dom directions that change with time.SeeCurieWeiss law; paramagnetic materials; susceptibil-ity, magnetic.

paraxial approximation Only paraxial raysare considered in imaging by a lens/optical sys-tem and the small angle approximationsin θ ≈tan θ ≈ θ is used.

Parseval theorem Theorem relating the in-tegral of the product of two functions,f1(x)andf2(x), to an integral over the product of theFourier transforms of the respective functions,g1(k) and g2(k), or to a sum over a productof Fourier coefficients. The statement of the-orem has distinct forms in the separate caseswhere functionsf1 and f2 are either periodicfunctions ofx in an intervalL, or non-periodicfunctions ofx in the interval between plus andminus infinity. The respective cases correspondto representations forfi(x)(i = 1, 2) in termsof Fourier coefficientsain or transform functions

gi(x) in the forms:

fi(x) =∞∑

n=−∞aine

inKx , K = 2π/L ,

fi(x) =

∞∫−∞

dk gi(k)eikx ,

from which the Parseval theorem can be derivedin the respective forms:

L2∫

−L2

dxf∗1 (x)f2(x) = L∞∑

n=−∞a∗1na2n ,

or

∞∫−∞

dx f∗1 (x)f2(x) = 2π

∞∫−∞

dk g∗1(k)g2(k) .

partially polarized light Most simply, a mix-ture of polarized and unpolarized light. (Seepolarization of light.) A more precise definitionderives from a general representation of light asa superposition of two linearly polarized com-ponents, characterized by polarization vectorsvibrating in mutually orthogonal directions inthe plane perpendicular to the direction of prop-agation of the light. Under the condition thatthe two components have equal amplitudes, anddiffer in phase by an amount that varies rapidlyand irregularly with time, the light is said tobe unpolarized;whereas, under the conditionthat the components have unequal amplitudes,and/or a phase difference that is not completelyrandom, the light is said to bewholly or par-tially polarized.The representation of light as asuperposition of linearly polarized componentsprovides for an operational definition of partiallypolarized light in terms of measurements of theintensity of the light transmitted through a lin-ear polarizer oriented so as to define a directionperpendicular to the direction of propagation ofthe light. Specifically, rotation of the polarizerthrough all orientations in the plane perpendic-ular to the propagation direction, and measure-ment of the intensities of the transmitted light,


make possible (in principle) a determination ofa maximum and a minimum in the transmittedintensity,Imax andImin , in terms of which thedegree of polarizationof the light, P , can bedefined by the ratio

P ≡ Imax − Imin

Imax + Imin.

The definition assignsP the values 1 and 0 in thecases of completely polarized light and (com-pletely) unpolarized light, respectively, and al-lows partially polarized light to be defined interms of values ofP unequal to either 1 or 0.

partition, transmission of sound throughTransmission of sound energy (transmissionloss) through single walls that are homogeneousand damped, dependent on the product of sur-face density and frequency. The thickness is notof importance for wall thicknesses below 30 cm.When walls are combined to form an enclosure,a small opening (thin windows, cracks aroundthe door) can render noise reduction measuresuseless.

Paschen-Back effect Named after Germanphysicists Louis C.H.F. Paschen (1865–1947)and Ernst E.A. Back (1881–1959). An effect inwhich the spectral lines of a light source are splitinto multiplets when the light source is placedin a strong magnetic field. The strong magneticfield modifies the atomic structure of the atombreaking the coupling of the orbital angular mo-mentumL and spin angular momentumS lead-ing to the observed multiplets.

passive device Devices such as resistors, in-ductors, or capacitors without a built-in powersource, as opposed to active devices such as tran-sistors.

pattern, acceptance (1) The acceptance pat-tern of an antenna is the distribution of the off-axis power relative to the on-axis power as afunction of angle or position. The acceptancepattern is the equivalent of a horizontal or verti-cal antenna pattern.

(2) The acceptance pattern of an optical fiberor fiber bundle, is the curve of total transmittedpower plotted against the launch angle, wherethe launch angle is the angle with respect to the

normal at which a light ray emerges from a fibersurface.

peak value (voltage/current) The maximumpositive or negative value of an alternating quan-tity (voltage or current). For example, a sinu-soidally varying voltage with time has a peakvalue equal to the amplitude. The peak valueof a sinusoidal voltage displayed on an oscil-loscope is obtained by subtracting the lowestvoltage from the highest voltage and dividingby two.

Peltier effect (1) The heating or cooling thatoccurs at a junction of two dissimilar metalswhen an electric current is passed through it.The direction of the current determines whetherthe junction will be heated or cooled. The de-gree to which the junction is heated or cooledis determined by the magnitude of the currentand the type of metals used. For a circuit con-sisting of two junctions of dissimilar metals (AandB, say) in series, one junction will be heatedwhile the other cooled when a direct current ispassed through the circuit. This is simply dueto the current passing from metalA to metalBat one junction, and vice versa. The origin ofthe Peltier effect can be understood by thinkingof electrons as being evaporated from metalAand condensing on metalB or vice versa. Heat-ing or cooling will result if the energy requiredfor evaporation is not equal to the energy re-quired for condensation. Many Peltier devicesare made from two dissimilar semiconductorsand are used to cool miniature electronic com-ponents or provide temperature control for elec-tronics for which the performance is criticallydependent on temperature stability.

(2) The production or absorption of heat ata junction between two dissimilar conductorswhen a current is passed across the junction ap-pears as power generated or absorbed at the junc-tion. The latter is directly proportional to thecurrent.P = πabI, whereπab is the Peltier co-efficient for currentI passing from condutorbto conductora.

penetration depth (low temperature) Dealswith the penetration of magnetic flux into thesurface layer of a superconductor. The penetra-tion depth is temperature dependent, decreasing


from a value corresponding to total field pene-tration at the critical temperatureTc to an ap-proximately constant value atTc/2. Typicallythe penetration depth is about a few tens of nmfor T << Tc.

periodic waves A train of waves in whicheach particle exhibits periodic motion. The sim-plest special case of periodic waves is a sim-ple harmonic wave, in that each particle is sub-jected to harmonic motion.See alsoharmonicmotion. More complex waveforms can be de-scribed in terms of infinite sums of simple har-monic waves, whose frequencies are integralmultiples of the fundamental frequency, usingthe Fourier series representation.See alsohar-monics.

peripheral vision Act of seeing (or vision of)images produced by light falling on areas of theretina outside its central (macula) region. Makespossible visual awareness of objects located oneither side of the column of space extending for-ward from the pupil of the eye.

periscope An instrument that allows obser-vation of objects not in direct line of sight. Inthe simplest form, it may consist of an opticalLshaped tube with lenses, and a mirror or prism atthe bend ofL, to enable a person in a submergedsubmarine etc. to have a view of objects on thesurface of water or over and around an obstacle.

Permalloy An alloy made up of Fe and Ni,and heat treated in a prescribed way to have ahigh permeability and low hysteresis so that itis a good soft magnet. Useful as a transformercore material.Seepermeability, magnetic; hys-teresis.

permanent magnet Magnetic material witha permanent magnetic moment; i.e., it has amagnetic moment in the absence of an appliedfield. Possesses large coercivity and usually hasa large magnetic moment. Useful in electric mo-tors, generators and many other electrical de-vices. Best permanent magnet currently knownis based onNd2Fe14B. It is prepared in ananocrystalline form and contains small crystal-lites ofα−Fe. Many other alloys are also used

as permanent magnets including Sm-Co and Al-nico. Seecoercivity; alnico.

permeability, diffusive During diffusivepermeability, a membrane allows passage of cer-tain, especially small, molecules or ions whoseprimary means of motion is of diffusive nature.

permeability, incremental (magnetic) De-fined at a particular bias fieldH. A small field∆H is applied on top of the bias field leadingto a small increase in the magnetic flux∆B.The incremental permeability is then defined as∆B/∆H.

permeability, magnetic The permeabilityµ is the proportionality constant between mag-netic inductionB and magnetic field intensityH and is defined byB = µH. If B is parallel toH thenµ is a scalar quantity. For free space thepermeablitiy isµ0 = 4πx10−7H/m. For para-magnetic materials the permeability can be anorder of magnitude or more larger thanµo. Forferromagnetic materials the permeability can beas much as six orders of magnitude larger thanµo. If B is not parallel toH then the perme-ability is a tensor quantity.Seesusceptibility,magnetic.

permeability, osmotic A membrane with os-motic permeability allows a rate of flow of asolvent to pass through during the case of an os-motic pressure difference between the two sidesof the membrane (seeosmotic transport).

permeance The reciprocal of the reluctanceof a magnetic circuit.Seereluctance.

permendur A magnetic alloy made of equalamounts of Fe and Co with a small amount ofV(2%) added and annealed in a prescribed way.This material has a high permeability and so is agood soft magnet.Seepermeability, magnetic.

permittivity Proportionality constant be-tween electric displacementD within a mediumand applied electric field thusD = εE. If D andE are parallel thenε is a scalar quantity.ε is re-lated the relative permittivity byε = εrε0 whereε0 is 8.85x10−12 farads/meter. Also related to


the electric susceptibilityχe by

ε = εo(1 + χe) .

persistence of vision Ability of the eye toretain image on the retina for a brief time aftertermination of the optical excitation that createsthe image. This allows the sequence of discreteimages used in cinematography (for example) toproduce an image on the eye that is continuousin time.

Petzval field curvature A monochromaticaberration of an optical system in which the im-age surface is not planar but curved. This imagesurface is called thePetzval surfaceand the as-sociated aberration is called curvature of field.If there are a number of thin lenses in air, thePetzval surface curvatureRP is given by:∑ 1

nifi=

1Rp

,

whereni andfi are the index and the secondaryfocal lengths of theith lens.

phantom circuit Telecommunication circuitobtained by superimposing an additional circuiton two existing physical circuits by means ofrepeating coils. It enables the transmission ofthree messages with only two pairs of wiring.

phase (1) The fraction of the period throughwhich the time variable of a periodic quantity(such as the sound vibration) has moved, as mea-sured from a point in time. It is commonly de-scribed in terms of angular measure, with oneperiod equal to 360 or radians.See alsophaseangle.

(2) The type of state of the system, suchas solid, liquid or gas, identified as having adistinct molecular arrangement that is homoge-neous throughout. When more than one phaseis present in a system, the phases are separatedfrom each other by easily identifiable phaseboundaries.See alsophase changes.

phase angle The argumentω(t± x

c

)+

φ of the sinusoidal wave described byp =pA sin

[ω(t± x

c

)+ φ

], obtained as a particu-

lar solution of the wave equation. The constant

φ is called the zero phase angle of the wave,the factorpA is the peak value (amplitude) ofthe pressure, the coefficientω is the angular fre-quency,t is time,x, the direction of propagationof the sound wave andc, the speed of sound inthe medium.See alsoharmonic motion and pe-riodic waves.

phase changes Transitions of a substancefrom one phase to another, such as gas to liquid(condensation), solid to gas (sublimation), etc.Release or absorption of energy accompaniesphase changes.See alsophase.

phase conjugation Process in which agiven optical field is mixed with two counter-propagating electromagnetic beams in a nonlin-ear medium to produce radiation that propagatesin a time-reversed manner with respect to thesignal field. Process corresponds todegener-ate four-wave mixingin which three fields ofa single frequency mix in a nonlinear medium,characterized by a third order nonlinear suscep-tibility χ(3), to produce a fourth field at the samefrequency with an amplitude equal to the com-plex conjugate of the amplitude of one of thefields. Termphase conjugationderives from thefact that time-reversed form of signal field

E(r, t) = E(r, ω)e−iωt + E∗(r, ω)eiωt ,

represented by

E(r,−t) = E∗(r, ω)e−iωt + E(r, ω)eiωt ,

differs from fieldE(r, t) only by replacementof the amplitudeE(r, ω) by the complex con-jugate amplitudeE∗(r, ω) (corresponding to afield propagating in a direction opposite to thedirection of propagation of the field with ampli-tudeE(r, ω)). With phase conjugation, when asignal field enters the four-wave mixing regionafter passage through an aberrating medium,the distortion produced in the signal field is re-moved from the time reversed (phase conjugate)field after passing through the same aberratingmedium in the opposite direction.

phase constant (1) A rating of a line ormedium through which a plane wave of givenfrequency is being transmitted. It correspondsto the imaginary part of the propagation con-stant, and describes the rate of change of phase


of a field component in the direction of propa-gation in radians per unit length.

(2) The constantφ in the argument of thesinusoidal wave described byp = pA sin [ω(t± x

c

)+φ]. See alsophase angle.

phase delay Ratio of the total phase shiftmeasured in radians of a sinusoidal signal prop-agating in the transmission line to the frequencymeasured in radians/second.

phase difference The difference between thephasesϕ of two sinusoidally varying quantitiesy1(t) = sin(ωt + ϕ) andy2(t) = sin(ωt) thathave the same frequencyω. Also calledphaseangle.

phase discriminator An electronic devicethat generates an output signal that is propor-tional to the phase difference between an oscil-lator signal and a reference signal. It is used tocontrol the oscillator and maintain it in synchro-nism with the reference signal. Also known asphase detector.

phase, in AC circuits The displacement ofa periodic waveform (usually sinusoidal) withrespect to a specific reference time or anotherperiodic waveform with the same angular fre-quency that does not rise and fall in unison. Usu-ally expressed as an angle ranging from 0 to360. Two waveforms are said to be in phase, inquadrature, and in opposite phase (or antiphase)if the phase angle is 0, 90, and 180, respec-tively. It is commonly used in many circuitswhere the current is not in step with the appliedalternating potential difference.

phase lag The negative of phase differ-ence between a sinusoidally varying quantityy1(t) = sin(ωt + ϕ) and another quantity,which varies sinusoidally at the same frequencyy2(t) = sin(ωt), when this phase difference isnegative.

phase sensitive detector (PSD) A detectorthat gives DC output proportional to the phaseshift between a reference signal and the inputsignal. It is also known as asynchronous rec-tifier, synchronous detector,or synchronous de-modulator.

phase shifter A device used to create a phaseshift between the input and output signals. Asimple type consists of a resistor and capacitor inseries with the output taken across the capacitor.

phase splitter A device used to produce twooutputs, one in phase and one 180 out of phase,with an input signal.

phase velocity The velocity of a point thatmoves with a sound wave at constant phase. Ifthe acoustic disturbance is represented as a setof harmonics, a phase angle can be assigned toany point of a component. The phase veloc-ity is the distance covered per unit time of thispoint along the direction of propagation. In anondispersive medium the phase velocity of allharmonics is the same, which is not the case in adispersive medium. The phase velocity appearsin the acoustic wave equation that describes thepropagation of sound in a medium. In an un-bounded, homogeneous medium the magnitudeof the phase velocity corresponds to the speedof sound. The speed of sound is a function ofthe adiabatic bulk modulus and the density ofthe medium; its magnitude depends on temper-ature, pressure and material composition.Seealsomodulation, acoustic.

phasor A vector representation of sinusoidalwaveforms. It is often used in the analysis ofAC signals.

phonetics Study of the sounds of humanspeech, their generation and the signs used torepresent them. The phonetic transcription pro-vides symbols for each phoneme of a languagetranscribed, as well as additional symbols tospecify differences between variations of thesame phoneme depending on the situation.

phonic chronometer An electric chronome-ter driven by a phonic motor. Phonic motorsallow the conversion of vibratory motion intorotary motion of constant speed using an electri-cally maintained tuning fork and a phonic wheel.

phonodeik An instrument used to record themovement of air caused by sound. It consists ofa horn that delivers the sound on a fine glass di-aphragm. A fine wire attached to the diaphragm


is wound around a vertical steel staff and main-tained taut by a spring. A small mirror is fixedto the staff. Pressure oscillations caused bythe sound waves set the diaphragm into motion,and its deflections are transferred to the mirrorthrough the wire and the staff. A beam of lightfrom a fixed source is reflected by the mirrorand falls on a vertical film strip. In this way,the deflection of light caused by the motion ofthe mirror can be recorded on the film. This in-strument is used to record curves representingspeech and different musical instruments.

phonograph An instrument for recording orreproducing acoustic signals, such as voice ormusic, by transmission of vibrations from or toa stylus that is in contact with a groove on a ro-tating cylinder or disk. The early phonographformed a groove of varying depth in a cylin-der made of wax, while the stylus moved in andout of the wax following the motion of the di-aphragm exposed to the sound waves. This mo-tion was then magnified by retracing the grooveon the cylinder or disk with a stylus attached toone end of the light lever, and allowing the otherend of the lever to trace the curve on a smokeddrum. See alsogramophone.

phonometer Instrument that measures soundintensity. Webster’s phonometer is based onthe resonance method and consists of a tunablecylindrical resonator with a diaphragm mountedon the cylinder opening. The diaphragm is tunedto the sound whose intensity is measured byvarying the tension of the wires supporting it.A small mirror attached to the diaphragm al-lows the experimenter to observe and measureits motion. The pressure amplitude of the soundimpinging on the diaphragm is proportional tothe displacement amplitude of the diaphragm.

phonon Quantum of sound representing ex-citations or energy levels in liquid Helium II inthe form of longitudinal sound waves. The con-cept was explained by the peculiar behavior ofliquid helium II by Landau in 1941. Thermalvibrations in a crystal lattice can be calculatedby this quantum of thermal energy.

photodiode A commonly used diode thatconverts a photon input into a current output.

photoelasticity The effect in certain mate-rials (also termedstress birefringence) whereinapplication of a mechanical stress causes mate-rials to exhibit birefringence, made evident bythe appearance of colored fringes when mate-rials are observed through crossed polarizers.The phenomenon can be interpreted as the ef-fect of stress-induced strain in altering densityand polarizability of a material so as to producean alteration in its dielectric tensor. Serves asthe basis of technique for detection of internalstresses in mechanical structures by examina-tion of structures through crossed polarizers forevidence of strain related birefringence.

photoelectric effect The effect wherein elec-trons (termedphotoelectrons) are ejected froma material surface by incident electromagneticradiation of sufficiently high frequency (usuallyin the visual part of the spectrum). The phe-nomenon is most pronounced where the materialsurface is metallic. The fact that maximum ki-netic energy of photoelectrons is linearly depen-dent on the frequency of the incident radiationbut independent of its intensity led Einstein to anexplanation of the effect based on the interpre-tation of radiation in terms of localized packetsof radiation energy calledphotons.An extendeddefinition of the photoelectric effect refers to thebroad category of radiation-induced changes inthe electrical properties of a material, includingchanges in its conductivity. The effect is madeuse of in light beam-activated circuits of the typecommonly used inelectric eyedevices.

photoemissive cell Device that detectsand/or measures light or other electromagneticradiation by the measurement of radiation-induced emission of electrons from the surfaceof a photocathode via thephotoelectric effect.Represents the essential element of aphotomul-tiplier tube.

photogrammetry The process of makingmaps or scale drawings from photographs. Ithas particular importance in the drawing of mapsfrom aerial photographs.

It also relates to the general process of mak-ing precise measurements by means of photog-raphy. Typical fields that use this technique in-


clude archeology, architecture, medicine, andengineering.

photography The process (and art) of pro-ducing images of objects on a photosensitivefilm by the collection of reflected radiation, usu-ally in the form of light, from the surfaces of theobjects.

photography, clinical Use of photography tohelp in the diagnosis or treatment of diseases orother physiological conditions (seephotogram-metry).

photography, color Type of photography inwhich the images of the photographed objectsreproduce the colors of the objects. Requiresthe use of color sensitive film.

photography of sound waves Sound wavescan be visualized by taking advantage of the de-pendence of the refractive index of light on thedensity of the medium. Optical measurementtechniques, such as the shadowgraph, Schlierenmethods, and optical interferometry, have beenused to visualize compressible flow fields, suchas supersonic flows and shock waves around air-foils, bullets and projectiles. Because of the highspeed of the process, short exposure times are es-sential for successful photography. Short expo-sure times are accomplished using stroboscopicillumination or high-speed cinematography.

photography, spark Type of photographyin which illumination of objects to be imaged isprovided by a spark to (severely) restrict the ex-posure time of the film. Allows for the produc-tion of sharp images of rapidly moving objects.

photolysis Most generally, a process in whichlight (or other radiation) produces a chemicalchange in a substance. A common use of theterm defines the process as one in which ab-sorption of light causes decomposition of themolecules of a substance.

photomagnetism Modification of magneticproperties, e.g., magnetic susceptibility, of amagnetic material by application of light (elec-tromagnetic waves). Light modifies the elec-

tronic structure by exciting charge carrier.Seesusceptibility, magnetic.

photometer An instrument for measuringthe luminous intensityand/or flux produced bya source of visible light, usually in comparisonwith the luminous intensity of a reference lightsource. The original-type photometer comparesluminous intensities,I1 and I2, of a sourceand reference source on an observing screenby varying distanced between reference andscreen until two sources produce equal lumi-nance on the screen’s surface. The ratio of in-tensities of sources follows from the relationI1/d

21 = I2/d

22. The modern photometer in

general measures the intensity of a light sourcevia a calibratedphotoemissive cell.

photometer, integrating (Commonly in theform of anintegrating-sphere photometer.) Aninstrument for measuring the total luminous fluxemitted in all directions by a lamp or other lightsource. Allows for the determination of thelu-minous efficiencyof a light source, given by thetotal luminous flux emitted by the source di-vided by the total power to the source. Thisintegrating-sphere device makes use of a hol-low sphere that can encompass the source to bemeasured and has a diffusely reflecting interiorsurface, the illumination of which from the re-flected light is proportional to the total flux fromthe source.

photometry The process (and/or science) ofmeasuring the luminous intensity, flux, color,spectral or angular distribution, reflectance ortransmittance of visible radiation (representinglight). Contrasts withradiometry,defined to bethe process of measuring the intensity of non-visible as well as visible radiation.

photometry, grease spot Measurement ofthe intensity of a light source compared to a ref-erence source by observing the effect producedwhen two sources illuminateoppositesides ofan opaque white screen containing a central spotmade translucent by treatment with a lower re-fractive index substance (such as oil or grease).The fact that unequal illumination of the screenresults in the appearance of a dark central spot onbright surroundings, or the reverse, allows rela-


tive intensities of the sources to be determinedfrom a “balance position” of the screen betweensources for which the central spot disappears.Seephotometer.

photometry, heterochromatic Branch ofphotometryconcerned with a comparison of theilluminating effectiveness of light sources of dif-ferent colors.

photomicrography The process of makingphotographs of images of minute objects formedby a microscope. In general, this makes use of aninstrument containing aphotomultiplierfor am-plification of light from the separate segments ofthe image to be photographed.

photomultiplier A device in which aradiation-induced photocurrent is amplified byfocusing initial photoelectrons onto a successionof electrodes (called dynodes) so as to induceemission of secondary electrons. The acceler-ation of electrons between successive dynodescauses each secondary electron to produce ad-ditional secondaries which multiply the initialphotocurrent. The resultant gain in electron cur-rent can equal (or exceed)108. The device al-lows for detection of low levels of (visible ornear-visible) radiation.

photon The basic unit (orquantum) of lightor other electromagnetic radiation. Plays therole of carrier of the electromagnetic field. Anentity characterized by zero rest mass and a ve-locity of propagation along a particular directionwith magnitude equal to the speed of light. Cor-responds to a quantity of electromagnetic en-ergy hf and angular momentum along the di-rection of propagationh/2π, wheref is the fre-quency of the electromagnetic radiation andh isPlanck’s constant (6.626075× 10−34 J·s). Theexistence of a photon is manifested by the factthat excitation and de-excitation of atoms andmolecules takes place only via the absorption oremission of integer numbers of photons. An-other (vision related) meaning ofphotonis theamount of light received by the retina of the eyefrom a surface with a luminance of 1 candela/m2

when the area of the pupil is 1 mm2 .

photonics The field concerned with the gen-eration, propagation, processing, and detectionof light or other radiant energy, often in connec-tion with the transmission or detection of sig-nals. Contrasts withelectronicsin that, controlof electrons is replaced by control of photons,which displace electrons as the primary carriersof signals. The term emphasizes the quantizednature of a basic unit of radiation. The area de-fined by the term includes, for example, the pro-duction and amplification of radiation via lasersand other radiation sources, the design and fabri-cation of optical waveguides and interconnects,and the use of nonlinear optical effects in ma-terials relevant to the generation and control oflight and other electromagnetic radiation.

photosensitivity Property of a material ororganism whereby it is sensitive to visible (ornear-visible) radiation. The term in general ap-plies to materials or organisms readily affectedby light. Photosensitivity in materials is exhib-ited via effects such asphotolysis,increased con-ductivity, the emission of photoelectrons, or thephoto-voltaic effect.

photosynthesis In the most general sense,photosynthesis is the process in which greenplants and certain other organisms use sunlightenergy to manufacture carbohydrates from car-bon dioxide and water with the help of chloro-phyll. In most cases of photosynthesis, oxygenusually results as a by-product. In this sense,photosynthesis is the reverse of respiration inwhich carbohydrates are broken down to releaseenergy.

In the first stage of photosynthesis, also calledlight reaction,direct light is required. Water isbroken down into oxygen (released as gas) andhydrogen. Also, ATP molecules are produced.Next is the second stage calleddark reaction,where the hydrogen and carbon dioxide (CO2)are converted into intermediary compounds thatultimately yield the organic compound glucose(C6H12O6) plus water. The chemical reactioninvolved is given by

6CO2 + 12H2O + energy →C6H12O6 + 6O2 + 6H2O .

The second stage does not require light to oc-cur, using instead the energy released from the


hydrolysis of ATP, where ATP reacts with waterto yield ADP, inorganic phosphate, and energy.

Chlorophyll, exhibited mainly by the greenpigment in plants, is contained in thechloro-plasts (organelles contained in the cytoplasmof plant cells). Chlorophyll is the only sub-stance in nature able to trap and store energyfrom sunlight. Because the red and blue-violetparts of the visible spectrum are the ones thatget absorbed by chlorophyll, the rest of the spec-trum, mainly green, gets reflected, thus givingthe green color to plants.

phototherapy Therapy of diseases or typesof disorders, especially of the skin, using light.Ultraviolet and infrared radiation are of partic-ular use in this kind of therapy.Seered light,healing effect; light, monochromatic; biologi-cal action.

photovoltaic efficiency Measure of the elec-tric potential (or voltaic response) produced in anonhomogeneous material by exposure to lightor other electromagnetic radiation. Measuresthe process in which radiation absorbed by amaterial structure in the region of a potentialbarrier (such as at ap-n junction or metal-semiconductor contact) produces electric poten-tial differenceVp in the region (e.g., by separa-tion of electron-hole pairs). Photovoltaic effi-ciency of a process is defined by the ratio ofpower associated with (latent) voltage-inducedcurrent,Ip, expressed byVpIp, divided by powerin input radiation,Pin.

pianoforte The pianoforte action refers tocommunicating the entire energy to be radiatedas sound by a vibrating string or wire duringa very short time interval, 1/500 s, while thehammer is in contact with the string. In the pi-anoforte action the hammer is projected againstthe string, and it is free from the system of leversthat have set it in motion. In addition to the pointof impact and the velocity of the hammer head,which determine the loudness, the pianoforteperformer can also influence the quality of thesound generated by the string. The way in whichthe motion has been initiated by the performerinfluences the vibrations of the hammer and thequality of the sound produced in this way.

piano, sound from The piano is a stringedkeyboard instrument in which a hammer strikingthe string is used to excite the sound; the ham-mer immediately rebounds after the action. Thenonlinear elasticity of the hammer plays a keyrole in determining the character of the pianosound. An iron frame maintains the tension ofthe strings. The vibration of the string is trans-mitted to a soundboard that serves as the mainsource of acoustic radiation into the surround-ing air. The frequencies generated by the pianospan somewhat over seven octaves (fromA0 toC8) with frequencies between 55 and 8360 Hz.When the piano key is struck and held, the soundbegins to decay at one rate and then “breaks” tocontinue decaying with another, slower rate thuscreating the prompt sound and the aftersound.This double decay characteristic is an importantfeature of the piano tone.

picture tube, color A cathode ray tube usedto display images in color by variation of thebeam intensity. It uses a system of three differ-ent colored pixels in adjacent locations to pro-duce the different colors.

piezoelectric effect The piezoelectric effecttakes advantage of the properties of crystallinequartz in that mechanical deformations can begenerated along the mechanical axis of the crys-tal to induce longitudinal vibrations along thisaxis by applying an alternating electric fieldalong the electric axis. As the frequency of theapplied electric field approaches the natural fre-quency of any longitudinal vibration mode of thequartz crystal, the amplitude of the mechanicalvibrations increases. Since the electroacousticefficiency of these vibrations is high, the piezo-electric effect is frequently utilized to generateultrasound in gases, liquids and solids. Con-versely, when a stress applied to the quartz crys-tal results in a strain either along the optic orthe mechanical axis, the crystal becomes elec-trically polarized and piezoelectric charges ofopposite sign form on the two surfaces perpen-dicular to the optic axis. This effect is exploitedin the operations of certain types of sensors. Themost commonly applied vibration transducerstake advantage of the piezoelectric effect. X-cutquartz crystals are typically used for piezoelec-tric energy conversion.


pigments Finely divided particles of natu-ral or synthetic substances used in suspensionin materials to contribute to the optical (and/orphysical) properties of the materials. Distin-guished from dyes, used in solution, by insol-ubility in the host materials. Pigment particlesare characterized by the ability to re-radiate lightof particular wavelengths on absorption of lightof other wavelengths so as to produce reflectedand transmitted light of the same color. Com-monly used to affect the color and opacity ofpaints and coatings.

pinch effect Effect whereby electric chargecarriers moving in the same direction are at-tracted toward each other by a magnetic forcecreated by their movement. Results in a radi-ally compressive force. In liquids and gases theflowing charge may pinch off so that no currentwill flow. Seemagnetic force on moving charge.

pinch-off voltage The gate-source voltagein a JFET at which the drain current approacheszero due to a meeting of the depletion regions.

ping-pong mechanism The ping-pongmechanism is a two-step chemical reaction inwhich a reactant reacts with a molecule thatleads to a chemical resultant. Then the samemolecule reacts again with the same kind of re-sultant to yield the original reactant. The impor-tance of this mechanism lies in the importanceof its by-products and of the fact that it is cyclic.

Examples of a ping-pong mechanism in-clude mechanisms where membrane proteinsexchange one charged molecule or atom foranother, such as Cl− for HCO−3 , or Na+ forH+. These proteins play a role in cellular pro-cesses that include volume and pH regulation,and transport of ions across the membrane.

The protein that catalyzes the exchange ofCl− for HCO−3 functions by a ping-pong mech-anism, in which the transport protein can existeither in a conformation with the transport sitefacing outward (Eo) or with the transport sitefacing inward (Ei). Conversion fromEo to Ei

or vice versa occurs only when a suitable an-ion, such as Cl−, is bound to the transport site.Thus, the system is confined to tightly coupledone-for-one exchange of ions.

Also CO2, coming from metabolic processesin tissue, is a participant in a ping-pong mecha-nism when it diffuses into the red cell and intra-cellular carbonic anhydrase catalyzes its trans-formation into bicarbonate. After, intracellu-lar bicarbonate is exchanged for extracellularchloride as bicarbonate flows passively down itsconcentration gradient. Once in the lungs, theprocess is reversed, and CO2 diffuses into theatmosphere, thus completing the cycle.

pipes, sound from Vibrations of air columnsare the source of sound in the organ as well asin most wind instruments. In the theoreticalanalysis it is assumed that the walls of the pipeare rigid, the diameter is small compared to thelength of the pipe and large enough to justifyneglecting viscosity effects. The two types ofpipes of practical importance are the open pipe(open at both ends) and the closed pipe (closed atone end). The modes of the organ pipe are char-acterized by displacement antinodes at the openends and by an appropriate number of nodes(separated by antinodes depending on the vi-bration mode) in the middle region. A displace-ment node at the closed end and an antinode atthe open end characterize the closed pipe. Theclosed pipe thus has a fundamental frequencythat is one octave lower in pitch than the openpipe. See alsoorgan pipes, vibrations in.

piston source A vibrating piston mountedin an infinitely large rigid wall (baffle) or in theend of a long tube used to approximate soundsources.

pitch, acoustic An aspect of the subjectivesensation of sound that allows sounds to be or-dered on a musical scale from “low” to “high”.The variations in pitch lead to the sensation ofmelody. Pitch, measured in mels, correspondsto the frequency for a pure tone and to the funda-mental frequency for a periodic complex tone.Pitch as a subjective attribute cannot be mea-sured directly; a value is assigned to a sound byspecifying the frequency of a sinusoidal soundvibration that has the same subjective pitch asthe sound. In the perception of pure tones, twotones separated by an interval of one octave (thehigher tone has twice the frequency of the lowertone) sound similar and have the same name on


the musical scale. The concept of pitch is impor-tant in accounting for the perception of complextones.

Pitot tube An instrument that measures thestatic and/or stagnation pressure of a flowingfluid, consisting of a slender tube equipped withholes along the perimeter of the tube or one atthe tip, pointing into the fluid and connected toa pressure indicating device. The static pres-sure probe is equipped with small measuringholes serving as pressure taps along the perime-ter. The manometer connected to the tube indi-cates the static pressurep along the streamline atthe location of the taps. The stagnation pressurep0 is obtained when the fluid is decelerated tozero speedV0 at the tip of the stagnation pressureprobe. The pressure is sensed through the holeat the tip of the probe. The Bernoulli equation,p0ρ + V 2

02 = p

ρ + V 2

2 , relates the changes in speedto the changes in pressure along the streamlinein incompressible flow of densityρ. The combi-nation of the static and stagnation probes in thePitot tube allows determining the flow velocity

V =√

2(p0−p)ρ from the measured difference

between stagnation and static pressures,p0− p.

Pitot tube.

plane wave A wave in which the wavefront isa plane surface. The equiphase surfaces of planewaves form a family of parallel planes. Planesound waves have the same acoustic propertiesat any position on the plane surface that is per-pendicular to the direction of propagation of the

sound wave. Plane sound waves can exist in astraight pipe or duct; they are one-dimensionaland vary with timet and one Cartesian coordi-natex. They can be described with the one-dimensional equation of motion, the acousticwave equation, which relates the second deriva-tive of sound pressurep along the direction ofpropagationx to the second derivative of thesound pressure with respect to timet through thesquare of the speed of soundc, ∂2p

∂x2− 1c2

∂2p∂t2 = 0.

The general solution of the acoustic wave equa-tion is p(x, t) = f(x− ct) + g(x+ ct), wheref andg are arbitrary functions.

Most generally, waves characterized by “pla-nar wavefronts” having amplitudes and phasesthat have the same values for all points withinany plane perpendicular to the direction of prop-agation. The less general, more usual definitiondefines plane wave to correspond to a solutionof the scalar wave equation

∇2Ψ− 1v2

∂2Ψ∂t2

= 0 ,

of the form

Ψ(r, t) = Aei(k·r−ωt+φ) ,

whereA and(k · r−ωt+φ) are referred to, re-spectively, as the “amplitude” and “phase” of thewavefunctionΨ, and|k| ≡ k = ω/v. The har-monic variation of a wavefunction with distancek · r and timet results in a wavefront that ad-vances in the direction of vectork at the “phasevelocity” ω/k. The planar character of wave-fronts results from the dependence of functionei(k·r−ωt+φ) only on a component of coordinatevectorr along vectork, (k · r), and the absenceof dependence on a component ofr perpendic-ular tok, (k × r). The resulting wave has in-finite extent in the directions perpendicular tothe propagation direction, infinite extension intime, and corresponding infinite length along thedirection of propagationk.

plastic deformation This is a permanent andirreversible deformation in crystals due to linedefects.

plates, vibrations in Objects in the class ofplates, which can also include surfaces that arenot flat, can emit sounds classified as “noises”


with “metallic” character rather than “notes”.Vibrations in a thin plate are described by thefundamental differential equation of motion forfree vibrations (with the assumption that themiddle layer of the plate is physically inexten-sible) as∇4ξ − k4ξ = 0, with k4 = ω2/c4. ωis the angular frequency and the parameterc isdefined asc4 = qh2

3ρ0(1−µ2) (with q as Young’smodulus,2h the thickness of the plate,µas Pois-son’s ratio andρ0, the volumetric density of theplate.

plethysmograph, capacitance The capaci-tance of a body part can be determined from aplethysmograph.In a plethysmograph, the ca-pacitance of a body part can be measured whenthe reactance of this body part is determined asa function of the frequency of the external al-ternating current (seeplethysmograph, imped-ance).

plethysmograph, impedance An imped-ance plethysmograph measures and records vol-umetric variations of an organ or body part onthe basis of variations in the electrical resistanceand reactance of the body or its segment as afunction of the frequency of an input alternatingcurrent. The electrical variations in the outputcan be related to the amount of volume pulsingvariations coming from blood and other intra-and extravascular fluids passing through the or-gan. With this technique, the flow of electrolytesinjected in the blood flow can be easily followedby electrical conductivity measurements.

Considering the case where the input currentis constant, the electrical resistanceR of a por-tion of a conductor of uniform cross sectionaand lengthl is

R =ρl

a

=ρl2

l · a

=ρl2

V

whereρ is the resistivity of the material andV ,the volume of the segment.

If the length of the conductor is constant andthe volume is increased by increasinga, thenδR

is proportional toδV as

δR = R1 −R2

= ρl2(1V 1

− 1V 2

)

= − ρl2δV

V 1 · V 2or

δR ≈ −ρl2δV

V 2

for small changes, so that

δR

R= −δV

V.

This resulting equation is the basis for thevolumetric variation measurements from an im-pedance plethysmograph. The equation resultscan be interpreted to mean that a small increasein volume brings a decrease in the electrical re-sistance.

p-n junction A junction formed by the con-tact of p and n doped semiconductors. TheFermi level difference in thep andn type mate-rials produces a built-in potential in the device,which for silicon is∼ 1.1 V. Seen-type silicon,p-type silicon.

P.O. Box (post-office box) One of the olderforms of the Wheatstone bridge that consists ofan arrangement of resistance coils, brass blocks,and tapered plugs that form three arms of thebridge. The resistance coils are inserted in thecircuit by removing plugs from tapered holes be-tween adjacent blocks. Each of the holes has aresistance associated with it. Each arm of thebridge contains several such holes; the resis-tance of an arm is read by adding the resistancesof the unplugged holes.

Pockels effect Electro-optic effect in certaincrystalline materials wherein the application ofan electric field produces a change in the re-fractive properties of the materials proportionalto the first power of the applied electric fieldstrength. The effect occurs in crystals in whichthe applied electric field produces slight defor-mation of the ionic lattices (piezoelectric effect)and/or a redistribution of the bond charges, re-sulting in a change in the dielectric tensor of thematerial. First studied by F. Pockels in 1893.


polarimetry The process (and/or science) ofmeasuring the rotation of the plane of polariza-tion of visible or near-visible radiation producedby passage of the radiation through a materialmedium. Serves as a basis for measurement ofoptical activityor circular dichroism of materi-als (such as sugar solutions).

polariscope An instrument, consisting usu-ally of a plane polarizer through which lightpasses and falls on a transparent sample mate-rial to reach a rotatable analyzer. It is used toinvestigate the effect of the sample material onthe state of polarization of the emergent lightand also to study strain in transparent materialsamples.

polarization, circular Special type ofellip-tical polarizationof light or other electromag-netic radiation in which the electric vector of theradiation rotates in time in the plane perpendic-ular to the direction of propagation of the radiantenergy such that the tip of the vector describesa circular helix with its axis along the directionof propagation and with a period equal to the re-ciprocal of the frequency of the light. Circularpolarization is referred to as “right handed” or“left handed” depending on whether the sense ofrotation of the electric vector coincides with thecurl of the fingers of the right or the left hand re-spectively when the thumb of the hand is pointedin the direction of thePoynting vector.

polarization, degree of Measure of the ex-tent to which light or other electromagnetic ra-diation can be said to be polarized. Represen-tation of light as a superposition of linearly po-larized components provides for an operationaldefinition of degree of polarization in terms ofmeasurements of the intensity of the light trans-mitted through a linear polarizer oriented so asto define a direction perpendicular to the direc-tion of propagation of the light. Rotation of thepolarizer through all orientations in the planeperpendicular to the propagation direction, andmeasurement of the intensities of the transmit-ted light, make possible (in principle) a deter-mination of a maximum and a minimum in thetransmitted intensity,Imax andImin , in termsof which thedegree of polarizationof the light,

P , can be defined by the ratio

P ≡ Imax − Imin

Imax + Imin.

The definition assignsP the values 1 and 0 in thecases of completely polarized light and (com-pletely) unpolarized light respectively.Seepar-tially polarized light.

polarization, electric The electric dipolemoment per unit volume of a dielectric. Electricpolarization occurs when a dielectric is placedin an electric field that tries to align the electricdipoles parallel to each other. This results in aseparation of electric charge in the assembly ofdipole moments which in turn produces chargeson the surface of the dielectric. The degree ofpolarization is dependent on temperature sincethermal agitation tends to oppose the order pro-duced by the electric field.

polarization, elliptical State of polarizationof light or other electromagnetic radiation inwhich the electric field vector of the radiationat a particular point in space rotates in time inthe plane perpendicular to the direction of prop-agation of the radiant energy such that the tipof the vector traces out an ellipse. Ellipticallypolarized light can be represented as a super-position of two linearly polarized light wavesof unequal amplitude, which have mutually or-thogonal directions of polarization, and whichare out of phase with respect to one another bya non-integer multiple ofπ.

polarization, linear State of polarization oflight or other electromagnetic radiation in whichthe electric vector of the radiation oscillates intime along a fixed direction in a plane perpendic-ular to the direction of propagation of the radiantenergy. Also known asplane polarization.

polarization, membrane Seepotential, rest-ing; repolarization (cell); nerve impulses, prop-agation of.

polarization of light Property of light andother electromagnetic radiation defined by anon-random orientation of the electric (and mag-netic) vector of the radiation field in the planeperpendicular to the direction of propagation of


the radiant energy. A specific state of polar-ization is in general specified in terms of thedirection of the electric vector,E, in the planeperpendicular to thePoynting vector.Types ofpolarization can be categorized aslinear, ellip-tical or circular.

polarization, plane Alternate term forlinearpolarization.

polarizer An optical device whose input isnatural light and whose output is polarized light(attained usually with the help of a Nicol prism,Polaroid sheet, etc). Depending on the nature ofpolarization of the outcoming light (e.g., planepolarized, circularly polarized), the polarizer iscalled aplane polarizer, circular polarizer,etc.

Polaroid Trade name for a transparent sheetof dichroic material which transmits light that islinearly polarized along a particular direction.A common type of polaroid material consists ofcolorless plastic sheet treated with an iodine so-lution that creates parallel chains of polymericmolecules containing conductive iodine atomswhich produce a plane of polarization by dichro-ism. Commonly used to reduce glare in opticaland lighting devices. A generalization of theterm relates to a range of photographic and op-tical products based on polymeric materials.

Polaroid camera Trade name for a camerathat makes use of film containing its own de-veloping and printing agents that make possiblethe production of a finished positive print withinminutes after the photograph is taken. Devel-oped by Edwin H. Land in 1948. Also knownasLand camera.

pole piece Magnetic pieces that attach to op-posite ends of a magnet to finish a magnetic cir-cuit. Often have an air gap between them. Theirsize and shape determine the magnetic flux dis-tribution in the gap. Can concentrate magneticflux creating a large magnetic field in a smallvolume or spread flux lines out uniformly creat-ing a small uniform magnetic field over a largervolume.Seeflux, magnetic.

pole strength A measure of the strength ofa magnet. The pole strengthp has MKS units

of A.m and is defined bym = pl wherem themagnetic moment of a bar magnet with northand south poles of strength+p, and−p andl isthe separation of the poles.

polling A technique for coordinating accessto a shared medium. A master checks whethereach slave has data to send, and if it does, givesit a chance to use the medium.

potential, contact The potential differencethat develops between two dissimilar metalsplaced in contact. It is given by the differencebetween work functions for the two metals andvaries with the temperature of the junction. Twosuch junctions placed in series and kept at dif-ferent temperatures produce a net electromotiveforce through the circuit which forms the basisof thermocouple thermometers.See alsother-mocouple.

potential, demarkation The demarkationpotential is the threshold potential beyond whichthere is the initiation of an action potential. Ifa stimulus provided by sensory information orneurotransmitters changes the local membranepotential by as much as the demarcation poten-tial of approximately−60 mV, then the initialsignal leads to the rapid opening of Na+ chan-nels and to the initial steps toward creating anaction potential.Seenerve impulses, propaga-tion of.

potential difference In electrical circuits, itis the work required to transfer unit charge be-tween two points in the circuit. The SI unit isjoules per coulomb but it is commonly referredto as volts. The potential difference across aresistance in an electrical circuit is obtained byapplying Ohm’s law. In an electric field, it is thework required to move a unit charge betweentwo pointsA andB. It can be obtained by cal-culating the difference in electric potentials atpointsA andB, by

V = −∫ B

A

E · dl

whereV is the potential difference,E is the elec-tric field, anddl is a path element betweenA andB. See alsoOhm’s law; potential, electric.


potential divider A circuit used to transmita selected fraction of the input potential to theoutput. It typically consists of two resistors inseries with the output taken across a single re-sistor.

potential, electric The work required tobring a unit positive charge from infinity to acertain position. This is given by

V =W

q,

whereV is the electric potential,W is the workdone by an external force, andq is the test chargebrought from infinity at a constant speed to therequired position. The SI unit of electric poten-tial is joules per coulomb, otherwise known asvolts i.e.,

1 volt = 1 J/C .

If the electric fieldE is known, the electric po-tential can be calculated from the line integral

V = −∫ x

∞E · dl

wherex is the position at which the electric po-tential is required,dl is a length element alongthe path taken by the test charge coming frominfinity to pointx.

potential, evoked When certain areas of thebrain are driven toward electrical activity understimulation of specific sensory pathways, the po-tentials coming from them are calledevoked po-tentials. These evoked potentials are recordedby placing wires on the scalp over the areas ofthe brain being stimulated with a particular stim-ulus.

Common examples of evoked potential testsare the visual evoked potentials(VEP), thebrainstem auditory evoked potentials(BAEP),and thesensory evoked potentials(SEP). In VEPthe patient sits before a screen and responds toalternating visual patterns. In BAEP the audi-tory part of the brain is tested by presenting tothe patient a series of clicks to each ear. In SEPa small electrical impulse is administered to thepatient on an arm or leg. Not common but ex-isting, aremotor evoked potential teststhat candetect lesions along motor neuron pathways ofthe central nervous system.

Evoked potential tests are often used to helpmake a diagnosis of multiple sclerosis (MS), be-cause they can indicate dysfunction along neu-ronal pathways caused by demyelination of theaxons of neurons.

potential, extracellular Potential that ariseswhen an action potential crosses the synapse andenters the post-synaptic membrane. The currentthat flows into post-synapse and into the mem-brane closes the current loop by flowing out ofthe cell along the length of the walls of the mem-brane and into the extracellular space, and sub-sequentially re-enters the synapse.

Because the extracellular resistanceRex isso small compared with the large resistance ofthe membraneRm, the voltage across the mem-braneδVm is effectively equal to the currentImultiplied byRm. Also, sinceRex Rm, theextracellular potential dropδVex is going to bemuch smaller thanδVm.

By equating currents

δVm

Rm=δVex

Rex,

we get typical values

δVex =δVm

Rm·Rex

=5× 10−3V

1× 105Ω· 50Ω

= 2.5µV .

The extracellular potentials of populationsof neurons can be recorded and form the basisof the electroencephalographic measurements(EEG).

potential, graded (membrane) A gradedpotential is a potential whose value depends onan external parameter. An example of a gradedpotential is thepostsynaptic potential(PSP).The postsynaptic potential is a transient changein the electric polarization of the membranecaused by the influx of neurotransmitters as a re-sponse of an action potential at the presynapticmembrane. The PSP is a graded potential be-cause its degree of hyperpolarization (increasein negative charge inside the cell membrane—inhibitory) or depolarization (decrease of the


Extracellular potential.

negative charge—excitatory) varies on depen-dence of the activation of the ion channels bythe neuro-transmitters (seeneuro-transmitters).

potential gradient The spatial rate of changeof electric potential in a conductor, dielectric orfree space. It is obtained by evaluating the po-tential difference per unit length along the direc-tion of the electric field vector. In three dimen-sions, the electric fieldE is related to the spatialderivative of the potentialV as follows:

E = −(i∂V

∂x+ j

∂V

∂y+ k

∂V

∂z

).

potential, Henderson In the acid-base bal-ance equation

HB → H+ +B− ,

whereHB is a weak acid and the reaction showshow it dissociates, the reaction rate is given bythe constantKa. The reaction constant is in turngiven by the ratio[H+][B−]/[HB]. From thereaction it is evident that strong acids dissoci-ate more readily and have highKa while theopposite is true for weak acids.

In 1908 Henderson studied a metabolic reac-tion involvingCO2 and applied the law of massaction to get[

H+] [HCO−3

]= Ka [CO2] [H2O] .

Later, Sorensen (1909) introduced the “po-tential for hydrogen” notation wherepH =− log[H+]. Using thepH notation also for therate constantKa (pKa = − logKa), Hassel-balch in (1916) rewrote Henderson’s equationto get the Henderson–Hasselbalch equilibriumequation

pH = pKa + log

[HCO−3

][dCO2]

,

wheredCO2 stands for dissolvedCO2.The pH andpKa notations are particularly

good since the hydrogen concentrations and thereaction constantsKa for weak biological acidsare very small numbers. The acid-base balanceequations above indicate then that strong acidshave a lowpKa, and vice versa.

potential, interaction (1) An interaction po-tential between two particles comes from theirmutual interaction force. The interaction poten-tial at pointB, relative to pointA, is proportionalto the integrated mutual force if one would takeone particle along a path that joins both points.

More specifically, the interaction potential isproportional to the work doneWAB in bringing aparticle from pointA to pointB along a specificpath,

WAB =∫ B

A

~F · ~dl ,

whereF is the force between the particles, andl specifies the path of approach. For there tobe a potential associated to a specific force, thework done has to be independent of the partic-ular choice of the path. These forces are saidto be conservative. Examples of conservativeforces are the gravitational force and coulombicforces.


(2) The membrane potential arises from thecharge separation that occurs between the extra-cellular and the intracellular regions. Becauseit requires electrical work to take an ion againstthe electric field created by the disbalance ofcharges, there is a corresponding potential dif-ference across the membrane.

potential, intercellular (1) Potential differ-ence between different cells.

(2) Before transmission of an action poten-tial across a synapse, the pre- and post-synapticmembranes lie at different potentials. The re-lease of neuro-transmitters propagates the sig-nal by equalizing the potentials and provokingan action potential at post-synapse.Seepoten-tial, extracellular; potential, membrane; neuro-transmitters.

potential, junction The junction potential isthe potential difference at the boundary betweentwo solutions with different ionic compositions.It arises from the different diffusion constants ofthe ions in each particular solution.

The junction potential may give significantcontributions in electrophysiological measure-ments of the order of mV. Namely, it may beimportant in measurements of ion permeationthrough ionic channels using the patch-clamptechnique, in which extraneous ions with lowmobilities are used in fairly high concentrations.Seevoltage clamp, ionic current in cell.

potential, liquid (1) The thermodynamicsof simple molecular liquids are usually welldescribed by pair interparticle potentials. Anempirical potential for simple liquids is foundto have a repulsive interaction at very smalldistances that is inversely proportional to thetwelfth power of the distance between them. Asthey separate, an attractive interaction due toa weak charge-polarization or induced dipole-dipole interaction comes into effect. The inter-particle potential can be written as

U(r) = 4ε[(σr

)12

−(σr

)6],

whereε andσ are the depth of the interactionwell at equilibrium and the effective radius of theparticles, respectively. These two parametersare experimentally obtained.

(2) Both sides of the interface between twoionic solutions may be at different potentials dueto the differences in diffusion coefficients of theions. The drop in potential gives rise to a junc-tion potential.Seepotential, interaction; poten-tial, junction.

potential, membrane The membrane po-tential, or transmembrane potential difference,is the electrical potential difference across aplasma membrane (seepotential, resting; actionpotential).

The membrane potential arises from both theion pumps and from the flow of ions throughchannels that are open in the cell plasma mem-brane (seeosmotic equilibrium (cell)). The pas-sive flow of ions through the membrane channelsdepends on the osmotic gradient of ions acrossthe plasma membrane. The ion pumps use en-ergy from ATP hydrolysis to actively transportions across the plasma membrane in favor oragainst their osmotic gradient.

Because of the different mechanisms in-volved in distributing ionic composition in andout of the cytoplasm, the intracellular fluid issubstantially different from that of extracellularfluids. The relationship between ion concen-tration and membrane potential is given by theNernst equation. SeeNernst equilibrium poten-tial.

potential, miniature end plate (MEPP) Theminiature end plate potential arises from smallfluctuations (typically 0.5 mV) in the resting po-tential of post-synaptic cells. The profile of thepotential is of the same shape, although muchsmaller than, the end plate potentials caused bythe stimulation of the pre-synaptic cell. MEPPsare considered as evidence for the quantal re-lease of neuro-transmitters at synapse, a singleMEPP resulting from the release of the con-tents of a single synaptic vesicle.Seeneuro-transmitters.

potential, receptor, hair cell In general,the dermis consists of several types of tissue:glands, nerve endings, fat cells, hair follicles,and muscles. The nerve endings, calledrecep-tors,perform an important sensory function, re-sponding to various stimuli including: touch,pressure, pain, heat, and cold.


In particular, the receptor hair cells in the in-ner ear are responsible for the marked sensitiv-ity of mammalian hearing. These hair cells areresponsible for the transformation of the me-chanical sound into a neural signal. Mechanicalinput, from acoustic pressure waves, deflect thehair cell’s receptive organelle, the hair bundle.Bending of these hair cells (stereocilia) causesthe opening of small channels in the cell mem-brane to which the hair cells are attached. There-fore, the movement of stereocilia controls theion current flow into the cell. Bending the stere-ocilia in the direction away from (toward) thecenter of the cochlea leads to an increase (de-crease) of the intracellular potential. Intracellu-lar voltage changes as small as 0.1 mV are ableto cause neuro-transmitter release at the synap-tic contacts and in this way action potentials areevoked in the fibers of the auditory nerve.

potential, resting The resting potential of amembrane is the electrical potential of the insideof a cell membrane relative to its surroundings.In almost all types of animal cells the inside oftheir membranes are negative with resting po-tentials in the range of−20 to−100 mV, with−70 mV being the typical value.

Although the membrane resting potentialsare initially caused by the action of sodium ionpumps, their value comes primarily from thesubsequent diffusion of potassium out of the cellthrough potassium leak channels. The restingpotential is thus close to the Nernst potential forpotassium.Seeaction potential; osmotic equi-librium (cell).

potential well A region in which a chargedparticle experiences a lower potential energy andconsequently becomes trapped in that region.The particle can escape from the potential wellprovided it gains sufficient energy equal to thedifference between its kinetic energy and thewell depth. This is known as thebinding en-ergy. According to quantum physics, any par-ticle trapped in a potential well can have onlydiscrete energy levels; i.e., the levels are quan-tized. The concept of a potential well is mostlyused to describe the physics of trapped atomicand subatomic particles such as electrons.

potentiometer An instrument used to mea-sure precisely an unknown potential differenceof an EMF by comparing it to the known poten-tial difference of a standard cell under the con-dition in which there is zero current from theunknown potential difference. The termpoten-tiometer is alternately used to refer to a three-terminal voltage divider. Two of the terminalsare connected by a fixed resistance while thethird is a variable sliding contact that can bemoved by a rotatable shaft. This forms a vari-able resistance between the sliding contact andthe other two terminals. The changing resis-tance with changing position of the sliding con-tact may be linear, logarithmic, sinusoidal, etc.In everyday language, the term has been short-ened topot.

power density The power density spectrum,W (ω), is defined as

W (ω) = limT→∞

1T|FT (ω)|2 ,

whereT is the time interval of interest andFT (ω)is the Fourier transform of the given randomfunctionf(t), given by

FT (ω) =∫ T

0

dtf(t) exp(−jωt) .

The functionW (ω) is an even function of fre-quency:

W (−ω) = W (ω) ,

since for a real functionf(t),

FT (−ω) = F ∗T (ω) .

Sometimes the spectrum is limited to positivefrequencies by considering

W ′(ω) = 2W (ω) for ω > 0= 0 for ω < 0 .

The power in a band extending fromω1 toω2 is∫ ω2

ω1

dωW ′(ω) .

power detection A form of detection inwhich the demodulator supplies a substantialpower output directly to the load without using


an intermediate amplifying stage. A demodula-tor is a circuit, apparatus, or circuit element thatis used in communication to demodulate the re-ceived signals, i.e., to extract the signal from acarrier with minimum distortion.

power factor The ratio of the power,P (inwatts) dissipated in a circuit to the effective volt-amperes,VeffIeff, applied to it. The power dissi-pated is given by

P = VeffIeff cosφ ,

wherecosφ is the power factor,φ is the phaseangle between the alternating current and volt-age,Veff andIeff are the root-mean-square val-ues of the sinusoidal current and potential dif-ference, respectively. The power dissipated in apure inductance or pure capacitance is zero sinceφ = 90 in both cases, which makes the powerfactor equal zero. The power factor of a circuitis usually expressed in percent;cosφ = 1 isspoken of asunit power factoror 100 percentpower factor.

power gain The ratio of the output power tothe input power in a device or circuit.

power, in AC circuits This can be dividedinto apparent power and actual power. The ap-parent power is the product of the effective volt-age and effective current. Actual power is givenby the product of apparent power and the powerfactor. Apparent power is expressed in volt-amperes while actual power is in watts. Fora sinusoidal voltage and current, the effectivecurrent,Ieff, and voltage,Veff, usually called theroot-mean-square values, are given by

Ieff =I0√2

Veff =V0√

2,

whereI0 andV0 are the peak values of the cur-rent and voltage respectively. The power ex-pended by a circuit is given by the actual power.It is common practice for an AC circuit or ap-paratus to be rated by the apparent power.Seealsopower factor.

power in wave motion The rate at which en-ergyE, transported by the wave at speedv, is

delivered by the wave. For a vibrating string, theaverage rate of energy (averaged over the timeinterval of motion) transported across a point onthe stringP is the average energy density thatcrosses the point in a unit time, multiplied bythe length of the wave at that point (wave speedtimes the unit time),P = v

⟨dEdx

⟩= 1

2µω2y2

0v,with µ as the mass density of the string,ω theangular frequency andy0 the displacement am-plitude. The power delivered by a wave is pro-portional to the square of both amplitudey0 andfrequencyω. See alsooscillations, energy of.

power rating The maximum power outputavailable from a device, or the power it requiresto operate.

power spectral density For a specified band-width of radiation consisting of a continuous fre-quency spectrum, the total power in the specifiedbandwidth divided by the specified bandwidth isthespectral density.

Letx(t) be a random process such that at timet1, x(t1) is a random variable having a probabil-ity density,px1(η), which statistically describesthe process att1. The probability that the pro-cessx(t) will have a value in the range(a, b) attime t1 is then given by

Prob[a ≤ x (t1) ≤ b] =∫ b

a

px1(η)dη .

One simple measure of the degree of random-ness of a processx(t) is indicated by the pro-cess’s autocorrelation function

Rx(t, τ) = cE[x(t)x(t+ τ)] ,

wherecE denotes a statistical average over thejoint density of the processx(t) at time t andx(t) at time t + τ . The frequency character-istics of a stationary random processx(t) areexhibited by its spectral density,Sx(ω), definedas the Fourier transform of the process’s auto-correlation function

Sx(ω) =∫ ∞

−∞Rx(τ) exp−jωτ dτ ,

whereω is the frequency. The functionSx(ω)is called the spectral density or power spectrumof the stationary random processx(t).


Poynting vector Vector quantity, denotedS,the magnitude and direction of which determinethe magnitude and direction of the energy trans-ported by light or other electromagnetic radia-tion across a unit area in a unit of time. Thevector is explicitly defined in terms of the crossproduct of the electric and magnetic vectors ofthe radiation,E andB, via the relation

S =1µ

(E ×B) ,

whereµ is the “permeability constant” of themedium. The significance of vectorS derivesfrom the fact that the integral of the normal com-ponent ofS over a given surface equals the rateof flow of electromagnetic energy through thesurface.

pre-amplifier An amplifier operating beforethe main amplifier to boost the signal. It is nor-mally used before signal processing occurs toavoid amplifying the noise.

precooling This is a stage in an experimentused to speed up the overall cooling process.It is common practice to introduce a little airas exchange gas before the subsequent transferof liquid helium into the cryostat for cooling.Cryostats in the temperature range above 1 Kthat use4He for cooling, usually use nitrogenfor precooling.

presbyopia A reduction in the ability of theeye to accommodate to bring close objects to afocus on the retina. This is a naturally occurringconsequence of aging.

primary cell An electrochemical cell thatcannot be recharged is called aprimary cell.They usually have high energy density and goodshelf life. They are widely used in electronicequipment and are disposable. Typical exam-ples arezinc-carbon cellandalkaline cell.

principal focus The point of intersection ofa focal plane with the optic axis of the sys-tem. Corresponding to the first and second focalplanes, we have thefirst andsecond foci.

principal maxima The peaks of largest in-tensity in the interference pattern produced by

interfering beams of light or other electromag-netic radiation. Generally correspond to thezeroth and first-order maxima in an interfer-ence pattern, as, for example, the central andtwo neighboring maxima in the intensity patternproduced by the passage of coherent radiationthrough two slits.

principal planes/points Every lens/lens sys-tem has two principal points/planes and are twoof six cardinal (or Gaussian) points/planes of theoptical system. A ray entering a thick lens fromthe first focal point will emerge parallel and aray parallel to the axis on the object side willpass through the second focal point. The exten-sions of the incident and emergent rays, in eachcase will intersect, by definition, the principalplanes that cross the axis at the principal points.Principal planes in general do not coincide andmay sometimes be located outside the opticalsystem.

principle, convolution The convolution oftwo functions,ψ1(x)andψ2(x), is by definition,the functionψ(x) equal to:

ψ(x) =∫ +∞

−∞dy ψ1(y) · ψ2(x− y) .

The Fourier transform ofψ(x),ψ(p) is the ordi-nary product of the respective transforms ofψ1

andψ2. Seeprinciple, deconvolution.

principle, cryodyne Seeprinciple, Gifford-McMahon.

principle, deconvolution In general, the de-convolution principle is the inverse of the con-volution principle (seeprinciple, convolution).The goal in this procedure is, given a single func-tion ψ(x), two functionsψ1 andψ2 (that whenconvoluted together yieldψ(x)) can be sepa-rately determined.

The method is of practical use to imagerestoration, enhancement, reconstruction, andsignal filtering. Numerical algorithms existthat can reconstruct image objects by iterativecoded-source image deconvolution. In manycases, a sharper image is sought when the sourceimage has contributions from noise.

Novel techniques in the recording and imag-ing of X-ray and gamma rays use neural net-


works for the deconvolution phase mixed withnonlinear filtering for noise removal and edgeenhancement.

principle, Gifford-McMahon This is an ex-ample of expansion-cooling, and external workcycle. The cycle consists of four phases; in thepressurization phase, the warm volume is at amaximum. In the subsequent intake phase thevalve remains open to enlarge the cold volumeand reduce the warm volume. The expansionphase then takes over, when the exhaust valve isslowly opened and the cold volume is cooled byexpansion. The final exhaust phase occurs whenthe displacer is moved downward to displace theremaining cold gas. This is also known as thecryodyne.The cryodyne is used to provide smallrefrigerators that cool microwave equipment oroptical devices to 70, 20 or 4 K.

principle of complementarity The wave na-ture (electromagnetic wave) and particle nature(collection of photons) of light are manifestedin different experiments. Similarly, matter alsoexhibits both wavelike and particlelike proper-ties. Niels Bohr proposed that matter has a dualnature and that the wave and particle aspectsof matter complement each other. Einstein ex-tended this duality concept to electromagneticwaves and photons.

principle of reversibility Any ray in an opti-cal system, if reversed in direction, will retracethe same path.

printed circuit A pattern of conductors ona board of insulating material to which compo-nents are added to form a circuit. A printedcircuit is often created by photolithography.

prism A block of optical material with flatpolished sides that are arranged at precise an-gles to each other. Prisms do not form imagesbut can be used to deviate beams of light, invertor rotate an image, disperse light into its com-ponent wavelengths, or isolate separate states ofpolarization.

prism, achromatic Seeaberration, chro-matic.

prism binocular Binoculars in which Porroor other prisms are used to produce erect finalimages and to reduce the length of the instru-ment. The distance between the objective lensescan thus be made greater than the interpupillarydistance, thereby enhancing the stereoscopic ef-fect produced by ordinary vision.

prism, combination of A combination ofprisms used to give a final erect image whilesimultaneously removing the left-right reversalproduced by a typical telescope. Use of prismsavoids the problems of aberration and of in-crease in length of optical systems associatedwith lenses.

prism, Litrow If a Brewster prism (a prismin which both a particular wavelength and lin-ear polarization rays at minimum deviation enterand exit the prism at a Brewster angle) is cut inhalf along the plane in which there is a bisectorbetween the two Brewster entrance/exit faces, aLitrow prism is used as the planar element of ahemispherical laser resonator.

prism, Nicol A polarizing prism. The princi-pal section is perpendicular to the entrance face,but the optic axis is neither parallel nor perpen-dicular to the face. It transmits the extraordinaryray, but the ordinary ray is totally internally re-flected.

prism, Porro A reflecting prism, commonlyused in binoculars to provide erect images. Itconsists of two right angle prisms such that lightentering perpendicular to the hypotenuse sur-face is totally reflected in turn by the two op-posite surfaces, to emerge from the hypotenusesurface parallel to the incident light.

prism, Rochon A common type of polariz-ing beam splitter when a ray of light, incidentnormally at the entrance face, travels along theoptic axis in the first half of the prism. Bothordinary and extraordinary rays are undeviatedand have the same refractive index. The secondhalf of the prism has its optic axis at right anglesto that in the first half, but the ordinary ray isundeviated since its refractive index is the samein both halves. The extraordinary ray has min-


imum index in the second half and is refractedat the cut.

prism, Wollaston Wollaston prisms are usedto provide double images of single sources andsplit a beam of light into two naturally orthogo-nal linearly polarized beams.

programming The process of preparing aset of coded instructions that can be executedby a digital computer to yield the solution to aspecific problem or perform a specific function.

progressive wave A wave that transfers en-ergy from one location in space to another (incontrast to a standing wave). Typical progres-sive waves are waves traveling along a stretchedstring and compression and rarefaction wavestraveling along a tube. For example, a wave de-scribed by the shapey = f(x) at the positionx and at some time instantt = 0 traveling inthex direction with a constant velocityv is de-scribed by the equationy = f(x − vt) at anytime instantt. Also known as atraveling wave.

projection effect Due to the three-dimensional nature of objects, only such pointsof objects that lie in the focal plane are imagedas sharp point images in the screen plane. Otherpoints on the screen plane are depicted by smallluminous areas that are sections cut out of thisplane from the cone of image rays emanatingfrom the off-focal plane points.

propagation constant (1) A characteristic ofa transmission line that summarizes the effectson the wave being transmitted by the line. It isa complex number, the real part of which (theattenuation constant) measures the signal losswhile the complex part (the wavelength or phaseconstant) measures the shift in phase the waveundergoes.

(2) A complex quantity that measures the at-tenuation and phase change of a sinusoidal trav-eling wave along a transmission line. The realpart of the propagation constant is the attenua-tion constant in nepers per unit length; the imag-inary part is the phase change constant in radiansper unit length. The propagation constant,γ, ata specific frequency is given by

γ = logeI1/I2

whereI1 andI2 is the current at two differentpoints separated by unit length along the trans-mission line,I1 is closer to the signal source,andI1/I2 is the vector ratio of the currents. Thepropagation constant can also be written as

γ = α+ iβ

whereα is the attenuation constant,β is thephase change constant, andi =

√−1. The am-

plitude of the vibration of a wave,E, at anydistance,x, along the transmission line, is re-lated to the initial amplitude,E0 (i.e., atx = 0),of the wave by

E = E0e−γx .

The propagation constant is also known asprop-agation coefficient.

propagation constant, acoustic The angularwave numberk∗, which is a complex number ina porous material, multiplied by the imaginaryunit i =

√−1, ik∗ = γ = α + iβ. α, the real

part of the propagation constantγ, is called theattenuation constant,and it quantifies the ex-ponential amplitude decay of the acoustic wavein the direction of propagation. The imaginarypart ofγ is the phase constantβ. The complexwave numberk∗ is obtained from the solutionof the wave equation.

propagation loss The loss of energy froma beam of electromagnetic radiation due to ab-sorption, scattering and spreading of the beam.

propeller sound Noise generated by pro-pellers — devices equipped with rotating bladesmounted on a shaft — is of note as, for example,the major source of ship noise. Acoustic radia-tion is due to the complex dynamic interactionof propeller blades and nonuniform flow fieldsthat can, in extreme cases, lead to cavitation.The sound characteristics depend on the bladeform and operating conditions; noise control isan issue of continuing interest. Noncavitatingpropeller noise can be classified as (a) mechani-cal blade tonals that depend on angular velocityand blade number, (b) broadband noise causedby the interaction of propeller blades with tur-bulent structures and trailing edge vortices thatcause a vibratory response, and (c) propeller-singing occurring at matching vortex shedding


and blade resonant frequencies. Cavitation gen-erates broadband noise, caused by growing andcollapsing sheets of bubbles forming on the pro-peller blades.

prosthesis An artificial replacement for amissing body part. Prostheses include artificiallimbs, false teeth, hearing aids, artificial kid-neys, and implanted pacemakers. In the con-struction of artificial organs, the use of biomate-rials (materials that are biocompatible) are used.

Modern devices include structural improve-ments that allow them to be of lighter materials,more realistic appearance, and greater flexibil-ity. In some cases prostheses can permit thepatient to participate in sports activities.

protocol Two or more parties at the samelevel of communication are referred to aspeerentitiesand a protocol is a set of rules and for-mats that govern the communication betweenpeer entities.

proton pumps Proton pumps are ion pumpsthat promote active transport of H+ (protons).Because they involve active transport, theyspend energy in their function by hydrolizingATP. Proton pumps find their most use in ar-eas of the cell where metabolic activity is be-ing carried out, like intracellular organelles, en-dosomes, lysosomes, synaptosomes, chromaf-fin granules, golgi membranes, and endoplas-mic reticulum where production of H+ is thehighest.

Proton pumps are also of use in the pro-duction of ATP from glucose and other nutri-ents in the process calledcellular respiration.An important step during cellular respiration isthe electron transport chain (ETC), over whichATP is produced during the many steps in theprocess that involves oxidation of NADH andFADH2 and reductions of O2 into H2O. Thecytochromes, the typical protein carriers in theETC, can accept electrons, pump protons, andgenerate H2O from O2 and H+.

In plant cells, vacuoles contain proton pumpsin their membranes that, by transporting protonsto one side, create a gradient in potential that isused later to move sugars and other ions in andout of the vacuole.

pseudo sound Pressure fluctuations associ-ated with flow fluctuations that propagate at thespeed of the bulk flow (or a fraction of this speed)rather than at the speed of sound. Such fluctu-ations develop when, above a certain Reynoldsnumber characteristic for the particular flow ge-ometry, more or less steady vortices form atlower flow velocities. At higher flow veloci-ties turbulence sets in, and the pressure fluctua-tions associated with turbulent flow are irregu-lar. Small alternating pressures accompany therelatively large velocity amplitudes, and only afraction of the energy carried by the flow is emit-ted as sound (noise). Pseudo sound is generatedin heat exchangers in the flow over tube bun-dles, in blood vessels (blood flow), and fans (airflow).

p-type silicon Silicon that has been dopedwith an acceptor type impurity (boron (B), gal-lium (Ga), etc.) and in which holes, positivelycharged, are the majority carriers (hole concen-tration is much higher than electron concentra-tion).

pulse generator A circuit used to generatepulses. A pulse generator will often allow vari-ations in the repetition rate, amplitude, width,polarity, etc., of the pulses. They are commonlyused to produce inputs for digital circuitry.

pulse height discriminator A device thatpasses pulses higher than a predetermined pulselevel, but blocks pulses lower than that level.

pulse, longitudinal A pulse in a compres-sional zone (generated, for example, by giving apiston confined in a long tube a short, rapid in-ward stroke) traveling at a speedv, with particlesof matter displaced in the direction of propaga-tion (longitudinally). The fluid in contact withthe piston subjected to the pulse is compressed;its density and pressure rise above the undis-turbed values. The compressed fluid moves for-ward and compresses adjacent fluid layers, caus-ing the compression pulse to advance down thetube. For a longitudinal sound wave in a gas,

the velocity isv =√

γp0ρ0

, whereγ is the ratio

of specific heats for the gas, andp0 andρ0, theundisturbed pressure and density, respectively.


pulse operation A method of operation in acircuit in which the signals are passed in the formof discrete pulses rather than as analog signals.

pulse rate (Also known as for pulse-repetition frequency.) The rate at which pulsesare transmitted in a pulse train. It is the recip-rocal of the period and is measured in units ofHertz.

pulse shaper A circuit used to shape a pulseto a desired form, or to modify its characteristics.

pulse train A series of pulses with similarcharacteristics occurring at regular intervals.

pulse, transverse A disturbance (created, forexample, by applying a single sidewise move-ment to a stretched string) characterized by themotion of the particles of matter perpendicularto the direction of propagation of the disturbance(pulse). In a transverse pulse each particle re-mains at rest until the pulse reaches it, moves forthe duration of the pulse and then returns to itsinitial state. For the stretched string, the speedof propagation of the disturbance isv =

√F/µ,

whereF is the elasticity measured by the ten-sion in the string andµ the mass per unit lengthof the string.

puncture voltage The voltage gradient thatleads to breakdown or “puncture” across an elec-trical insulator. This leads to a sudden decreasein the resistance of the material to such an extentthat high-value currents may flow through localregions. In the case of solid insulation, punc-ture represents damage to the material, whereasin the case of liquid or gaseous insulation, the

interruption of the current flow and the removalof the offending overvoltage restores the mate-rial close to its original condition. The break-down of insulation is time dependent; the higherthe voltage above the critical value, the shorterthe time for breakdown.

pupil Pupils are derived from knowledge ofthe field stop in an optical system and are usedto determine the limiting cone of rays from theobject point to the conjugate image point. In theanalysis of optical systems, two types of pupilsare considered: the entrance pupil and the exitpupil.

1. entrance pupil:the image of the control-ling aperture stop formed by all the imaging el-ements preceding it.

2. exit pupil: the image of the controllingaperture stop formed by all imaging elementsfollowing it.

In visual optics, the pupil is the aperture in theiris, normally circular and contractile, throughwhich light enters the posterior portion of theeye.

pupil, entrance Seepupil.

pupil, exit Seepupil.

Purkinje effect Perceptual variation in rel-ative lightness of different colors as illumina-tion changes from daylight (photopic) to twi-light/night (mesopic/scotopic). That is, duringthe day when vision is photopic, objects close to550 nm wavelengths will tend to appear lighterthan objects of 500 nm. When vision becomesscotopic, the situation is reversed. Also knownasPurkinje Shift.


QQ-factor (1) Also known as the quality fac-tor. It is a figure of merit for an electrical cir-cuit (or any energy storage system), and is given

by Q = 2π average energy storedaverage energy dissipated per half cycle. For a

resonance system with high Q values, it is alsoequal tof0/∆f , wheref0 is the resonant fre-quency, and∆f = f2−f1 is the frequency band,defined as those frequencies that give more than50% of the total power delivered at resonance.

(2) Symbol: Q. Also known as the qualityfactor. A figure of merit for a tuned circuit. Itdetermines the rate of decay of stored energy.The decay time becomes longer with increasingvalue of the Q-factor. For a tuned circuit, it isgiven by

Q =2πf0LR

=1

2πf0CR

whereL = inductance,C = capacitance,R =resistance associated with a real inductance orcapacitance, andf0 = resonant frequency. Theresistance may also be included as part of thetuned circuit to reduce the Q-factor. It is alsogiven byQ = f0/∆f , where∆f is the band-width at the−3dB points of the voltage vs. fre-quency response function of a resonant circuit.

Q-meter Laboratory instrument that mea-sures the Q-factor of a circuit or circuit elementby determining the ratio of reactance to resis-tance.

quadrature State of being separated by 90,or one quarter cycle. Also known asphasequadrature.

quadripole The field that results from twomagnetic dipoles arranged as a unit. If the dipo-lar axes point in opposite directions, then, atlarge distances compared to the physical dimen-sions of the quadripole, the magnetic field pat-tern has a quadrupolar symmetry.

quadrupole, electric In its simplest form,an arrangement of two equal electric dipoles inopposite orientation.

Electric quadrupole configurations.

The electric field falls off as1/r4, wherer isthe distance from the center of mass.

quadrupole sound sources This refers tothe arrangement of the sound sources of whichthere are two basic types: lateral and linearquadrupole sources. In thelateral quadrupolearrangement,the monopole sources with alter-nating phases are at the corners of a square.Sound is radiated in a cloverleaf pattern, withstrong projection in front of each source andwith the sound being canceled at points equidis-tant from adjacent opposite monopoles. In alinear quadrupole arrangement,the two oppo-site phase dipoles lie along the same line, as ina tuning fork, for example. In this arrangement,in the near field there are four maxima and fourminima, while in the far field there are two max-ima and two minima.

quality, sound See alsotimbre. A string canbe plucked, bowed or struck, e.g., in a harp, vi-olin or piano, respectively. Quality depends onthe relative amplitude of various overtones tothe fundamental tone. Fundamentals are indis-tinguishable; overtones introduce the necessaryquality that is important for the ear. Notes of thesame pitch and loudness produced by differentinstruments can be distinguished by the quality.

quantization of charge Charges, whetherpositive or negative, come in certain specificamounts, namely,0,±q,±2q,±3q and so on.For example, the charge of one electron is equal


to−q, while the charge of one proton is equal to+q. Robert A. Millikan used an oil-drop appa-ratus to demonstrate the quantization of chargeand won a Nobel prize in 1923.

quarter-wave film Film whose thickness,multiplied by the index of refraction of the filmmaterial, equals one fourth (or an odd integermultiple of one fourth) of the wavelength of theradiation incident on the film. The significanceof quarter-wave film derives from the perfectconstructive or destructive interference that oc-curs between parts of a wavetrain reflected atfront and rear surfaces of the film (depending ondielectric constants of media on the two sides ofthe film), resulting in a maximum or a minimumin the reflectivity of the film.

quarter-wave line A section of transmis-sion line that is one quarter-wavelength longat the fundamental frequency being transmitted.When shorted at the far end, it has a high im-pedance at the fundamental frequency and forall odd harmonics, and a low impedance for alleven harmonics. It is often used as an imped-ance matching device between two transmissionlines with different impedances (Z1, Z2) suchthatZ2

0 = Z1Z2 whereZ0 is the impedance ofthe quarter-wave line.

quarter-wave plate Plate of birefringent ma-terial with ordinary and extraordinary indices

of refractionn1 andn2, cut with its surfacesparallel to the optic axis and with its thicknessd adjusted so that the difference(n1 − n2)dequals (exactly) one-quarter of the wavelengthof the radiation of interest. Passage of this radi-ation through quarter-wave plate, perpendicularto the plate surfaces, produces a difference be-tween the phases of the ordinary and extraordi-nary components of the radiation exactly equalto π/2, which serves to effect the conversion oflinearly polarized light into circularly polarizedlight, and vice versa.

quiescent component A device that haspower applied to it, but is receiving no inputsignal.

quiescent current A current in a circuit ordevice to which power has been applied, but noinput signal has been applied. For example, thecurrent in an AC amplifier circuit with no ACinput.

quincke tube A commonly used device fordemonstrating the interference of sound wavesand measuring their wavelength. Sound wavesfrom a high frequency source enter the apparatusat S and the energy divides betweenA andB.The two waves reunite atC and are picked upby detector atD.


Rradiance The radiant energy per second perunit solid angle passing through a unit area per-pendicular to the bisector of the solid angle con-taining the measured radiation. Measured inunits of watts/steradian/m2.

radiation (1) Propagating electromagneticenergy,interpreted classicallyto consist of os-cillating (time-varying) electric and magneticfields that propagate through space with a speeddependent on the properties of the matter thatoccupies the space, andinterpreted quantum-mechanicallyto consist of units of electromag-netic energy known as photons with energiesequal tohf , wheref is the frequency of theradiation andh is Planck’s constant. Classicalinterpretation of radiation applies where photonenergies are small compared to the energy sen-sitivities of the measuring equipment such thatthe observed effects involve large numbers ofphotons that act on the average to produce clas-sical fields. Types of electromagnetic radiation,classified by distinct ranges of frequencies, in-clude radio, microwave and infrared waves, vis-ible light, and ultraviolet, X- and gamma-rays.

(2) The term is alternatively used to refer toa stream of energetic nuclear particles or elec-trons.

radiation carcinogenesis By definition, acarcinogen is any agent that causes cancer inanimal and human tissue. Carcinogens may beinorganic (asbestos, arsenic) or organic (certainmolds, viruses). Other types include X-rays,UV, and gamma rays. Contamination may bethrough air (radon, smoke), skin absorbed (pes-ticides), or ingested (nitrites).

Radiation carcinogenesis is then the devel-opment (from the beginning) of malignant can-cer tumors exclusively from radiation exposure.Radiation carcinogenesis studies explore the re-lationship between radiation dose and responseas well as dose rate effects. The studies pro-vide assessment of dose response in individual

organs and usually extrapolate results obtainedfrom animal models to humans.Seeradiationeffect, cumulative.

radiation damping, acoustic Sound waveswill undergo an exponential decay if there is ab-sorbing material present in their path of prop-agation. The amplitude of the wave decreaseswith time due to the loss of mechanical energyto the medium or its surroundings. This effect ismore significant, for example, in a room wherestanding waves are set up and absorbing materialis present at the corners of the room. The acous-tic resistance and the coefficient of absorption ofthe room need to be taken into account.

radiation effect, cumulative Radiation ef-fects are counted above the tolerance dosageof approximately 5 Rems per year (whole bodydosage). The classification of doses range frommild and acute, to chronic, as a function ofdosage. Effects may occur as a result of cumu-lative small doses of radiation or exposure to ra-diation from accidents. Mild radiation sicknessis a common side-effect of radiation therapy forcancer.

Mild doses of radiation are 25 to 50 Remsin less than one week; these do not produce de-tectable clinical effects on the body. Doses of upto 150 Rems may show blood changes and mayprove to be a longtime hazard. A dose up to 250Rems is considered a moderate dose that willprovoke nausea and vomiting within 24 hours.Injury may vary from slight to serious and therecovery is subject to good health and no com-plications. A dose of 250 to 350 Rems bringsnausea and vomiting in under four hours withthe possibility of mortality in two to four weeks.Higher doses, up to 600 Rems, are considereda median lethal dose with 50% probability ofdeath or a very good chance of incapacitation.Doses greater than 600 Rems will provoke im-mediate nausea and vomiting with mortality inone to two weeks. The doses usually have theacute effects, the immediate effects within 48hours, and delayed effects of injury or incapac-ity in the range of one to five weeks. There arechronic effects that result in changes to the skinand vascular changes.Seeradiation exposure;radiation standards; radiation effect, genetic.


radiation effect, genetic Genetic effects ofradiation are readily understood in terms of theeffects that ionizing radiation has on living cells.Upon the irradiation of biological tissue, the ra-diation reacts with molecules and atoms causingthe release of free electrons. This release comesfrom the interaction of high energy photons withthe electron cloud of atoms and the transferal ofthe photon energy to the electrons, thus ionizingthe atom and setting the electron free. Free elec-trons in turn react with water dissociating it andcreating free radicals. Biological molecules, es-pecially DNA, are very susceptible to radicalsthat cause strand breaks in them, thus alteringthe genetic code of cells.

The use of ionizing radiation to create freeradicals is exploited in radiotherapy, wheretreatment and elimination of cancerous tumorsis achieved by destroying the genetic code ofmalignant cells, thus causing a disruption of cellactivity. The success of the therapy lies in theratio of tumor cells vs. healthy cells that are af-fected.Seeradiation exposure; radiotherapy.

radiation exposure Exposure to radiationemitting substances is becoming an increasinglycomplex problem due to increased exposure thatexists both from natural as well as industrialsources.

Naturally occurring radiation sources comefrom the radioactive decay of radium and radongas, found in some groundwater sources. Expo-sure dangers from radon, however, are mostlyfrom breathing the gas after it is released into theatmosphere, rather than from drinking. Othersources are from thorium and uranium. Thereare also some biologically internal sources of ra-diation like potassium40 and carbon14 that occurnaturally in living cells. Other types of radiationcome from cosmic rays that have outer space ori-gin.

The total annual dose received by a personfrom these naturally occurring radiation types atsea level is approximately 0.91 mSv. Twice thisamount may be received by a person who livesat higher elevations where cosmic rays are moreintense, or by people who live in a geographicplace with soil with a high radium content. Inthis last case, radon from the decay of radiummay accumulate indoors, and, if the house isnot well-ventilated, the person may be exposed

to doses as high as 100 mSv per year in theirlungs.

Artificial radiation may come from nuclearpower plants, uranium mining, and medical re-search, and may contribute substantially to wa-ter contamination. Nuclear weapons testing hasbeen responsible for strontium90 and tritiumcontaminants in water. It has been estimatedthat the average annual radiation dose receivedfrom medical and dental irradiation in developedcountries approaches in magnitude the dose re-ceived from natural background radiation.

Other artificial sources of radiation includeradiation from radioactive minerals in crushedrocks, phosphate fertilizers, radiation frombuilding materials, radiation-emitting compo-nents of television sets, and smoke detec-tors, among others. Total exposure of radia-tion from artificial sources amounts to approx-imately twice that of radiation from naturalsources alone.Seeradiation standards.

radiation impedance, acoustic Fundamen-tal characteristic of medium in which sound istransmitted. It refers to the opposition of thetransmission of sound in a particular medium.It is generally numerically given byρc (g/cm2

sec), whereρ is the density of the medium andc is the velocity of sound in that medium whichis determined solely by the physical propertiesof the medium.

radiation, isotropic Diffuse radiation suchthat the radiant energy is propagating in manydifferent directions through a given small vol-ume of space. The diffuse radiation has the sameintensity in all directions.

radiation of sound The propagation of soundwaves from a source outward in a medium.There can be plane wave propagation or spheri-cal wavefronts. Point sources produce sphericalwavefronts.

radiation pressure The pressure exerted byradiation on a given surface as a result of the rateof transfer of (radiant) linear momentum acrossthe surface per unit area (producing a force perunit area on the surface). Electromagnetic radi-ation pressure can be interpreted to derive fromthe force of the magnetic field of the radiation in


the direction of propagation. Pressure producedby the absorption (reflection) of electromagneticradiation by a surface equals two times the timeaverage of thePoynting vectorof the radiationdivided byc.

radiation pressure, acoustic Travelingsound waves, on striking a surface, become ab-sorbed or reflected. In doing so, they exert aforce on the surface on which they fall. Thiscan be used as a mechanism for measuring soundenergy.Seeradiometers; rayleigh disk.

radiation protection Protection against ra-diation has become increasingly important asmore and more techniques and applicationsmake use of radioactive materials in applicationsthat involve materials quality testing to medicaldiagnostics and treatment of disease.

The most common type of radiation that peo-ple are exposed to is ultraviolet radiation fromthe sun. An increase of skin cancer, cataracts,and other effects from UV exposure have madeUV exposure an important health issue. In ef-forts to provide meaningful ways of protection,the UV Indexhas been established. The indexis meant to associate levels of exposure to risksof disease from radiation damage to tissue. TheUV Index ranges between 0 and 10+, and is inproportion to the amount of UV radiation thatreaches the earth’s surface over a one-hour pe-riod at noon. Other useful indices used by suntanlotion and sunscreen companies is the sun pro-tective factor (SPF). SPF numbers in these prod-ucts indicate the amount of protection againstthe different types of UV.

Radiation protection against radioactive ma-terials handling and managing is given by lim-iting personnel exposure to radiation from thesubstances. Storage is usually provided in leadsealed cabinets or cannisters. X-ray rooms areusually shielded as well as other facilities thatexpose doses of radiation to detectors. Pocketdosimeters provide a way of measuring the totalaccumulated dosage received, thereby monitor-ing radiation safety limits. Seesun exposureand skin cancer, sunburn, radiation standards,radioactive waste, radioisotope storage.

radiation quality Seeradiation standards;radioactive waste; radiation protection; radia-tion exposure.

radiation, resonance Electromagnetic radi-ation with a frequency matched to a transitionfrequency of the atoms or molecules of a givenmaterial. Alternatively refers to the radiationemitted by a gaseous material when the atomsor molecules of the material are excited by inci-dent radiation of the same or higher energy.

radiation, selected Electromagnetic radia-tion of a particular frequency. Generally refersto radiation that is selected by reflection from agrating in a particular direction, or by transmis-sion through an etalon via constructive interfer-ence at a selected wavelength.

radiation standards Patients being treatedfor cancer are usually administered a dose of 50Sv or more in daily exposures in periods that canlast from four to six weeks. Protection for thenormal tissue of the patient as well as protectionto the medical personnel against excessive oc-cupational exposure to stray radiation are takenin order to prevent or diminish damage and con-tamination. Comparable safeguards are utilizedto minimize the exposure of workers employedin other activities involving radiation or radioac-tive material. Similarly, standards of safety havebeen developed for the handling and disposal ofradioactive waste.

In the U.S., the FDA has regulatory powerover radiation and radioactive materials man-agement. This includes radiation control overconsumer products that may be able to radi-ate excess radiation in, for example, X-ray ma-chines, televisions, microwave ovens, etc.

For personnel who work in an environmentthat deals with radioactive materials usage andhandling (with radiation doses that exceed 20mR/hr at one meter), pocket dosimeters give aninstantaneous measure of the total accumulateddosage. These dosimeters are able to detectX-rays as well as gamma radiation.

There are standards on the permissibledosage to which people may be exposed. Occu-pational exposures should not exceed 500 mSvper year, compared with public exposures of lessthan 5 mSv per year.Seeradioactive waste.


radio (1) An apparatus for receiving, broad-casting, or transmitting radio signals withoutconnecting wires or waveguides.

(2) The process of transmission and recep-tion of information by electromagnetic wavesof radio-wave frequencies.

radioactive waste Radioactive waste is pro-duced in increasing amounts as byproducts ofnuclear weaponry, nuclear-power generation,and research. Because much of this waste re-mains radioactive for long periods of time, it isparticularly hazardous and difficult to store.

Fuel from nuclear power generators is con-sidered waste once it cannot be reprocessed any-more. At this point the waste is usually pre-pared for storage. In the U.S., the Departmentof Energy has the responsibility of receiving thespent fuel from utilities. The process starts bykeeping the waste in temporary storage that thenare packaged in corrosion-resistant canisters ofsteel. Some other types of radioactive wastecoming from fission in solution are completelyevaporated, leaving the waste in solid form thatis subsequently heated until all the constituentnitrate salts are converted to oxides. The ox-ides are then put into a glass-forming oven andmixed with materials that form a borosilicateglass. Lastly, the glass melt is poured into steelcanisters. After the waste has been prepared incanisters, the planned method for ultimate dis-posal is calledgeologic disposal,which meansthat the waste is deposited in underground minedtunnels.

Radioactive waste disposal is a world-wideproblem that usually demands large amountsof money and resources to keep from doingharm. The lack of appropriate disposal has ledsome countries to dump radioactive pollutantsin the oceans. There is evidence that links vari-ous radioactive pollutants to human health prob-lems such as cancer, birth defects, and geneticchanges.Seeradiation exposure.

radio channel A radio channel refers to afrequency band in the electromagnetic spectrumdedicated for radio communication. The band-width of any radio channel typically depends onthe application.

Radio signals in the medium radio frequencyrange of 525 to 1700 kHz are coded in amplitude

modulation (AM). Medium frequency AM radiowaves may propagate between the transmitterand the receiver as a ground wave, a space wave,and/or a sky wave.Ground wavesembody sur-face waves traveling along the earth-atmosphereboundary. Ground waves suffer signal attenua-tion as the wave penetrates the ground or watersurface, losing an amount of energy dependentin part on the electromagnetic wave frequencyas well as the conductivity and the permittivityof the ground/water surface.Space wavesrep-resent that part of the transmitted electromag-netic wave traveling through the atmosphere.Space waves are often blocked or impeded fromreaching the receiver due to geographic topol-ogy or the earth’s curvature.Sky wavesrepre-sent reflections of upward-bound space wavesoff the ionosphere. Sky waves seldom occurduring the day, when solar radiation ionizesthe ionosphere’sD layer (about 40 miles fromthe earth’s surface) which thus absorbs or scat-ters the upward-bound sky waves. During thenight, when theD layer becomes less ionized,the upward-bound sky waves reach and reflectground-ward at theE layer (60 miles aboveground level) andF layer (130 miles above theearth’s surface). Medium-frequency AM radiocommunication depends primarily on groundwaves and sky waves; space waves, due to shad-owing, are relatively irrelevant.

Radio signals in the very-high frequencyrange of 88 to 108 MHz are coded in frequencymodulation (FM). Very-high frequency radiocommunications depend primarily on spacewaves and relatively little on ground waves orsky waves. Due to their very long wavelength,very-high frequency electromagnetic waves cantypically diffract round or reflect off geologi-cal and human-made structures that would haveblocked a medium-frequency wave. Such re-fractions and reflections result in multiple time-delayed propagation paths between the transmit-ter and the receiver. The signal power attenua-tion and multipath profile resulting from refrac-tions and reflection may in principle be calcu-lated using physics principles, but may vary ina very complicated manner with time as the re-ceiver (such as a radio set installed in an auto-mobile) travels amidst a complex surroundingof reflectors and refractors. Global broadcast-ing in four shortwave sub-bands lying between


5950 kHz and 26.1 MHz, and FM broadcastingin the very-high frequencies between 88 and 108MHz.

The radio channel may be further corruptedby thermal noises of the electronic hardware,co-channel and adjacent channel interference,multipath propagation effects, non-stationaryreflectors and refractors near the transmittersand/or the receivers, obstructions by human-made structures, or by the geographical topol-ogy. These physical effects translate into ad-ditive stationary and non-stationary impulsenoises, time-varying multiplicative distortion tosignal amplitude, spectral distortion and inter-symbol interference, depolarization, and pro-longed deep attenuation of signal power. Thesechannel non-idealities may be mitigated by vari-ous diversity techniques at the transmitter and/orthe receiver — spatial diversity by deployingmultiple displaced antennas, polarization diver-sity by deploying antennas of different polar-izations, time diversity by interleaving the in-formation symbols, or frequency diversity bymodulating the transmitted signal with differentor time-varying carrier frequencies. Sophisti-cated high-speed signal processing techniquesare also essential and commonly used to miti-gate channel non-idealities.

radio communication Radio communica-tion represents communications between two ormore geographical points using as the transmis-sion medium unguided electromagnetic wavesin the radio frequencies. Radio communicationin the frequency range of 525 to 1700 kHz usesamplitude modulation (AM), with 10 kHz al-lowed for each AM channel in the Americas and9 kHz for much of the rest of the world.

Radio communication in the short-wave fre-quency range between 5950 kHz to 26.1 MHzare primarily for long-distance internationalbroadcasting. Shortwave radio channels haveonly a 5 kHz bandwidth, sufficient for speechcommunication but not for music transmission.

Radio communication in the very-high fre-quency range of 88 to 108 MHz uses frequencymodulation (FM), with a 200 kHz allowed foreach FM channel in the United States. FM chan-nels typically transmit in stereo, with a left sub-band and a right sub-band on either side of thecarrier frequency for each stereo channel.

radio, for navigation Radio waves propagateat 300,000 km/s and thus permit measurementsof distance as a function of time and directionas a function of differential distance to two ormore known points. In free space, radio naviga-tion is capable of considerable accuracy. Alongthe surface of the earth, however, the effect ofmultiple propagation paths between a transmit-ter and receiver reduces the accuracy. Since air-craft and ships may move over large areas, sys-tems that involve cooperation between a vehicleand a ground station required a high degree ofinternational standardization.

radiography The process of producing ra-diographs using X-rays as a probe. A radio-graph is in simple terms a photograph usingX-rays. Radiography has found wide accep-tance and use in diverse areas like medicine, bi-ology, civil engineering, the aerospace industry,and environmental protection.

The main difference between radiographyand other branches of nuclear medicine is that,in radiography, the patient is subjected to anexternal source of X-rays. Analysis is basedon scattered and transmitted radiation. In otherbranches, the patient is usually the source of theradiation. Also calledskiagraphy. Seeradiol-ogy.

radiography, neutron Non-destructiveimaging technique that involves the detectionof the attenuation of a neutron beam by the ob-ject being radiographed. The technique givesinformation about internal structure of materi-als because not all parts of the object providethe same attenuation, thus giving an image ofthe internal attenuation regions.

Neutron radiography is similar to X-ray ra-diography, but complementary in terms of theinformation obtained through it. While X-raygives information about the electron cloud sur-rounding the nucleus of an atom, neutrons giveinformation about the nucleus itself. Neutronsare either scattered or absorbed by the atomic nu-clei. There are two typical detection methods.In the indirect methodof obtaining an image,a screen is put opposite to the neutron beam onthe other side of the sample. After neutron bom-bardment, the screen becomes radioactive and,after getting it into contact with an X-ray film,


a radiograph is obtained where the darkening ofthe X-ray film corresponds to the highest radi-ation decay regions of the screen. In thedirectmethod,the X-ray film and a conversion screenare both put in the neutron beam, along with thesample.

An example of a neutron source isCalifornium252, in which one gram emits2.3×1012 neutrons per second.

radioimmunoassay Radioimmunoassay(RIA) is a technique in biochemistry andmedicine to measure extremely small amountsof substances (e.g., antigens, hormones, en-zymes, steroids, vitamins, immunoglobulins,drugs) in body fluids. In general terms, thetechnique consists of injecting a subject with asubstance or antigen that will cause the bodyto produce antibodies. Later, serum from thesubject is extracted and the present antibodiesare obtained and treated with a radioactiveantigen and later with a non-radioactive antigen.

The technique provides then a method of de-termining the amount of the substance presentin the body. Radioimmunoassay is used by hos-pitals to help diagnose diabetes, thyroid glanddisorders, and other diseases. It is also used tomeasure plasma renin activity.

radioiodination A technique in which bio-logical molecules (peptides, proteins) are radio-labelled with isotopes of iodine — radioiodineslike I125 or I131 — in tracer studies. The successof the technique lies in the fact that radioiodineis very well suited for radiolabelling hydrophilicand lipophilic compounds widely used in biol-ogy and medicine. The versatility of these ra-dioisotopes has made possible the present statusof diagnostic nuclear medicine.

In this technique, it is critical that after amolecule is radioiodinated, any remaining ra-dioiodine is removed from the sample to pre-vent radioactive detection other than the desiredmolecules. Radioiodine is by itself widely usedas a tracer in the diagnosis and treatment of thethyroid gland. Seeradioisotope tracers; radio-labelling; radiology.

radioisotope storage Storage place to putshort-lived or other radionuclides used in radi-ological studies. Depending on the usage, ra-

dioisotope storage should be accessible but atthe same time provide proper shielding and se-curity to prevent accidents and mishandling.

Types of radioisotope storage range fromsimple multi-drawer shielded modules to stor-age safes. Usually, shielding is provided by0.5to 1 inch of lead on all sides, depending on thetypes of radionuclides used.Seeradionuclides.

radioisotope tracers A radioactive sub-stance that, when introduced into a biologicalsystem, can be followed in time and space. Usu-ally, a biological process or structure is beingexamined and the tracer provides informationabout the time development or distribution ofthe tracer that may be assembled to give an un-derstanding of the process or a diagnosis of adisease. Tracers are located in the body by de-tecting their radioactive decay radiation, includ-ing the emitting of alpha or beta particles, orgamma rays.

Typical tracers in medical applications thatinvolve diagnosis and treatment include isotopesof iodine, technetium, astatine, and scandium.Seeradionuclides; radiolabelling; radiology.

radiolabelling A technique in which a ra-dioactive tracer is tagged to another molecule(e.g., hormone, enzyme, protein) in order to findor follow its location in the body. The levelof radioactivity is a direct measurement of theamount and number of the molecules that arelabelled. The selectivity with which the taggingmolecule has to work requires specific studies inorganic-inorganic chemical synthesis, biochem-istry, and receptor binding studies.

The technique is used for the imaging of tu-mors in the diagnosis of cancer with gamma-emitting radionuclides that are attached tobiomolecules that preferentially target tumorcells. The same technique is also applied in can-cer therapy. It is also used in the assessment ofhow the body metabolizes drugs in radiophar-maceutical studies.

radiology The branch of medicine that dealswith the use of radioactive substances and radi-ation which, based on their ionizing radiation,are used to produce images and give treatmentto disease. The most common example is the


use of X-rays to produce non-invasive imagesof bones and dense tissue in the body.

Examples of sub-branches are radiologicanatomy, medical imaging, magnetic resonanceimaging, computed tomography, cardiovascu-lar radiology, mammography, musculoskeletalradiology, positron emission tomography, fluo-roscopy, ultrasound, radiation medicine, nuclearmedicine, neuroradiology, quantitative radiol-ogy, gastrointestinal radiology, and genitouri-nary radiology.

radiometers Mechanical devices for measur-ing sound energy that are large in comparison totheir associated wavelength. If sound waves hita disk or plate hung on a torsion balance at nor-mal incidence, a pressure is exerted on the disk.Some of the energy is reflected and the acousticpressure causes the disk to rotate. The amountof acoustic energy is indicated by the amount oftwisting or motion.See alsorayleigh disk.

radiometry (1) The science of measuring op-tical radiation at any wavelength. All fundamen-tal radiometric measurements measure opticalenergy. Since optical energy induces heat intoan absorber, it follows that a thermal-sensitivedetector, calorimeter, can be used to measureoptical energy.

(2) Also refers to an instrument for measuringthe intensity or force of radiation.

radio, mobile A small mobile radio terminalused for short-distance radio links.

radionuclide generators Apparatus for theproduction of radionuclides. Radiation therapy,radionuclides for diagnostic and investigativemedicine, and positron emission tomographystudies have made radionuclide generators anessential component of hospitals. Their impor-tance lies in providing radionuclides that cannotbe transported because of their intrinsic shorthalf-lives.

Radionuclide generators can be of the typeof fission reactors that produce a broad range ofradionuclides or in-hospital medical cyclotronreactors. Seetomography, positron emission,ventriculography, radionuclide, radionuclides,radiotherapy.

radionuclides Radionuclides are isotopesof atoms that decay producing radioactivity.They emit various ionizing radiations, for exam-ple, electrons, positrons, alpha particles, gammarays, and X-rays. The particular emissions de-pend on the radionuclide.

Radionuclides are of importance in medi-cal applications where diagnosis and treatmentof disease requires the intervention of tracersand/or ionizing particles and radiation. Thereare three predominant uses for radionuclides inmedicine: in radiographic imaging techniques,in studies of metabolism, and in radiation ther-apy.

In imaging techniques,the purpose is to visu-alize the distribution of an injected radionuclidewithin a given organ as a means of studying theanatomic structure and pathological conditionsof the organ. Inassessments of metabolism,thegoal is to quantitate the absorption and retentionof radionuclides in organs, as a function of timeand other medical variables.

Examples of radionuclides include: Car-bon11, Nitrogen13, Oxygen15, and Fluorine18,which are important in PET studies. Othergamma emitting radionuclides are Technetium99

and Iodine131, used in imaging and metabolicstudies, respectively.Seeradionuclide genera-tors; tomography; positron emission; radiother-apy.

radio sonde A miniature radio transmit-ter that broadcasts meteorological information,such as pressure and temperature, and other sci-entific data from various levels of the atmo-sphere to ground. A radio sonde is usually car-ried by a balloon or kite.

radiotherapy Radiotherapy refers to thetreatment and management of malignant diseasewith radiation. The treatment usually involvesirradiation with X-rays or other ionizing radi-ation, and by ingestion of radioisotopes. Theionizing radiation results in the production offree electrons that react with water in the tis-sue to form reactive free radicals. The radi-cals then interact with the surrounding biologi-cal molecules, especially DNA, causing strandbreaks. Radiotherapy is a local treatment andmay be used when cancerous cells are containedin a few areas of lymph nodes.


Radiotherapy is more effective and safest ifthe radiation is given in multiple small doses(fractions) over a long period of time. In gen-eral, the reason is that small doses separated intime allow normal tissue to repair the damagedone to healthy tissue, and this repair is usu-ally faster than the repair to tumors. This tech-nique produces an overall reduced toxicity tonormal tissue. Also, the tumor will have bet-ter oxygenation, allowing for tumor shrinkageas the treatment progresses (poorly oxygenatedtumor is less sensitive to irradiation). Regard-ing the well-being of the patient, there is bettertreatment tolerance. Another advantage in theplanning of the treatment is that the amount ofradiation can be reduced as the size of the tumorshrinks. This usually leads to the delivery of ra-diation in multiple fractions over a time periodthat typically spans one to two months.

RAM (random access memory) Memoryin a computer used to read and write data. Thelocations may be accessed in any order.

Raman effect Seescattering, Raman.

Raman scattering, coherent anti-Stokes(CARS) In CARS, a sample is subjected totwo collinear strong laser beams at different fre-quencies,ν1 andν2. If the frequency differenceν1 − ν2 (assumingν1 > ν2) coincides with thefrequency of a Raman-active rotational or vibra-tional modenuR, then there is enhancement ofthe signal atνA = 2ν1 − ν2 (anti-Stokes) andνS = 2ν2 − ν1 (Stokes). In Stokes, a phononis produced from the scattering while in anti-Stokes a phonon is absorbed.

Either νA or νS can be used for analysis,but the anti-Stokes signal offers the advantagethat its frequency is well aboveν1 and can bereadily separated from the incident beams andspontaneous fluorescence signals by optical fil-tering. In CARS, intensities of up to 104 to105 can be achieved over those of conventionalRaman spectroscopy, which allows for fasterdata acquisition. Other advantages over con-ventional Raman spectroscopy include the factthat smaller samples can be used, better spatialdiscrimination can be obtained, and different re-gions within a sample can be examined. The

last two reasons are of special interest to CARScompositional studies of flames and plasmas.

Raman scattering, electrophoretic The useof Raman scattering techniques can be com-bined with electrophoretic separation in orderto study effects of an electric field on the vibra-tional modes of the molecules in solution. Also,any property that can be related to the vibra-tional modes can be analyzed in dependence ofthe applied electric field.

Electrophoresis by itself is a technique basedon the mobility of particles suspended in anelectrolytic solution. Because different parti-cles (e.g., proteins) move in the same electricfield at different speeds, the difference in speedcan be utilized to separate the contents of a sus-pension. When used in conjunction with Ramanscattering techniques, conformational changesof polymers in solution (for example) can bedetermined as a function of the applied electricfield. Raman spectroscopy of vibrational spec-tra from the polymers is very sensitive to slightchanges in the local structure (changes in bondlengths and bond angles). Shifts and changes tothe Raman spectra are then reflections of con-formational changes.

Modern electrophoretic techniques includecapillary electrophoresis in which the solute iscontained inside a capillary and subjected to anelectric field parallel to the length of the cap-illary. Its advantages over normal techniquesare due to its heat dissipation characteristics andsensitivity.

random code A random, or pseudo-random,code is used as the signature spreading sequenceto modulate the data signal of a particular userin code division multiple access (CDMA). Thisrandom code is used to modulate (or spread) thatparticular user’s message bits via either an am-plitude modulation scheme (i.e., direct sequenceCDMA) or a frequency modulation scheme (i.e.,frequency hopping CDMA). The same noisespreading code is used at the receiver to correlatethe received signal so as to recover the originaldata signal. Different users in a CDMA sys-tem are typically assigned orthogonal or near-orthogonal signature spreading code sequence,such that the transmitted digits from different


users have no or little interference against eachother at the receiver after de-spreading.

rating The specified limit to operating con-ditions of an electrical or electronic component,e.g., the power rating of a transistor, resistor, etc.

ratio detector An electric circuit that has twoinput terminals and one output terminal. Theoutput signal is only determined by the ratio ofthe two input signals.

ray, bound In an optical fiber, bound raysare those that are confined to the fiber core.

ray, extraordinary In double refraction (seeray, ordinary), the extraordinary ray does notobey Snell’s Law on refraction at the crystal sur-face. The extraordinary ray can be describedin terms of components polarized in directionsboth perpendicular and parallel to the optic axis,propagating with different velocities. The emer-gent extraordinary ray is polarized in a directionperpendicular to the polarization of the ordinaryray.

rayleigh disk (1) A thin disk suspended bya wire into a fluid, either liquid or gas. If thedisk makes an angleθ with the direction of fluidflow, the hydrodynamic forces on the disk tendto align it perpendicular to the fluid flow (i.e.,θ → 90). This occurs regardless of the ab-solute value of the direction of flow (e.g., left-ward or rightward), so it is particularly suitedto measuring the presence of small sound fieldspresent in the fluid. In superfluid helium, it isalso possible to create thermal waves, denotedas second sound. Early experiments in super-fluid helium used Rayleigh disks to sense sec-ond sound waves and test the two-fluid model.See alsohelium-4, superfluid.

(2) These are small paper disks, whose sizesare much smaller compared to the wavelengthof the sound under investigation. They are sus-pended on a fine fiber at 45 and can be used toshow streamline flow. Instruments based on thisdesign are also used for comparing intensities ofsounds of definite pitch.

rayleigh’s criterion An arbitrary, but use-ful criterion which relates the lens diameter,D,

wavelength,λ, and the limit of the resolution,∆θ, for just resolvable images. This requiresthat the centers of the two image patterns be nocloser than the angular radius of the Airy disk;i.e., the principal diffraction maximum of oneimage falls directly over the first minimum ofthe other.

∆θ =1.22λD

.

rayleigh waves Acoustic waves associatedwith earthquakes that travel in a thin layer closeto the surface of the earth along a great circlefrom the epicenter. The wavelength is a fractionof the size of the plate the wave travels on. Suchwaves can also be used for exploration of defectsin material.

ray of sound The concept is used in geomet-ric acoustics in which a sound emanating from asource is assumed to travel in rays of unchangingfrequency.

ray, ordinary When light is propagatingthrough a crystal whose optical axis is at anarbitrary angle with respect to the beam direc-tion, the light would experience double refrac-tion — two refracted beams calledordinaryandextraordinary rays emerge. The ordinary rayobeys Snell’s Law and would emerge linearly,polarized perpendicular to the optic axis.

ray, principal A ray that passes through thelens/optical system undeviated is called theprin-cipal ray (also called thechief or undeviatedray).

rays, pencil of Usually, the section of a ray-bundle made by a plane containing the chief ray(any ray from an off-axis object point that passesthrough the center of the aperture stop). Also, abundle of rays diverging from or converging toa single point.

reactance The imaginary part of the imped-ance associated with energy storage. Differsfrom resistance, which is the real part of theimpedance and is associated with energy dis-sipation. The unit of reactance is the ohm. Fora pure inductance or capacitance, it is given by


2πfL and1/(2πfC), respectively, wheref isthe alternating current frequency.

reactive current In the phasor representa-tion of an AC current, the component of the ACcurrent perpendicular to the AC voltage is calledthereactive current.

reactive factor The ratio of the reactivepower of a circuit (i.e., the product of reac-tive voltage and amperes) to the apparent power(equal to the product of the root-mean-squarecurrent and voltage).

reactive voltage That component of the pha-sor representing the voltage of an AC circuit thatis in quadrature (i.e., 90) with the current.

reading glass A large-aperture, simple bi-convex lens held such that the object to beviewed remains between the focal point of thelens and the lens itself, producing a magnifiedvirtual image.

read only A signal is stored in an electriccircuit. This signal can only be used as an inputsignal to outside electric circuit. Any change inoutside circuit cannot alter this signal.

receiver, radio A radio receiver captures am-plitude modulated or frequency modulated radiowaves and converts them to signals that drive anoutput transducer such as a loudspeaker. A sim-ple radio receiver consists of six stages. Thefirststageis a radio-frequency section that providesa coupling between the antenna and the radio re-ceiver. It also provides any pre-selection or am-plification before the frequency of the incomingsignal is changed. Thesecond stageis a mixerand local oscillator section that converts the in-coming signal to a predetermined fixed inter-mediate frequency, which is usually lower thanthe signal frequency. Thenext stageis an in-termediate frequency amplifier section, whichprovides most of the radio receiver’s amplifica-tion and selectivity. Themain stageis a seconddetector section that either detects amplitude-modulated signals or frequency-modulated sig-nals. Thenext stageis a modulation frequencysection consisting of either an audio or video am-plifier that provides the additional amplification

to drive an output transducer. Thelast stageisan output transducer, such as a loudspeaker usedto provide an audio signal.

reception, diversity A communication sys-tem that has two or more paths (referred to aschannels). An example of space-diversity re-ception is the use of two antennas at differentheights to provide a means of compensating forchanges in electrical-path differences betweendirect and reflected rays. The outputs of the twochannels are combined to give a single receivedsignal and thus reduce the effects of fading. Fad-ing is the variation in signal strength at a receiverdue to variations in the transmission medium.

reception of sound For acoustic waves tobe detected, a receiver must either partake orotherwise influence the motion of the particlesof the medium or must respond in some wayto the pressure variations on its surface. Thisforms the basis of sound detectors such as theear, where the pressure variations are felt on theear drum that vibrates with the same frequencyas the impinging sound.

reciprocal network A circuit whose outputis reversibly proportional to its input.

reciprocity Deals with the reciprocal rela-tion between transmitters and receivers of soundwaves. If there are any obstacles between thepoint of origin and reception of sound, the soundemanating at the origin is perceived with thesame intensity at the point of reception as ifan equal sound had originated at the point ofreception and recorded at the original point ofemanation.

recording of sound Mechanical vibrationsin a medium need to be converted to electricalsignals for recording. Several different mediafor recording exists such as records, tapes andcompact disks, as well as film.

recording, quadraphonic These sound sys-tems possess a frequency range that includesall the audible components of the sounds be-ing reproduced. It is necessary that the intensityrange associated with the recording sounds bedistortion-free. The spatial sound pattern and


the reverberation characteristics of the originalsound should also be preserved.

rectification (cell) The rectification behaviorof the cell membrane is exhibited more readilyin the behavior of the current as a function of themembrane potential. Because the current acrossthe membrane is a direct measure of the ion fluxthrough it, then, as a function of the membranevoltage, the permeability of the membrane todifferent ions will show a nonlinear behavior.

The ion channels conduct ions more readilyin one direction than in another when the direc-tion of the driving force is reversed. This is thebehavior of an electrical rectifier. The rectifyingcharacteristics of the membrane are usually de-picted by the plot of the current vs. the voltagefor specific channels.

In the figure below (a), the I-V linear curvemakes the channel an ohmic channel, while in(b), the channel is a rectifying channel. Theunits are usually measured in mV for the voltageand pA for the current.

rectifier An electric circuit that changes analternative input signal into a unipolar outputsignal. Most of the rectifier makes of the diodes.The figure shows a block diagram of a rectifier.

rectifier, bridge This is a specific rectifier.The figure shows this rectifier made of four diodes.

The input signal is alternating and the output sig-nal is only one direction.

Rectifier, bridge.

rectifier, electrolytic A rectifier that useselectrolytic mechanisms to change an alterna-tive electric signal to a unipolar electric signal.When the alternate signal is applied to the elec-trolyte, the molecules in this electrolyte becomepositive and negative ions. These ions producea unipolar electric signal. Usually this signal isa current or voltage.

rectifier, full-wave A rectifier that has aunipolar output signal during both halves of theinput sinusoid.

rectifier, half-wave A rectifier that only hasa unipolar output signal during one half of theinput sinusoid.

Rectifier, half wave.

rectifier, mercury vapor A rectifier in whichthe mercury vapor is filled in a tube.

rectifier, metal A rectifier whose anode andcathode terminals are enclosed in a metal cham-ber.

rectifier, selenium A rectifier in which a se-lenium layer is deposited on an aluminum plate.Electron flow from a selenium to an aluminumplate is easier than from the opposite direction.

rectilinear propagation In geometric optics,the propagation of light is described in termsof rays. This is stated in terms of thelaw ofrectilinear propagation of light;namely, lighttravels in straight lines. That mode of propaga-tion where the light path can be described by an


V inV out

V in V out

V in V out

infinitely thin pencil, the light ray, which propa-gates perpendicular to the wave front. This limitis commonly known as thelimit of geometricalopticsand can be obtained by setting the wave-lengthλ → 0. Any deviations from this limitare proportional toλ, leading to the existenceof diffraction effects for radiation of sufficientlylong wavelength.

red light, healing effect Dating back to NielsRyberg Finsen in the last part of the 19th century(founder of modern phototherapy: treatment ofdisease by the influence of light), light has beenused to relieve ailments. As part of his discover-ies, he found that lengthy exposure to red lightby smallpox victims prevents the suppurationof the pustules. Separate studies make use ofinfrared and ultraviolet light in heat lamps totreat neuritis and arthritis conditions to relieveinflammation.

On UV irradiation treatments, the heating oftissues by IR after the UV tends to suppressthe observable manifestation of dilation of theminute blood vessels in the dermis, or erythema(reddening of the skin). If the IR heating occursbefore the UV, there is an increases in the degreeof erythema.

Biostimulation of biological tissue has beenobserved during photo-stimulation. Wave-lengths of about 660 nm seem to cause an in-crease in the speed of tissue repair. Different re-sults may be obtained depending upon whetherthe light source is continuous or pulsating.Seelight, monochromatic, biological action.

red light stimulation and bacteria growthWhile the part of the spectrum from 600 nmto 700 nm is important for chlorophyll synthe-sis and photosynthesis, bacterial photosynthesistakes place close to 900 nm, deeper into the red.

For bacterial photosynthesis, there is a re-action center with the pigment bacteriochloro-phyll, which absorbs light of longer wave-lengths. These bacteria require some electrondonor other than water, and they do not releaseoxygen. The green bacteria (Chlorobiaceae)and purple sulfur bacteria (Chromatiaceae) useelemental sulfur, sulfide, thiosulfate, or hydro-gen gas as the electron donor, whereas the purplenonsulfur bacteria use electrons from hydrogenor organic substrates. All these bacteria require

exposure to red light as well as anaerobic con-ditions for photosynthetic activity.

reed A musical box composed of a thin rodclamped at one end, and excited by air pres-sure from the mouth being blown in through themouth piece, which in turn makes the free endvibrate.

reflectance (Also known asreflectivity). Theratio of the reflected to the incident power (flux).Depending on the nature of the incident radia-tion, one talks of specular, diffuse or total re-flectance. A large value of reflectance can causeserious loss of light in a multi-component opticalsystem. To reduce the losses due to reflectance,the optical surfaces are coated with film of atransparent substance with a thickness equal toone-quarter wavelength of light in the film.

reflection, acoustic This occurs when a pro-gressive plane wave in one fluid medium im-pinges upon the boundary of a second mediumcausing the acoustic disturbance to bounce backinto the first medium.

reflection coefficient Ratio of the reflectedvoltage to the incident voltage when a transmis-sion line of characteristic impedance,Z0, is ter-minated with an impedance,ZR. The reflectioncoefficient is given by

Z0 − ZR

Z0 + ZR.

Note that when the characteristic impedancematches the termination impedance (i.e.,Z0 =ZR), the reflection coefficient is zero so there isno reflected voltage.

reflection coefficient, acoustic This is givenby the ratio of the reflected flow of sound energyto the incident flow of sound energy for the trans-mission of acoustic waves from one medium toanother. The reflection coefficientαr = Ir/II ,whereIr andII are the reflected and incidentintensities respectively.

reflection density The negative of naturallogarithm of the reflectance. Equivalently, thenatural logarithm of the ratio of the luminanceof a non-absorbing perfect diffuser to that of the


surface under consideration, both surfaces be-ing illuminated at45 to the normal and viewednormal to the surface.

reflection, laws of For any small region ofa surface that can be considered smooth, (a) theincident ray, the normal to the surface at the pointof incidence, and the reflected ray, all lie in theplane of incidence, and (b) the angle of incidence(between the incident ray and the normal to thesurface) and the angle of reflection (between thenormal to the surface and the reflected ray) areequal to each other for any wavelength of light.

reflection, multiple Caused by multiple pas-sages of a light ray between two reflecting sur-faces (e.g., in a thin film). The interference pat-terns provided by transparent material like a filmof oil on water, soap bubbles, etc. result in col-orful effects.

reflection, selective Reflection by a bodyshowing selective (wavelength-dependent) ab-sorption. The color of specularly reflected lightfrom such materials is complementary to thecolor of a thin film of the material viewed bytransmission.

reflection, specular Reflection of a well-defined narrow beam from a surface, when thebeam can be approximated with a ray and thelaws of reflection are followed.

reflection, total internal When light from amedium of higher index is incident on an inter-face at an angle greater than the critical angle, itwill be totally internally reflected. No light willemerge into the lower index medium.

reflective power The ratio of the energy fluxof the beam reflected from a surface to the energyflux of the beam incident on the surface.

refracting edge The edge along which tworefracting plane surfaces of a prism intersect (onextension, if necessary).

refracting face The two non-parallel planesconstituting the boundaries of a prism where re-fraction takes place.

refraction Continuation of the part of the fluxassociated with a wave (of light) in a mediumdifferent from the one in which the wave is trav-eling currently. For specular incidence, and asmooth surface of separation of the two me-dia, the laws of reflection and refraction are fol-lowed.

refraction, acoustic The bending of soundwaves due to a change in velocity. When a trav-eling wave strikes a second medium at an anglein which its velocity is different from that whichit is traveling in, the wave continues at a differ-ent angle. This effect is significant in the oceanand depends on changes in temperature, salinityand depth in sea. Wind also causes refraction ofsound waves.

refraction, atmospheric Changes in direc-tion of light rays due to a gradual increase in airdensity (and hence index of refraction) on com-ing closer to the surface of the earth. The appar-ent direction of a star as seen through a telescopeis thus different from its true direction. The con-tinuous thermal and density fluctuations in theatmosphere cause fluctuations in the apparent di-rection of a star. Some other manifestations ofatmospheric refraction are that stars are visiblea short time after they have actually set belowthe horizon, and the disks of the sun and moonnear the horizon appear oval instead of circular.

refraction, conical The situation in which re-fracted light spreads out in a hollow cone insidea biaxial crystal, resulting from the incidenceof a beam of unpolarized light such that it isrefracted along one of the optic axes (internalconical refraction). External conical refractionis the emergence of a hollow cone of polarizedlight when a hollow cone of incident unpolarizedlight is refracted within a biaxial crystal into anarrow pencil or ray of light.

refraction, double Seeray, ordinary.

refractivity The name given to some quan-titative measure of refraction, usually (n − 1),wheren is the refractive index of the medium.Its dependence on wavelength causes the phe-nomenon of dispersion.


refractometer An instrument to determine arefractive index for various purposes like iden-tification of gems and stones, suitability of ma-terials for lenses, and for analysis of gaseoussamples. Physical properties — like total inter-nal reflection, angle of minimum deviation froma prism, shift of fringes in an interferometer, etc.— are used to help determine the refractive in-dex. In the case of solid samples, it is convenientto have at least one polished surface. Otherwise,a solid sample with irregular shape is immersedin a mixture of liquids that is adjusted to havethe same refractive index as the solid (till theoutline of the solid disappears).

refractometer, Pulfrich A device used tomeasure the refractive indices of solids and liq-uids, which uses right angle prism and theprin-ciple of internal reflection.

refrigerants, nuclear Relies on the princi-ples of magnetic cooling or adiabatic demag-netization of the nuclear spins in metals. It isa single-cycle refrigeration process and is effec-tive for producing sub-mK temperatures. Due tothe small nuclear moment in metals, the dipole-dipole interactions are reduced and in principlespin temp of less than 1 mK can be reached bynuclear cooling. Copper in wire form or powderform is most commonly used as the nuclear re-frigerant. The required polarization is producedby precooling to as low a temperature as possiblewhile applying a large external field.

refrigeration This involves the process ofdrawing heat from substances to lower their tem-perature. Mechanical refrigeration systems arebased on the principle that absorption of heatby a fluid known as the refrigerant as it changesfrom a liquid to a gas lowers the temperature ofthe surrounding air. In the compression system,a compressor exerts pressure on a refrigerant gascausing it to pass through a condenser. It thusloses heat and liquefies. On circulation of theliquid through the refrigeration coils, it vapor-izes, drawing heat from the air surrounding thecoils. The refrigerant gas then returns to thecompressor, and the cycle is repeated.

refrigeration cycle, Stirling This is a re-versible cycle that gives rise to one of the most

useful type of refrigerators. It is a four cycleprocess:1 → 2: the gas or refrigerant is com-pressed isothermally at temperatureθH and re-jects heatQH to a hot reservoir.2 → 3: Gas isforced through the regenerator at constant vol-ume, giving up some heatQR to the regenera-tor. 3 → 4: An isothermal expansion occursat the low temperatureθc during which heatQc

is absorbed by the gas from the cold reservoir.4 → 1: Gas is forced at constant volume fromthe cold to the hot end through the regenerator.

Stirling’s refrigeration cycle.

refrigeration, nuclear A nuclear coolingstage is employed in a dilution refrigerator. Itprovides a high cooling power at a low base tem-perature.Seerefrigerants, nuclear.

refrigerator, dilution A technique that em-ploys solutions of3He and4He which is capableof maintaining continuously low temperaturesas low as 2 mK. This is an improvement overthe single refrigerant cryostat which tends to belimited to base temperature of 1 K for4He and0.3K for3He. The method depends on the phaseseparation of3He-4He mixtures below 0.9 K.The 3He rich phase floats on top of the moredense phase. When the3He atoms move acrossthe boundary from the concentrated to the dilutephase, a heat of the solution is taken from the liq-uids. The dilution process can be operated as asingle cycle.

refrigerator, helium This system uses he-lium for cooling below temperatures of 1 K. Theprocess involves evaporation cooling of liquid,Pomeranchuk cooling by adiabatic solidification


of the liquid, and dilution cooling by mixing3Heand4He. Seerefrigerator, dilution.

refrigerator, Joule-Kelvin These are mi-crominiature refrigerators based on the Joule-Kelvin effect. These devices have no movingparts except for the remote compressors. Thefine capillaries are about 63µm wide and areproduced by photo-lithography. Relatively hightemperatures are achieved using nitrogen gas asthe working fluid. Other units attempt to em-ploy a three-stage cooling process involving ni-trogen, hydrogen and helium to achieve heliumtemperatures.

refrigerator, Leiden This employs the con-cept of circulating4He rather than3He in a di-lution refrigerator. With a circulation rate of∼ 10−3 mol/s, temperatures as low as 10 mKhave been achieved.

refrigerator, nitrogen System that uses liq-uid nitrogen for cooling or as the refrigerant.This type is often used in hospitals where it isnecessary to cool to temperatures near 77 K forthe preservation of human tissues, blood andbone marrow.

refrigerator, Pomeranchuk This involvesusing a method for obtaining temperatures be-low 1 K, and close to 2 mK using the by-adiabaticsolidification of the liquid. The cooling power ofthis type of refrigerator is proportional to the ab-solute temperature over the temperature range.By suitable precooling, using a dilution refrig-erator or an adiabatic demagnetization stage andby the application of pressure, liquid3He isbrought to a certain point on its melting curve.Increasing the pressure in the cell produces so-lidification. The cooling power of this method issimply the amount of heat that can be absorbedwhenn moles /second of liquid are convertedinto a solid at constant temperatureT . One ofthe main problems with this method is devisinga method for compressing the helium withoutfrictional heating.

regeneration The input signal is amplifiedthrough the circuit block (seefigure). The out-put signal feedback increases the input signal.This increased signal is further amplified through

the circuit if the combined gain is greater thanone.

Regeneration.

regenerative depolarization (cell) (1) In thesynaptic junction between two neuron cells, theelectrical signal that comes down the membraneof one end is transmitted to the neighboring cellvia neuro-transmitters. The quantity of the neuro-transmitters is in proportion to the strength ofthe signal. The actual signal received at post-synapse can be either hyperpolarizing or depo-larizing. In the latter case there is a decreaseof the negative charge inside the cell membranecausing an excitatory stimulation.

The flow of neuro-transmitters from one mem-brane to the membrane of the other cell preventsthe signal from dying out and regenerates theaction potential or membrane depolarization atpost-synapse.

(2) An example of a regenerative depolariza-tion is in the generation of spike trains in the cen-tral nervous system (CNS) neurons. Sustaineddepolarizing input waveforms to neurons, whichhold the membrane potential above threshold,provoke rhythmic firing. Less likely, but possi-ble, are extra spikes that come after a depolar-izing after-potentialthat crosses threshold. Theextra spikes may subsequently cross the firingpotential threshold themselves, provoking an-other spike. The cycle may continue, thus ob-taining regenerative ordelayeddepolarizations.Seepotential, graded (membrane); repolariza-tion (cell).

reinforcement Superposition of waves thatare in phase undergo vector addition of the dis-placement of the particles leading to an augmen-tation in the amplitude of the acoustic waves.Superposition no longer holds if the intensity


V in V out

of the sound becomes very high, such as in anexplosion.

relative permeability Measure of magneticsoftness. Defined byµr = µ/µo, whereµ andµo are the permeability and the permeability offree space, respectively.Seepermeability, mag-netic.

relative permittivity Measure of electric po-larizability. Ratio of the permittivity of a mate-rial to εo, the permittivity of free space.Seepermittivity.

relaxation, acoustic Molecules are thoughtto have a certain stiffness that causes a delay inresponse to a vibration. The time lag betweena change in pressure and the corresponding dis-placement results in some absorption of acousticwaves.

Reproduction of sound involves the prelimi-nary process of reception, then recording, trans-mission and amplification. Electronics permitsthe reproduction of sound with fidelity.See alsorecording of sound.

relay An electrically activated switch. Gen-erally, it consists of an electromagnet that, whenenergized with a current, attracts a pivotedspring loaded lever. On moving toward the elec-tromagnet, the tip of the lever touches a fixedcontact. The pivot of the lever is one terminalof the switch while the fixed contact is the other.As a result, the switch is now closed. Whenthe current in the electromagnet is turned off,the lever will spring back to its original positionand break the electrical connection between thetwo terminals. The relay may consist of severallevers and can therefore open and close severalswitches simultaneously.

relay (in communication) A relay systemuses active satellites to retransmit informationto earth. The signals are usually amplified andthe frequency changed by a transponder. Atransponder is a combined transmitter and re-ceiver system that automatically transmits a sig-nal when a predetermined trigger is received byit.

reluctance The opposition presented by amagnetic circuit to the establishment of mag-netic flux in it. It is defined by the ratio ofmagnetomotive force to magnetic flux in a mag-netic circuit. The reluctance,R, depends on thelength,L, cross-sectional area,A, and magneticpermeability,µ, of a magnetic circuit by

R =L

A× µ.

Reluctance is the reciprocal of the permeance ofa magnetic circuit. It is analogous to the conceptof resistance in an electric circuit.

reluctance, magnetic Proportionality con-stantR between magnetomotive forceEm andmagnetic fluxφ in a magnetic circuit. Definedby

Em = RΦ .

Measures the opposition presented by a mag-netic circuit to magnetic flux.Seemagnetomo-tive force.See alsoflux, magnetic.

remanence Magnetic moment of a materialin zero applied field. Usually measured afterapplying a magnetic field to saturate the mate-rial and then reducing the applied field to zero.If all the magnetic moments remain completelyaligned then the remanence is close to the satu-ration magnetization. This is an important pa-rameter for permanent magnets where a largeremanence is required.

repeater A repeater is a device at the physi-cal level that receives signals in one circuit andautomatically delivers corresponding signals toone or more other circuits. A repeater is most of-ten used with telephonic or telegraphic circuitsto amplify the signal strength. In pulse telegra-phy, repeaters perform pulse regeneration of thetransmitted pulses. Repeaters can operate eitheron signals in one direction only or on two-waysignals. Telephone repeaters operate on four-wire circuits or two-wire circuits. Repeaters al-low a link to span a longer distance.

repolarization (cell) At the beginning of anaction potential, the sodium channels open veryquickly when the membrane potential reachesthe threshold potential. Once the action poten-tial is at its peak, the sodium channels close with


equal speed. This creates the sodium inactiva-tion; it is caused by gates within the channel thatare sensitive to the depolarization of the mem-brane. Following the sodium inactivation is theopening of the potassium channels, which al-low the diffusion of K+ out of the cell. Thecombined effect of sodium inactivation, whichblocks the influx of cations, and potassium acti-vation, which causes the efflux of other cations,is the immediate return of the cell membrane toa polarized state, with the inside negative in rela-tion to the outside, i.e., repolarization.Seenerveimpulses, propagation of; potential, membrane;potential, resting.

repulsion, electric The mechanical force thatmoves apart charges of the same polarity or ob-jects carrying charges of the same polarity.

residual charge The charge remaining on theplates or dielectric of a capacitor after an initialdischarge of the capacitor.

resistance A property of an electrical con-ductor that determines the size of a direct cur-rent that can flow on the application of a certainpotential difference across it. The relationshipbetween resistance, current and potential differ-ence is given by Ohm’s law. The unit of resis-tance is the ohm (Ω), which can be any valuebut in practice is between wide limits (10−6 to108 Ω) depending on the material and geometryof the body. Although it is generally a constantfor a particular body, a change in temperaturemay change its value. On the atomic level, re-sistance is due to collisions of electrons with theatomic lattice of the material, thus dissipatingheat energy in the process.See alsoOhm’s law.

resistance, contact Resistance across the in-terface of two conductors in contact. This re-sistance is generally higher than that of the bulkmaterial because the contact area is usually lim-ited to a number of contact points on the two sur-faces. Pushing the two surfaces closer togetherincreases the number of contact points and con-sequently decreases the contact resistance.

resistance, internal A small resistance in-herent in any AC or DC voltage source. Conse-quently, not all of the EMF of a source is avail-

able for use in its external circuit, and the loss isreferred to as the drop in potential due to internalresistance. It is given by

r =E − V

I,

wherer is the internal resistance,E is the emf,Vis the potential difference across the terminals,andI is the current.

resistance, minimum at low temperaturesObservations have shown that many metals havea negative temperature coefficient at low temper-atures and sometimes show a minimum in theirresistance curve, for example, metallic oxidesand sulfides and semiconductors. Existence oflocalized moments in dilute alloys that couple tothe conduction electrons affects electrical con-ductivity. The magnetic impurities act as scat-tering centers, and if they are responsible forlattice imperfection, then at low temperaturesthe scattering caused will be the primary sourceof electrical resistance. Nonmagnetic scatter-ers lead to resistivity dropping monotically withdecreasing temperatures. In magnetic alloys,the resistivity has a rather shallow minimum oc-curring at low temperatures around 10 K thatdepends weakly on the concentration of mag-netic impurities. The minimum arises when thescattering of conduction electrons shows unex-pected features when the scattering center has amagnetic moment according to Kondo theory.

resistance, Umklapp This owes its originto phonon collisions, in contrast to normal scat-tering. The Umklapp process presents a directresistance to the flow of heat. Umklapp scatter-ing is negligible at very low temperatures sincethere are no phonons of a sufficiently large wavevector in the crystal. An Umklapp process deal-ing with three-phonon scattering is defined asone in which the total crystal momentum is notconserved — a process more likely to occur athigh temperatures than at low temperatures. Atthe higher temperatures, the mean free path ofthe phonons is ultimately limited by interatomicspacing thus reducing the spread in thermal con-ductivity of different crystalline solids.

resistivity A property of the material fromwhich an electrical conductor is made that does


not depend on the physical shape of the con-ductor. It has the units ohm-meters (Ω-m). Itcan be used to determine the resistance,R, of aconductor from the following:

R = ρL

A,

whereρ is the resistivity,L is the length of theconductor, andA is the cross-sectional area.

resistivity, cytoplasm Because resistance isinversely proportional to the cross-sectional areaof the material through which current flows, cy-toplasmic resistance is rather high, especially indendrites that are long and thin processes of theneuron. In addition, the length also contributesdirectly to the resistance because the greater thelength, the higher the number of collisions be-tween the ions (that transport the current) andthe cytoplasm components. Values are usuallyan order of magnitude smaller than the mem-brane resistance.Seeresistivity, membrane.

resistivity, membrane The conductanceacross the membrane is directly proportional tothe number of open ion channels. Then, the to-tal conductance is given by the combination ofthe ion-gated K+, Na+, and Cl− channels. Themembrane resistivity is readily obtained fromthe inverse of the conductance. This resistanceis from the ionic (rather than the capacitive)membrane current. Typical values for the mem-brane resistance are in the proximity of 200kΩ.

resistor A component used in circuits to pro-vide a resistance of known value. Electrical en-ergy is converted into heat when current flowsthrough a resistor. Thus resistors are sometimesused as heating elements. They are also usedto control current and voltage in circuits. Typesare wire wound, metal film, etc. Many resis-tors used in electronic applications have colorbar codes on the outside that indicate the valueof the resistance.

Circuit diagram symbol for a resistor.

resistor, ballast (1) A resistor that mantainsa constant current in an electric circuit. It doesthis by having a resistance that is proportional tothe current which flows through it, and that canvary rapidly with the current. It usually consistsof a resistive element inside a gas filled tube.

(2) A resistor used in electrical circuits toregulate the current. It is made from a materialwith a positive temperature coefficient; i.e., anincrease in temperature will result in an increasein the resistance. It is usually connected in se-ries with a circuit, especially power supplies, sothat a sudden increase in the current leads to anincrease in temperature which in turn increasesthe resistance thereby reducing the current.

resistors in parallel Network of resistancesin which the two ends of each resistor are con-nected to the corresponding ends of every otherresistor.

Resistors in parallel.

The network can be replaced by an equivalentresistance,R, given by

1R

=1R1

+1R2

+1R3

+ · · · ,

whereR1, R2, R3, etc. are the values of theindividual resistances making up the network.The potential difference across each resistanceis the same, which enables the current in eachresistance to be determined using Ohm’s law.

resistors in series Resistances connected toeach other in a similar way to the links in a chain.

Resistors in series.


R

1R R2 R3

R1 R2 R3

These can be replaced by a single equivalentresistanceR such that

R = R1 +R2 +R3 + · · ·

whereR1,R2,R3, etc. are the values of the in-dividual resistances. The current through eachresistance is the same. This enables the poten-tial difference across each resistance to be de-termined using Ohm’s law.

resolution of an optical system The distancebetween two object point sources such that thetwo points can be distinguished in the image.The resolution is limited by diffraction effects,so that for any form of radiation it will be deter-mined by the wavelength and the aperture of thesystem.

resolution, principle of Refers to the use of aquantitative criterion to determine the resolutionof an optical system. That most commonly usedis theRayleigh criterion,which states that twoobject points are just resolved when the diffrac-tion pattern of the image of one falls on the firstminimum of the diffraction pattern of the other.This gives an angular separation of the objectpoints

(∆ϕ)min = 1.22λ/D ,

whereλ = wavelength andD = aperture of thesystem.

resolving power For two just resolved pointsin the image of an optical system of focal lengthf the center-to-center separation of the imagesis

(∆`)min = 1.22fλ/D .

The resolving power is then defined as1/(∆`)min. Seeresolution, principle of.

resonance This phenomenon occurs in thecase where a mass is maintained in a state ofvibration by a periodic force. It is a case offorced vibrations, and, when the frequency ofthe driving force equals the natural frequencyof the system, the amplitude of oscillations in-creases significantly.

resonance in AC circuits Occurs when thefrequency of the applied periodic voltage is ator near the natural frequency of the circuit. The

resonant circuit consists of an inductor and ca-pacitor in parallel or in series. The resonantfrequency,f , is given by

f =1

2π√LC

,

whereL is the inductance, andC is the capaci-tance. At the resonant frequency, the series cir-cuit has a minimum in the impedance, while theparallel circuit has a maximum.

resonance, optical When incidentmonochromatic light has a frequency equal tothe resonant frequency of a medium, there willbe optical resonance and strong absorption.This energy is then re-emitted as resonanceradiation. This gives rise to the close connec-tion of emittance and absorptance of materials,known asKirchoff ’s law.

resonance scattering, uses in biological stud-ies Resonance scattering studies provide awide range of possibilities in the analysis andstudy of diverse biological processes. Usually,resonance scattering studies the effect on a sam-ple of an incoming probe that could be in theform of radiation or particle bombardment. Theradiation “tunes in” with intrinsic energy modesof the substance in consideration, in contrastwith elastic and inelastic scattering experiments.The incident frequency of the probe is usuallytuned to a particular substance in the samplesuch that measurements of concentration, abun-dance, real-time tracking, diffusion rate, reac-tion rate, or other properties can be realized.The main results come from the analysis of theabsorbance of the incident radiation, due to res-onance, and of the scattering characteristics.

Seeparticular uses in Raman scattering, co-herent anti-Stokes (CARS), Raman scattering,electrophoretic, NMR nuclei in biological ma-terials, nuclear angiography. Other uses are incoherent nuclear resonance scattering studies.

resonance, sharpness of This depends onthe amount of damping in the system. The lessdamping there is, the sharper the resonance peak.See alsoradiation damping, acoustic.

resonators That which can vibrate to pro-duce acoustic waves. Examples are pipes and


Sharpness of resonance.

air cavities. They can act as sound producers ordetectors.

restoring force Simple harmonic motion willresult whenever this quantity, which is a prop-erty of the medium, is directly proportional tothe displacement caused. In acoustics the restor-ing force is provided by the medium. This forcealways acts to accelerate the particle in the di-rection of its equilibrium position.

retarded potential Electric potential at apoint which is delayed in reaching its maximumvalue with respect to another point due to thefinite speed of propagation of electromagneticwaves in the medium.

retentivity Describes the retention of a mag-netic moment by a ferromagnet once a magneticfield is removed. Can also refer to the reverse ap-plied magnetic field required to restore the zerooverall magnetization state.

retina The inner transparent near layer of theeye. This is the neurosensory layer of the eye.The light entering the retina encounters aboutten layers consisting of ganglion cells, bipolarcells, horizontal cells, and amacrine cell lay-ers, which are different types of neural cells be-fore encountering the photoreceptors (rods andcones) — the site of photon absorption. Theretina represents the terminal expansion of theoptic nerve.

reverberation The echo or reflection ofsound. Of great importance in architecturalacoustics.

reverberation chamber Small live roomsthat can be used to determine the absorption co-

efficient of a material by investigation of thereverberation time. The wall surfaces of suchchambers should be highly reflecting so as toproduce a large reverberation time in absenceof testing material. It should also have a largevolume to produce a large number of modes ofvibrations and should have irregular wall sur-faces to aid diffusion of sound waves.

reverberation time Relates the empirical re-lation between echo characteristics of an enclo-sure, its size, and the amount of absorbing mate-rial present and gives a measure of the acousticalproperties of a room. It is defined by Sabine asthe time in seconds that is required to reduce theintensity from a level of 60 db above the thresh-old of audibility to the threshold.

Sabine equation for reverberation timeT

T = 0.049V/a ,

whereV is the volume of the room anda its totalabsorption.

reverse leakage current A current that iscaused by a reverse bias voltage in an electric de-vice. For example, for a diode that has a positiveand negative terminal — if the positive terminalis connected to negative side of the voltage andthe negative terminal is connected to the posi-tive side of the voltage — the current flowingfrom the negative terminal to positive terminalis calledreverse leakage current.

rheostat An adjustable resistor used to con-trol the current in a circuit by varying the re-sistance. This results in the dissipation of un-used electrical energy as heat. Rheostats maybe constructed using coils or grids from a vari-ety of metals and alloys; they may also be madeof carbon, either pulverized and held in tubes orin the form of solid rods or disks. The resistanceis adjusted by a linear or rotary sliding contact.

right-handed rotation For a linearly polar-ized light beam passing through an optically ac-tive crystal, a right-handed rotation of the planeof polarization is that which is in a clockwisedirection when looking back toward the source.

right-hand screw rule When a rotating orcirculating quantity is related to a vector, the


right-hand screw rule relates the direction of cir-culation to the vector direction. In this rule, theextended thumb of the right hand gives the vec-tor direction and the direction of the curled fin-gers of the right hand give the direction of cir-culation. Examples are:

1. the direction of the magnetic moment of acurrent loop and the sense of circulation of thecurrent in the current loop,

2. the direction of the electric current in astraight wire and the direction (rotation sense)of the circles of magnetic field lines centered onthe wire, and

3. the direction of the magnetic field insidea solenoid and the sense of circulation of thecurrent in the solenoid windings.

rings, vibration of A set of frequency stan-dards used for producing sounds of definitepitches.

ripple tank Liquid in a wooden trough thatcan be used to demonstrate wave phenomena ofreflection, refraction, interference, and diffrac-tion.

ripple voltage The AC voltage superimposedonto the DC voltage from a rectifier, such as theoutput from a half- or full-wave rectifier. Ex-pressed as a percentage of the average voltage.It may be reduced by a series choke and shuntingcapacitor, or zener diode. In practice, it is elim-inated by including an active circuit (regulator)after the shunt capacitor.

rise time A time during which a signal is in-creased from a specific low value (usually 10%)to a high value (usually 90%). The figure showsa resister and capacitor circuit. The input signalis a DC voltage. The output voltage increasesfrom zero to the input voltage value. Rise timeis for the output voltage value changing from10% to 90% of the input voltage value.

Rise time.

rods, vibration in Propagation occurs bylongitudinal waves, i.e., as the wave disturbancemoves along the bar, the displacement of the par-ticles is parallel to its axis. The propagation issimilar to acoustic waves. Several acoustic de-vices utilize longitudinal vibrations in a bar; forexample, the sound of definite pitches can beproduced by bars of different lengths. Vibra-tions in nickel tubes are used to drive the vibrat-ing diaphragm of a transducer.

ROM Read-only memory, an electric chipthat permits reading but restricts the writing op-eration. In computer systems, ROM can be usedto store an operating system program.

roton Quanta of vortex motion or micro-scopic vortex rings. They can be characterizedby the parametersenergy, momentum,andeffec-tive mass.

routing The process of finding a path from asource to every destination in a network.

Rowland ring Named after H.A. Roland(1848–1901). A magnetic material, usually aferromagnet, formed into a ring. Magnetic fluxis contained entirely within the solid materialof the ring so that no demagnetization field ispresent.


t

Vs

∆t

V Vs

Vs10%

90%

Ssagitta The segment of a line between thevertex of a small curve and the chord joiningthe two symmetric ends of the curve, which isperpendicular to the chord. Kepler gave thisname “sagitta” because of the resemblance ofthis line with an arrow. Its length measures themaximum elevation (depression) of the convex(concave) surface containing the arc.

Sagitta.

sagittal focus The point of intersection of theline of secondary image and the sagittal plane.For a conical bundle of rays starting from anon-axial point, the cross-section of the beamafter leaving the lens is initially circular, thenit becomes gradually elliptical with the majoraxis in the sagittal plane, until the tangential orthe meridional focus where the ellipse degener-ates into a straight line. Beyond this point, thebeam’s cross-section first opens out into a circleof least confusion, and then deforms into a linecalled secondary image.This line image liesin the meridional plane and has a greater imagedistance than the primary focus.

sagittal plane The plane (also called theequatorial plane) containing the principal ray(or thechief ray) and perpendicular to the tan-gential or the meridional plane.

sagittal rays A bundle of rays cut by thesagittal plane to form the pencil of sagittal rays.

sagittal section A section cut by the sagittalplane.

sampling (1) A technique in which only someportions of an electrical signal are measured andused to represent the information contained inthe original signal. The sample measurementsare usually a set of discrete values of the origi-nal continuous signal. The rate of sampling ofa periodic quantity must be at least twice thefrequency of the signal if the output values areto represent the input signal without significantloss of information.

(2) A technique used in radio navigation sys-tems where information from the navigation sig-nal is extracted only when a sampling gate isactivated by a selector pulse.

sampling function The one-dimensionalsampling function can be expressed mathemat-ically as

S(x) =∞∑

n=−∞δ(x− n∆x) ,

where δ is the Dirac-delta function. A two-dimensional version of the sampling functioncan be written as

S(u, v) =∑

δ (u− un, v − vn) ,

which can be weighted to

S(u, v) =∑

RnTnDnδ (u− un, v − vn) ,

whereRn is a reliability weight,Dn is a densityweight andTn is a taper.

sampling gate A circuit that takes signalsfrom the input signal only when an externalpulse is active. This external pulse is appliedto the transistor gate.

sampling period A time interval duringwhich the input signal is picked up.

sampling window A time interval duringwhich the input signal is picked up and the out-put signal is the sum of the input signal and theexternal triggering signal.

satellite (1) An artificial body that is pro-jected from the earth and may orbit either theearth or another body of the solar system.


(2) Information satellites transmit signalscontaining many different types of informationto the earth. These satellites are typically usedto provide atmospheric and meteorological data,infrared, gamma-ray and X-ray studies of celes-tial objects, surveys of the earth’s shape, surfaceand resources, and as navigational aids.

(3) Communications satellites are used to re-ceive radio-frequency signals from the earth bymeans of highly directional aerials and returnthem to another earth location for purposes oflong-distance telephony, TV broadcasting, etc.These satellites provide links that traverse verylong distances at high bandwidth. They use the4 to 6 GHz bands, and their potential link band-width is in the order of Gbps.

satellite beacon A beacon satellite acts as aradio-navigation station. Its emissions are in-tended to enable a mobile-radio service to de-termine its bearing or direction in relation to thesatellite beacon. The primary use is to facilitatea search and rescue operation.

satellite, geostationary (Also known asgeosynchronous satellite.) A satellite orbitingabove the earth’s equator at an altitude of ap-proximately 35,000 km, such that its period ofrotation is equal to the earth’s rotational period.Thus, the satellite continuously views the sameportion of the earth’s surface.

Low-earth and geostationary satellites typ-ically rebroadcast messages that they receivefrom any of the earth stations. Data rates ofup to 500 Mbps are possible. The up-and-downpropagation delay for geosynchronous satellitesis about 250 ms, which is unacceptable for mostinteractive services (though acceptable for tele-vision broadcast).

satellite, synchronous A synchronous satel-lite orbits the earth approximately 35,900 kmabove the equator and moves in a west-to-eastdirection. At this altitude it makes one revolu-tion in 24 hours and thus is synchronous withthe earth’s rotation.

saturation The condition where the outputsignal of a physical measurement increases toa constant maximum value and cannot increaseany further due to subsequent increases in the

input signal. The phenomenon is generic in vir-tually all physical systems and corresponds tocomplete breakdown of the linear regime.

saturation, current/voltage/resistance Acurrent/voltage/resistance value that cannot in-crease further when the outside signal (volt-age/current) increases. In a transistor, the sourceor drain current cannot increase even though theapplied voltage increases further. This currentis calledsaturation current.

sawtooth wave A periodic wave whose am-plitude linearly changes with time between cer-tain time periods. The shape of wave is like thatof a saw.

Sawtooth wave.

scale, diatonic The natural musical scale inWestern music in the key ofC major, ascendingfrom middleC, which has a frequency of 256Hz, although it is represented musically as 261.2Hz.

scale-of-ten This is similar as scale-of-two,but it has ten inputs. Every input signal from teninputs is one and the output signal will be one.

scale-of-two An operator in a circuit in whichevery two input pulses produce one output pulse.This kind of circuit is normally used in comput-ers for mathematic operations.

scales, equally tempered On a keyboard in-strument like a piano or organ, the notesA toGare sounded by depressing white keys and addi-tional black keys are situated between each pairof consecutive white keys that differ from eachother by a tone. Each black key sounds a note


t

v

that is intermediate in pitch between the whiteones on either side of it.

scales, musical A definite series or succes-sion of tones ascending or descending accordingto fixed intervals. The range between any noteand the octave above it is divided into seven in-tervals by the insertion of six intermediate notes,the various pitches of which give a number ofconsonant intervals with each other and with thenotes at each extreme of the octave.

scanner An instrument that can convert theinformation on materials into an electric signal.Normally, the scanner is the picture converter. Itconverts the picture on the paper into the electricsignal that can be stored in computers.

scattering, acoustic Incident waves on re-flection as secondary waves undergo wavelengthshift in increasing proportion toward higher fre-quencies. It has been shown that the amplitudeof the secondary waves varies directly as the vol-ume of the scatterer, and inversely as the squareof the wavelength of the incident sound. The in-tensityI of the reflected sound varies inverselyas the fourth power of the wavelengthλ

The law of scattering I = 1/λ4 .

scattering, Brillouin Light of frequencyvi

incident on a solid is scattered at a frequencyvs = vi ± ve, due to inelastic scattering bythe medium. When the scattering is due to theinelastic phonons of the medium, the effect isknown asBrillouin scattering. In this case thescattering conditions limitve to ve < 100GHzor≤ 1cm−1.

The effect can be explained classically by thefact that the operative acoustic phonons set upan effective diffraction grating in the mediumso that the incident light is Bragg diffracted andDoppler shifted by the acoustic phonon travelingat the velocity of sound. The quantum descrip-tion is more exact and is based on conservationof energy and momentum for the scattered pho-tons and the emitted or absorbed phonon. For

incident photon frequencyvi, wave vector

k i,

scattered photon (vs,

k s) and phonon (Ω,

K),

this gives

k i −

k s =

K andvi − vs = ±Ω.

Brillouin scattering is mostly done on ther-

mal phonons; the phonon wave vector

K can

be determined by fixing

k i and

k s. The ex-periment is done with a laser source. Sincethe phonon lines are very close to the elas-tic (Rayleigh) peak, a very high resolutionFabry-Perot interferometer is used to resolvethem. The observed sidebands with decreased(increased) frequency are called Stokes (anti-Stokes) and each contains three peaks corre-sponding to a longitudinal phonon and twotransverse phonons.

Brillouin scattering is a useful technique forthe study of the elastic properties of materialssuch as layer compounds, amorphous materials,gels, phase transitions etc.

scattering, Coulomb The scattering of an in-cident charged particle by a stationary chargedtarget due to Coulomb repulsion between them.The classic example is that of Rutherford scatter-ing of alpha particles by gold foils. The experi-ments showed that the average scattering angleper atom was very small, of the order of 10−4

radians. This led to the Rutherford model ofthe atom where most of the positive charge andessentially all of the mass is assumed to be con-centrated in the nucleus at the center of the atom.This model was subsequently quantitatively ver-ified by experiment and led to a good estimateof the size of the nucleus, for copper atoms lessthan2× 10−12cm.

scattering, cross section The total cross sec-tion for scattering is the ratio of the scatteredpower to the incident intensity. The cross sec-tion has the dimensions of area, so that it gives anintuitive indication of the relative importance ofvarious scattering mechanisms. In general, thecross section is made up of the sum of that forabsorption by the target and that for scatteringout of the beam.

scattering, incoherent Scattering in whichthere is no phase relationship with the incidentradiation. Hence there are no associated inter-ference effects and the intensities of such scat-tered beams can be added to obtain the total scat-tered intensity.


scattering, inelastic That in which the inci-dent radiation is absorbed by the scatterer andre-emitted at a different frequency.

scattering, Mie The scattering of light bya single sphere or incoherently by a group ofspheres separated from each other by distancesmuch greater than the wavelength. The theory(G. Mie, 1908) is based on the diffraction of aplane electromagnetic wave by a homogeneoussphere in a homogeneous medium; it is validfor all types of spheres and mediums. The the-ory, which is very complex, involves the solu-tion of Maxwell’s equations in spherical polarcoordinates. The polar diagrams for scatteringdepend strongly on the ratio of sphere diameterto wavelength. The theory is of great practicalimportance and can be used in the study of lightscattering by colloidal suspensions, atmosphericdusts, clouds, fogs, etc. as well as to explainnatural phenomena such as rainbows and solarcorona. When the wavelength is of the order ofthe diameter of the sphere, the effect can be usedto monitor variations of the latter.

scattering of light in tissue Light transmit-ted by tissue is in a large proportion of longwavelengths (red), whereas backscattered andotherwise scattered light is of short wavelengths(blue). This corresponds to the observation thatskin looks red when transilluminated by whitelight. In contrast, blood in veins and melanindeep in the dermis preferentially absorb red andreflect blue light.

Because red and near IR light has a typicalabsorption length in biological tissue of 1 to 10cm, as opposed to 10 to 100 mm of scatteringlength, elastic scattering of light in the opticalwavelengths dominates the scattering phenom-ena, giving rise to turbidity. In general, turbiditydestroys phase coherence in the incident light,blurring and making it very difficult to make asensible image out of the scattered light.

With recent advances in laser technology, theunderstanding of turbidity has been brought intonew importance in the development of spectraldiagnosis of disease and optical tissue imaging.The importance of light imaging as comparedto X-rays, for example, is that in addition toproviding real time imaging, it is a non-invasive

procedure that may provide information aboutbiochemical constituents.

Current research in imaging using opticallight has been able to image inside tissue thatis up to 1 cm in thickness. The imaging isachieved by collecting specific light (transmit-ted) that goes through the tissue and discardingthe scattered light that reaches the other side witha delay relative to the unscattered light.

scattering, Raman An incident monochro-matic beam of light of frequencyvi is scatteredinelastically to a frequencyvs = vi±ve. Whenthe inelastic scattering is by electronic excita-tions or optical phonons, the scattering is calledRaman scattering. The sidebands at±ve aretypically in the range 10 to 1000 cm−1. Thelower frequency band is called Stokes and theupper anti-Stokes.

Raman scattering can be described classi-cally by the induction of a dipole moment in thecrystal by the electric field of the laser beam,hence by the polarizability tensor. Symmetryarguments show that a crystal with a center ofinversion cannot be both infrared and Raman ac-tive; it follows that both are allowed for piezo-electric crystals. Quantum mechanically, theRaman process, as for Brillouin scattering, canbe described by the conservation of momentumand energy for incident and scattered photon andthe participating phonon.

The experiment is carried out with a lasersource and a double monochromator with pho-tomultiplier tube detection. In chemistry, thetechnique is useful for measurement of rota-tional and vibrational molecular spectra, whilein physics it is generally used for phonon spec-troscopy in solids for such studies as structuralphase changes and soft modes, magnetic andamorphous materials, doped semiconductorsetc. Seescattering, Brillouin.

scattering, Rayleigh Scattering of electro-magnetic waves by small particles, of diametersless than the wavelength. The result for a classi-cal model, calculated by Lord Rayleigh in 1871,gives for the scattered intensityIs = I0λ

−4,whereλ is the incident wavelength. This well-known result has been verified in innumerablecontexts and accounts; it has been shown, for ex-ample, for blue sky and red sunset, attenuation


in optical fibers due to impurities and the centralelastic peak in Rayleigh and Raman scattering.

scattering, Thomson The classical scatter-ing of electromagnetic waves by electrons. Itwas applied by Thomson to the scattering ofX-rays by a metal foil. Under suitable assump-tions (the X-ray wavelength is small comparedto the atomic diameter and the energy of an X-ray quantum large compared to the binding en-ergy of the atomic electron but small comparedto its rest mass energy), the interaction can beconsidered as being between the oscillating elec-tric field of the X-ray and the charge of the elec-tron. The theory provides the scattering crosssection and the angular distribution of scatteredX-rays.

Schottky anomalies Heat capacity of solidsdecreases with decreasing temperature; how-ever, at extremely low temperatures other con-tributions to the heat capacity may become sig-nificant. Impurities in a crystal give rise to thistype of anamoly.

scintillation Emission of light by a solidbombarded with radiation. The process is tra-ditionally used to detect high energy particlessuch asα, β andγ radiation in scattering exper-iments. For the case ofγ rays, the scintillationcounter may consist of a scintillation detector(T` doped,NaI crystal and photomultiplier),amplifiers, and a discriminator. Theγ rays in-teract with the crystal by photoelectric effect,Compton effect, and pair production. For eachof these interactions the primary energy goesinto the kinetic energy of the electrons and theremainder into secondary photons that are de-tected.

Scott connection A connection betweentransformers that can convert two-phase powerto three-phase power or vice versa.

screened cable A cable with a flexible pro-tective screen of conductive material surround-ing it.

search coil A coil used to measure magneticfield properties. A stationary coil may measureproperties of a time-varying magnetic field. A

rotating coil may measure properties of a con-stant magnetic field. In both cases an inducedEMF due to the change in magnetic flux in thecoil is measured. Can be part of a ballistic gal-vanometer or a fluxmeter.Seeflux, magnetic.See alsoLenz’s law of induction, fluxmeter.

searchlight A powerful beam of light pro-duced and projected long distances, usually withthe help of an intense light source (e.g., carbonarc) positioned at the focus of a paraboloidal re-flector.

secondary cell An electrochemical cellthat can be electrically recharged many timesis called a secondary cell. They are alsocalled rechargeable batteries. Typical examplesare lead-acid cells in automobiles and nickel-cadmium batteries.

secondary focus Seesagittal focus.

secondary maxima These occur betweenthe principal maxima of interference fringes forFraunhofer diffraction byN identical apertures.The intensityI(θ) at a point on the screen mak-ing angleθwith the center of the grating is givenby

I(θ) = I0

(sinββ

)2( sinNαsinα

)2

,

where

I0 = flux density in theθ = 0 direction

emitted by one of the slits.

β =kb

2sin θ

k = wave number of incident radiation=2π/λ

b = slit width

α =ka

2sin θ

a = distance between slit centers .

Principal maxima occur whensinNα/ sinα =N or α = 0,±π,±2π, . . . , minima oc-cur for (sinNα/ sinα)2 = 0 or α =± π

N , ±2πN , ± 3π

N . . . and secondary maxima


appear between each pair of neighboring min-ima.

Seebeck effect This is the net EMF estab-lished in a circuit made from two junctions ofdissimilar metals connected in series while thetwo junctions are maintained at two differenttemperatures. The magnitude of the potentialdifference between the two junctions determinesthe magnitude of the EMF. The Seebeck effect isthe basis of a temperature measurement deviceknown as athermocouple. See alsothermocou-ple.

seismic waves There are two basic types ofwaves: P -waves or primary waves are longi-tudinal waves that can propagate equally wellthrough a solid and through a fluid medium.S-waves or secondary waves are transversewaves that can propagate through solids but notthrough liquids. These are body waves and travelwithin the body of the earth. Also, there areL-waves or large waves that travel in the uppersurface of the earth.See alsorayleigh waves,surface waves.

seismograph Very sensitive instrument usedfor recording earth tremors that relies on theprinciple of a pendulum driven by the earthtremors.

selectivity Quality or state of being selective.

selenium cell Can be one of two types:photovoltaicor photoconductive.The photo-voltaic selenium cell requires no external EMF.An EMF is generated across the terminals ofthe cell when light shines through an opticallytransparent window onto a thin film of gold over-laying the selenium film. The photoconductiveselenium cell requires an external EMF to op-erate. The cell also has a transparent windowto admit light. Changes in light intensity cre-ates changes in the conductivity of the seleniumwhich changes the current through the cell. Bothtypes of cells are used to measure light intensity.

self-capacitance Self-capacitance is the ratioof the charge to the potential of a conductor,C =q/V , whereq is the charge on the conductorandV is the potential of the conductor. Self-

capacitance is the measure of how much charge aconductor can hold per unit potential; it dependson geometry of the conductor and the mediumthat the conductor is embedded in.

self-diffusion coefficient The self-diffusioncoefficient is defined as the coefficient of pro-portionality that quantifies the strength of theflux of particles into one another. In a sense, ittells how good the particles in a substance mixwith one another.

More precisely: consider a substance com-posed of two species of particles whose onlydistinction is that one species is labeled (e.g., ra-dioactive) but are otherwise equal. Assume thatinitially the two species are separated into twodomains along thex axis. If the concentrationof labeled particles is described by a functionn(x), then after some time the two species willmix, tending to maken(x) more uniform andthus increasing the entropy of the fluid.

If we defineJx as the flux of labeled particlesalong thex direction, then one expects that

Jx = −D∂n(x)∂x

.

The coefficientD (D > 0) is the coefficient ofself-diffusion of the substance. Becausen(x)changes as a function of time, from particle con-servation, it follows that

∂n

∂t= −∂Jx

∂x.

Combining the two equations we get:

∂n

∂t= D

∂2n

∂x2,

which is the diffusion equation.Self-diffusion should not be confused with

mutual diffusion in which the labeled particlesare different from the others.

self-focusing Self-focusing of a laser beammay occur in a non-linear optical medium. Ifthe refractive index increases with the incidentintensity and if one assumes a Gaussian trans-verse intensity distribution, then the center ofthe beam will travel slower than the edges. Thenet effect is that the medium acts as a positivelens and self-focusing occurs, which can lead to


focal spots a few microns in diameter. If self-focusing is arranged to exactly cancel diffractionthen self-trapping of the light beam will occur. Ifthe intensity is high enough, self-focusing maylead to damage of the crystal.

semiconductors Materials in which the con-ductivity is between conductive metals and insu-lators. Their conductivity can be changed afterimpurity doping, such as crystal silicon, germa-nium, and three-five compound crystal materi-als.

semitone The interval between any two con-secutive notes of the musical scale such asCandC#.

sensitive flame A tall, steady, high pressuregas flame that ducks and roars in the presenceof sound waves.

sensitivity A minimum input signal thatcauses a distinguishable output signal. This sig-nal could be light, electric, stretch, etc.

sensitivity of ear The response of the earvaries with frequency. The hearing thresholdfor a person with acute hearing is 0 dB and thethreshold of pain at about 130 dB. The thresh-old for hearing, audibility and that of pain alsodepends on frequency.

sentence (logical) Logical words that areconstructed by the computer software and canbe executed in the computer.

sequential logic Logic in which the outputsignal is dependent on the previous input signalby a delay time.

server An entity that controls access to ashared resource.

servomechanism A feedback system whoseoutput signal represents mechanical motion.

shadow The shade of finite extent cast upona screen or another body by an opaque object in-tercepting the incident rays from a light source.Depending on the size of the source, the shadowis complete (umbra) for a point source and of

Sensitivity of ear.

variable intensity (umbra and penumbra) for anextended source. For macroscopic objects, theshadow can be constructed using the ray ap-proach. For objects with sizes comparable tothe wavelength of light, the wave nature of lighthas to be invoked for a correct description.

shadow, acoustic Region in which sound in-tensity is theoretically zero since no rays reachhere.

shadow, penetration of sound into Thediffraction of acoustic waves into regions ob-scured by obstacles allows sound to be propa-gated in such areas.

shear waves Type of wave, also known astransverse,in which displacement of a particleis perpendicular to the direction of motion.

shielding (low temperature) Shielding isnecessary to reduce heat leaks. Radiation shieldshelp inhibit connective oscillations in the heliumgas above the liquid that may be driven by thetemperature gradient down the cryostat. Thiscan transfer large quantities of heat to the coldpart of the cryostat. As such, built-in infraredshields and traps are used. The cryostat mustalso be protected from longer wavelength radi-ation and from emission caused by other equip-ment in the vicinity. An entire dilution refrig-erator can be enclosed in a large Faraday cage— an enclosed metal box known as ashieldedroom. Thermal conduction down support tubesand connecting leads are also major sources ofleaks.


shift register A digital circuit that can storea set of information in the form of pulses. Theregister has the capability of serially shifting thedata from each stage of the register to the adja-cent one. It may be a unidirectional or a bi-directional shift register, depending on whetherit can shift only from left to right, only fromright to left, or both.

shock wave A wave formed by compressionin a medium when an object moves violentlythrough the medium at a speed in excess of thatsound.

short-circuit When the load resistance of acircuit is zero we call the circuitshort-circuited.It can occur, for example, when a copper wire isconnected to the two terminals of a battery.

shortsighted Seeeye, near-sighted.

short wave A radio wave that has a wave-length in the range 10 to 100 m or a frequencygreater than 3 MHz, i.e., in the high-frequencyband (HF).

shunt A component placed in parallel acrossthe terminals of a circuit or device in order todivert a known current from it. For example, if acurrent to be measured by a galvanometer is toolarge for that instrument, a large fraction of thecurrent may be bypassed through a shunt resistorto reduce the sensitivity of the galvanometer ina known way and thus enable the current to bedetermined.

sidebands The waveform for simple sine-wave amplitude modulation is given by

e(t) = E(t)(1 +m) sinωst ,

whereE(t) = E0 sinωct is the unmodulatedcarrier wave with amplitudeE0 and frequencyωc. The frequency at which the unmodulatedamplitude is varied isωs andm is referred toas thedegree of modulation.The equation of awave with simple sine-wave amplitude modula-tion can thus be written as

e(t) =E0 sinωct+mE0

2cos (ωc − ωs) t

− mE0

2cos (ωc + ωs) t .

The last two terms in the above equation arecalled thelower andupper sidebandswith side-band frequencies of(ωc − ωs) and(ωc + ωs),respectively. In general, there can be a range offrequencies over which the average amplitude isvaried. All the information content of the signalis contained in the sidebands.

sidebands, double (DSB) Double-sidebandmodulation is a method of encoding a signal,g(t), with no DC component. The product

e(t) = Acg(t) cosωct

represents a double sideband suppressed carriersignal, whereAc is the amplitude of the unmod-ulated carrier. The radio-frequency envelopefollows the wave-form of the modulating sig-nal g(t). The spectral components of the DSBsignal,e(t), are given by its Fourier transform

E(jω) =12G [j (ω − ωc)]

+12G [j (ω + ωc)] ,

whereG(jω) is the Fourier transform ofg(t).Note that the upper and lower sidebands aretranslated symmetrically±ωc about the origin.

sidebands, independent (ISB) Independentsideband transmission is double-sideband trans-mission in which the information carried by eachsideband is different. The carrier may or maynot be suppressed.

sidebands, single (SSB) It is possible totransmit all the information represented in amodulated signal by transmitting only a sin-gle sideband. The carrier component of anamplitude-modulated wave contains no infor-mation since its frequency, amplitude, and phaseare not affected by the modulation. Thus thecarrier need not be transmitted. Moreover, eachsideband contains the same information aboutthe modulation and only a single sideband needbe transmitted. Single-sideband transmissionrequires a frequency band only half as wide asthat required to transmit the modulated waveconsisting of two sidebands and the carrier.Single-sideband transmission also requires one-third the power of transmitting all the compo-nents of the amplitude-modulated wave.


signal A varying electrical parameter, such ascurrent or voltage, that is used to convey infor-mation through an electronic circuit or system.A signal is usually transmitted as electrical im-pulses or radio waves.

signal, asynchronous Asynchronous signalsare transmitted by prefixing start and postfix-ing stop information to the original signal. Onthe other hand, the start and stop informationneed only appear for each group or block ofsynchronous signals. Thus asynchronous sig-nals can usually not be transmitted as fast assynchronous signals.

signal, binary A signal in the form of a binarycode, i.e., digital rather than analog.

signaling (1) A method of controlling com-munications. Signaling is used to send a signalfrom the transmitting end of a circuit to informa user at the receiving end that a message is tobe sent.

(2) In a telecommunications network, signal-ing is the information exchange that involves theestablishment and control of a connection andthe management of the network. This is in con-trast to the user information that is transferred.

signaling, common battery A common bat-tery is a single electrical power source usedto energize more than one circuit, component,equipment, or system. In many telecommuni-cations applications, the common battery is at anominal−48 VDC.

signaling, common carrier Common-carrier signaling is a term used in telecommu-nications that applies to a method employed bya telecommunications company that holds itselfout to the public for hire to provide communi-cations transmission service.See alsocarrier.

signaling, composite Signaling in which anarrangement is made to provide direct currentsignaling and dial pulsing beyond the range ofconventional loop signaling. Composite signal-ing permits duplex operation; i.e., it permits si-multaneous two-way signal synchronous signal-ing.

signaling, double current (Also known aspolar direct-current telegraphy transmission.)Double current signaling is a form of binary tele-graph transmission in which positive and nega-tive direct currents denote the significant condi-tions.

signaling, flashing light Flashing light signalsystems as designed for high visual impact andmaximum operating efficiency.

signaling, frequency exchange (Alsoknown as two-source frequency keying.)Frequency-changing signaling is when thechange from one significant condition to an-other is accompanied by the decay in amplitudeof one or more frequencies and by the build-upin amplitude of one or more frequencies.Frequency-exchange signaling applies tosupervisory signaling and user-informationtransmission.

signaling, multifrequency Multifrequencysignaling is often applied to trunk circuits forthe transmission of switching information. Itthus increases the speed of setting up inter-officeconnections. Digital information is transmittedby combinations of two of the following fiveaudio frequencies: 700, 900, 1100, 1300, and1500 Hz. A sixth frequency of 1700 Hz is usedin combination with the 1100 Hz frequency asa “priming” signal and in combination with the1500 Hz frequency as a “start” signal.

signaling path The signaling path in a trans-mission system is a path used for system con-trol, synchronization, checking, signaling, andservice signals used in system management andoperation. It is not the path for the data, mes-sages, or calls of the user.

signaling underwater Communication isprimarily done by transmitting ultrasonic waves.High efficiency transducers are used to allowsignals to be beamed from source to receiver.

signal, inhibiting A signal that prevents theoccurrence of an event. An inhibiting signalmay by used, for example, to disable an ANDgate. It thus prevents any signals from passing


through the gate as long as the inhibiting signalis present.

signal, mark A mark signal is a term in teleg-raphy, which represents one of the two signifi-cant conditions of encoding. The other, comple-mentary significant condition is called aspacesignal.

signals, AC, whole-organ (1) It has beenestablished that heart or cell activities have vari-ations in their electrical properties that can bemeasured by galvanometers. For example, ifan isolated frog heart (still beating) is placedin between two condenser plates, the rhythmicmovements of the heart cause variations in theelectrical capacitance and also in the impedanceof the condenser to the circuit.

(2) Low-frequency alternating current mea-surements indicate that activity from cells con-tribute to changes in the impedance of tissue.From these measurements it has been deter-mined that the frequency that causes the maxi-mum change in the impedance is in the audio-frequency range, and it is related to changes inthe permeability of the cell.

signal sampling Signal sampling is whenan initial continuous waveform is replaced bya finite discrete set of signal points that repre-sent samples of the continuous signal. The mostcommon application is when a continuous time-varying analog signal is transferred to a digitalsystem.

signals of specific frequencies The three(ideal) types arelow pass filters,which only al-low frequencies below a particular value to bepassed,band pass filters,which allow signalsbetween a lower and upper value to be passed,andhigh pass filters,which only allow frequen-cies higher than a particular value to be passed.Real filter circuits, which may allow some un-wanted frequencies to pass, approximate theseideals.

signal-to-noise ratio A measure of the noiseof a system, link, channel, etc. The signal-to-noise ratio (SNR) is usually expressed in decibel

(dB) units and defined as

SNR (in dB) = 10 log( sn

)2

,

wheres is the peak signal level andn is theroot-mean-squared noise level.

sign conventions A sign convention is usedin geometric optics to measure distances in anoptical system. This convention is essential forinternal consistency in solving problems. Acommonly used sign convention is theCarte-sian convention.If Cartesian axes are drawn atthe refracting (or reflecting) surface such that theorigin coincides with the vertex of the surface,then assuming that light travels from left to rightalways:

1. Object and image distances are negative tothe left of the origin and positive to the right.

2. The radius of curvature is positive if thecenter of curvature is to the right of the vertexand negative if to the left.

3. Vertical dimensions above the horizontalaxis are positive and negative below.

silencers Device used for deadening thesound of gas escaping from internal combustionengines or for deadening the sound of firearm.

silent zones Phenomenon common to largeexplosions. Waves of tremendous intensity re-veal abnormalities not evident with low inten-sity waves. Large explosions are accompaniedby a succession of pressure pulses arriving indifferent paths after reflection and refraction inthe atmosphere. There are regions where thesound is not audible, while at greater distancesthe sound is rather evident.

silsbee effect The superconductivity of awire- or film-carrying current can be quenchedor destroyed at a critical value. In thick speci-mens where the surface effects can be ignored,the critical current is that which creates at thesurface of the specimen a fieldHc. Smaller sam-ples remain superconducting with much highercurrents than those calculated in this manner.The size of the critical current depends on thenature and geometry of the specimen.


simple harmonic motion Deals with thestudy of motion of vibrating bodies and is anymotion that repeats itself in equal intervals oftime. The force acting on the particle is propor-tional to the displacement but is opposite to itin direction. The particle is attracted toward afixed point in the line of motion.

simplex A simplex communication channelor operation allows transmission of signals inone direction only.Compare withduplexing.

simplex code A set ofK simplex codewordsmay be formed from a set ofK orthogonal equal-energy codewords by subtracting from each oftheseK orthogonal codewords their geomet-ric mean. The resulting simplex codeword setwould have pairwise cross-correlation equal to

11−K , a codeword energy equal toM−1

M of theenergy of each of the original orthogonal code-words, and a pairwise distance identical to thatin original orthogonal code.

singing arc If a continuous current arc isshunted by an inductanceL and a capacitanceC in series, for certain values, a musical noteis emitted of frequencyN = 1/2Π

√LC. The

superposition of the oscillating current on thecontinuous current in an arc causes heating ef-fects in the gases around the arc.

siren An instrument consisting of a smallmetal wind chest with a flat top in which a num-ber of equally spaced holes are drilled aroundthe circumference of a circle. A metal wheel,which can rotate freely, is parallel and close tothe top of the wind chest and has a similar setof holes drilled in it. The wheel rotates by theslanting jets of air, which issue from the windchest. As the wheel spins, a puff of air passesthrough the holes every time two sets of holesare opposite to each other and this gives rise to asound of frequency equal to the number of timesthe holes coincide per second.

skin effect Effect by which alternating elec-tric current in a conductor flows near the surfaceof the conductor. This effect becomes importantat high frequencies and for good conductors andis most pronounced for superconductors. It re-sults in an increase in effective resistance since

less of the cross-sectional area of the conductoris used to transport current.

skin effect, anomalous This situation occurswhen the conducting slab is thick compared tothe skin depth — a thin layer on the surface in thematerial — but not compared to the mean freepath of the electrons. An electric field can influ-ence the electrons only when they are within askin depth of surface, and conversely, electronscan radiate energy back out of the metal onlywhen they are within a skin depth of the surface.If electrons have orbits in the magnetic field thatcarry them from within a skin depth of the topof a slab to within a skin depth of the bottom,then electrons in such orbits can reproduce onthe far side of the slab the current induced by thedriving electric field on the near side. Thereforethere is resonant increase in the transmission ofelectromagnetic energy through the slab when-ever the thickness and magnetic field are suchthat orbits can be so matched with the surfaces.

slew rate The change rate of the maximumabsolute output voltage in a circuit.

slip ring Provides electrical connection be-tween a continuously rotating object such as acoil and a stationary metal brush. Used in elec-tric motors and electric generators to make elec-trical connection to a coil that rotates in a mag-netic field.

small-signal approximation The signal pro-cessing in electric circuits is divided into tworanges: one is a small signal, which is usuallyfor AC signals, and the other is a large signal,which is usually for DC signals. For differentprocessing, the parameters for calculations aredifferent. The small signal processing is calledsmall-signal approximation.For small-signalprocessing, the change of the signal magnitudemust be smaller than the total signal magnitude.

Smoluchowski equation The Smolu-chowski equation relates the probability for arandom walker to go from an initial point to afinal point, using a determined number of steps,to intermediate probabilities. Smoluchowski’sformulation provides an alternate approach to


the problem of Brownian motion as provided byEinstein’s formulation.

Consider a one-dimensional Brownian par-ticle moving in a discrete system. Define theprobabilitypn(x0|x) as the probability that, in aseries ofn steps, the Brownian particle initiallyat pointx0 will reach pointx. Smoluchowski’sequation is given by

pn(x0|x) =∞∑

z=−∞pn−1(x0|z)p1(z|x) ,

for n >= 1.

Snell’s law When light goes from themedium of indexn1 to the medium of indexn2, it undergoes refraction. This is governed bySnell’s Law:

n1 sin θ1 = n2 sin θ2 ,

whereθ1 is the angle of incidence andθ2 is theangle of refraction. The angles are measuredwith respect to the surface normals.

sodar An acoustical system located on theground or on a ship that emits a sound impulseand receives itsechoesscattered by atmosphericturbulence. Sodars are used for acoustic re-mote sensing of the atmospheric boundary layer— in particular, different dynamic regimes ofthis layer — vertical profiles of wind velocity,and intensities of temperature and wind velocityfluctuations. A frequency band of sodars is fromone to a few kHz. The diameter of an acousticalantenna that is used to emit a sound impulse andreceive its echoes can reach a value of severalmeters.

software Software consists of logical sen-tences in computer science and can execute cer-tain operations.

solar battery An electric device that can con-vert solar energy or light energy to electrical en-ergy.

soldering fluxes (low temperature) The sol-der junction is produced by dipping a piece ofoxidized wire, usually niobium, into molten sol-der. When the solder freezes tight mechanicalcontact is established and the junction is ready.

The flux is seen to penetrate enclosing a regionaround the niobium oxide. The magnetic fieldin the area between the junctions is created bypassing a current through the niobium wire.

solenoid A coil of wire, usually uniformlywound with a length greater than its diameter,used to create a uniform magnetic field at itscenter. The magnetic field within the solenoidis given by

µoni ,

whereµo is the permittivity of free space,n is thenumber of turns per meter length of the solenoid,andi is the current in the solenoid. When thesolenoid is made long compared to its diameter,the field inside is uniform at positions not tooclose to the ends.

sonar Acronym for sound navigation andranging. Refers to underwater acoustics in-volved in the detection and tracking of sub-marines and surface vessels in naval warfare.

sonic barrier Also known assound barrier,when a bullet or shell travels with a velocitygreater than that of sound, a sound like a crackof an explosion is heard by the shock wave thatis created. When the ratio of velocity of air-craft/velocity of sound (Mach number) exceeds1, a shock wave develops.

sonic boom A loud explosive sound causedby the sudden dissipation of a pressure field builtup and around an airplane as it reaches the speedof sound.

sonobuoy An acoustic receiver and radiotransmitter mounted in a buoy, used to detectand transmit underwater sounds.

sonoluminescence The transformation ofsound energy into light energy by a process us-ing ultrasonic waves. These waves are aimedat an air bubble in a water cylinder, causing thebubble to oscillate vigorously thereby expand-ing and contracting to a maximum size of about50 microns. A near-vacuum situation at expan-sion causes catastrophic collapse of the bubblewhich is accompanied by a flash of light. En-ergy is concentrated by a factor of more than atrillion. See alsotriboluminescence.


sonometer Also known as amonochord.Aninstrument in which a wire, usually metal, isfixed to a steel peg at one end. The wire passesover a freely running pulley and tension is pro-duced by attaching weights to the free end. Usu-ally two fixed bridges and one movable bridgeare also provided to vary the length of the vibrat-ing segment. It can be used for the demonstra-tion of standing waves and resonant frequencies.

sonorous Deep, resonant or rich sound.

sonovox An electronic device used by a per-son whose larynx has been removed, for trans-mitting recorded sounds to the laryngeal area tobe emitted in turn as words through the mouth.

sound in gas The motion of a vibrating bodyis communicated to a gas by the production oflongitudinal waves that travel in the same direc-tion as the vibration of the gas particles. Thevelocity with which these waves travel in thegas are dependent on its elasticity and density.The velocity of sound in gases is given byC =√κ/ρo whereκ is the bulk modulus of elasticity

andρo is the density of the medium. The com-pressions and rarefactions in sound waves takeplace very rapidly so that the associated heatingand cooling does not have time to be transferredto the surrounding medium and, therefore, thechanges are adiabatic. The velocity in gases isindependent of pressure and varies as the squareroot of the absolute temperature.

sounding board Stringed instruments aregenerally connected by rigid supports to someform of base. The string itself transfers lit-tle sound energy into the surrounding medium.The vibrations are actually transferred to base orboard, which is more suitable for transmittingenergy into the surrounding medium. There-fore, a much larger vibrating area is in contactwith the surrounding medium, and the rate atwhich energy is transmitted is greatly increased.

sound in solid/liquid This scenario is morecomplex than acoustic waves in a gas and in-volves both shear and longitudinal stresses andstrains in the medium. The velocity of longitu-dinal wavesV , in an isotropic solid is

V =√

(B + 4)/3(G)/ρ .

WhereB is the bulk modulus,G, the shear mod-ulus, andρ, the density of the solid.

sound intensity The rate of flow of energyacross a unit area perpendicular to the directionof propagation. Also known asenergy flux.

sound intensity level Measure based on com-paring two intensities. It is a logarithmic func-tion, and the units are in decibels:

SL(dB) = 10 log I/I0 ,

whereI is the intensity at the threshold of hear-ing 10−12 W/ m2 at 1000 Hz.

sound level meters Instruments used formeasuring loudness. They generally consist ofa sensitive microphone of good stability, a lin-ear amplifier, one or more attenuators, a setof frequency-weighting networks and an indi-cating meter. Electrical voltages correspond-ing to sounds picked up by a microphone arefirst amplified and then passed through a suit-able frequency-weighting network, which en-sures the readings of sound level on meter cor-respond to observed loudness levels, to operatethe indicating meter.

sound, range of audibility The human ear issensitive to a range from about 30 Hz to 10,000Hz. Seeaudibility, limits of.

sound spectra The results of sound analy-sis are often represented with the frequencies ofthe harmonics represented on the horizontal axisand the amplitude of each particular harmonicrepresented on the vertical axis. Certain instru-ments will have specific spectrum.

Sound spectra.


sound synthesis Refers to the fact that asound of any quality can be regarded as a mixtureof harmonics of appropriate relative amplitudes.

source code Source coding represents an in-tentional condensation of a data record’s sym-bol rate to maximize coding compaction (i.e.,minimize data rate) with minimum distortionin the information content. The source coderepresents a compact (and thus more efficient)representation of the data (e.g., speech, images,videos) provided by the information source byremoving the inherent redundancy in the origi-nal data stream. This source coding compres-sion is performed prior to any additional pro-cessing by the communication system and isindependent of the particular communicationssystem to be used for transmission or storage.Source coding stands in contrast with channelcoding, which refers to the additional processingon the source code performed by a communica-tion system to render the transmission more ro-bust against channel distortions. At the receiver,the channel decoder will undo the channel cod-ing, followed by a source decoder to undo thesource coding to recover the original transmittedinformation.

source resistor An equivalent resistor, whichis contained in a power source.

space biology As part of its scope, spacebiology considers the study of the biological ef-fects of space travel on living organisms. Asthe studies involve longer and longer periods ofspace flight, space biology is more and moreconcerned on how well and for how long humansand other forms of life can withstand conditionsin space. Space biology is especially keen in de-tecting and studying re-adaptation on earth oncespace flight has ended.

Specific aspects of space travel under studyinclude weightlessness, inertial forces experi-enced during lift-off, radiation exposure, the ab-sence of day-night cycle (biorhythm), and heatproduced within the spacecraft.

The absence of the night-day division of timehas effects on the cyclic patterns of changesin physiological activities that are synchronizedwith daily, monthly, or yearly environmentalchanges. These include circadian rhythms that

respond to light and dark stimulus (opening andclosing of flowers, nighttime increase in activityof nocturnal animals). Circadian rhythms alsoinclude changes in blood pressure and urine pro-duction during the 24-hour period. In general,internal rhythms that are synchronized to exter-nal stimuli (light, temperature, etc.) may be af-fected and gradually drift out of phase from theoriginal earth environment.

space biology, cardiovascular effects Be-cause gravity plays a major role in determiningthe distribution of ventilation, blood flow, gasexchange, alveolar size, intrapleural pressures,and mechanical stresses within the human lung,the absence of gravity affects the cardiovascularsystem.

The removal of the force of gravity in thelungs during space flight has been linked to in-crease in diffusing capacity, pulmonary capil-lary blood volume increase, and increase inmembrane diffusing. This means that the lungsare much more uniformly soaked with blood.

Lack of gravity also has important effects oncardiac filling pressure and intravascular fluiddistribution. Without gravity, a major centralfluid shift occurs. Current data is contradic-tory regarding the behavior of the central ve-nous pressure, where there are discrepancies be-tween ground-based models and in-flight mea-surements.

space biology, effects on vision Althoughthere are no strong effects of space flights on vi-sion, there are changes regarding the ability toorient and position the body relative to the en-vironment. The knowledge of the relative posi-tion of the body is a function that involves inputfrom the eyes and inner ear, as well as receptorsin the muscles and joints. In space, the body isleft with input only from vision and the innerear.

In the inner ear, the vestibular system com-posed of the semicircular canals and the otolithsprovides information about the sense of rotationand tilting movements. The information com-ing from these systems are used to control eyemovements as a function of postural reflexes.For example, if the head is suddenly rotated, thevestibular system sends information to the brainto help stabilize the position of the eyes while


the head is moving. Also, information about thefinal position of the body is analyzed. All ofthese functions are affected in space since theinner ear needs gravity to properly measure thedifferent orientational changes. This may trans-late to symptoms of dizziness and nausea.

space biology, mechanical effects Long-term space flights negatively affect the structureof muscles and muscular function. Impairedmusculoskeletal functions as well as poor mus-cular coordination have been reported by astro-nauts after returning to earth. This effect maybe due in part to muscle atrophy, that is, the lackof the stimuli provided by the constant pull ofgravity. The decrease in muscle strength mayalso be due to other factors that relate to neuro-muscular control and changes in the contractileforce of individual muscle fibers.

There are also changes in the bone structure.Because the human skeleton is constantly undera renewing process, disruptions may cause im-balance in the growth-resorption process of thebone. Reduced mechanical workloads on thebody in space may induce the skeleton to dis-card bone it no longer needs. Also, hormonalchanges may be linked since some hormones(e.g., parathyroid hormone) are strong stimula-tors of bone resorption.

space biology, weightlessness Space stud-ies have revealed that space flight participantsinitially suffer from symptoms such as nausea,sensory disorientation, and poor muscular co-ordination. Prolonged flight and exposure toweightlessness affects physical functions thatare a direct result of gravity-driven adaptations.These include and affect weight-bearing mus-cles, gravity-sensing portions of the inner ear,and blood pressure.Seespace biology, me-chanical effects; space biology, cardiovasculareffects.

space charge Electric charges that distributein a material or an electronic device.

space craft A vehicle used for traveling inspace.

space-division multiplexing A form of mul-tiplexing where the data paths are separated in

space. The multiplexing is usually done withswitching where data samples in the switch areon separate data circuits.

spark The ionization and rapid discharge inair and other insulators placed between two con-ductors that produce a sufficiently high electricfield. A sharp snapping sound or a loud explo-sive noise is emitted depending on the lengthof the gap between the conductors. For exam-ple, thunder is a result of a very large sparkdischarge between the earth and clouds. Undersufficiently intense fields, such discharges mayalso take place in liquids and solid dielectrics,in which case the insulating property of the ma-terial will be impaired.See alsolightning flash.

spark gap Generally, two electrodes sepa-rated by a dielectric that breaks down into a sparkdischarge at a voltage determined by the typeof dielectric and the distance between the elec-trodes. It is mainly used as a voltage limitingsafety device (e.g., lightning arrester), genera-tor, of electromagnetic waves, and as a meansof depositing concentrated energy, e.g., sparkerosion.See alsolightning arrester.

spark sound A powerful electric spark canbe the source of a single intense pressure pulse,or, if the electric circuit is tuned, the spark maybe oscillatory with accompanying pulses of al-ternating pressure. The pressure effects producerarefactions and compressions (acoustic waves),which may be sonic or supersonic accordingto the frequency of the electrical oscillations inspark discharge.

speaking arc Seesinging arc.

specific charge The charge to mass ratio of anelementary particle is called thespecific chargeof the particle. For example, the specific chargeof an electron is1.759×1011 coulomb/kg whilethe specific charge of a proton is9.578 × 107

coulomb/kg.

specific conductance The ratio of the elec-trical current to the electric field in a given ma-terial, also known asconductivity.


specific heat at absolute zero The third lawof thermodynamics states thatthe entropy of allsystems and of all states of a system is zero atabsolute zero.This implies that the specific heatcapacity at constant volume and at constant pres-sure goes to zero as temperature goes to absolutezero.

specific heat at low temperatures The spe-cific heat capacity of all solids decreases withdecreasing temperature and at 4.2 K for a typ-ical solid it will be between 10−3 and 10−4 ofthe value at room temperature. The very lowheat capacity at low temperatures also meansthat only small cooling powers are needed inorder to further decrease the temperature of asample, for example, in a dilution refrigerator.However, at these low temperatures, other con-tributions may become significant from impuri-ties. SeeSchottky anamolies.

specific heat, cooperative, anomaly Spindisorders in the paramagnetic state give an in-verse relationship with temperature for specificheat at high temperatures. This spin disordershould vanish at 0 K, which can be due to Schot-tky peak or a cooperative singularity. The for-mer occurs in dilute systems. The latter is morelikely if the exchange interactions are strong,thus causing transition to the ferromagnetic andantiferromagnetic states.Seesuperconductors;specific heat.

specific heat, Debye’s theory Model for lat-tice heat capacity at low temperatures that cor-rects for inadequacies of the Einstein’s modelwhere observation shows aT 3 dependencerather than aT−1 dependence. Debye assumedthat a crystal havingN lattice points could beexcited by3N acoustic vibrational modes. Thefactor of three here refers to the three polariza-tions associated with each wavevector. Debye’stheory predicts that the lattice heat capacity is auniversal function scaling for all solids throughthe parameter known as theDebye temperature.

specific heat, electronic Heat capacity is ofparticular significance at low temperatures whenthe electrons at the Fermi surface are involved.The electronic-specific heat capacity gives ameasure of the electrons at the Fermi surface.

The electronic-specific heat has a shallower tem-perature variation than thermodynamic-specificheat and only begins to dominate for most metalsbelow a few Kelvins.

specific heat, Schottky The heat capacityof all solids decreases with decreasing temper-ature. However, impurities in a crystal at theselow temperatures contribute to the heat capac-ity significantly. This is theSchottky anomalyand can increase the heat capacity between 10to 1000 times the lattice value.

specific impedance, acoustic The ratio of theacoustic pressure in a medium to the associatedparticle velocity. It has a magnitudeρc, whereρ is the constant mean density of medium at apoint andc is the velocity of propagation of thewave.

spectacles A pair of ophthalmic lenses ofsuitable power mounted in a frame to correctfor ametropia.

spectra, acoustic The variation of intensitywith frequency which can be represented graph-ically. Seesound spectra.

spectra, band Bands of semi-continuousspectral lines separated by gaps and associatedwith transitions between molecular energy lev-els. Each band is actually composed of manyfine lines, corresponding not only to electronictransitions but also to vibrational and rotationalenergy levels.

spectra, continuous Composed of an infinitenumber of spectral lines corresponding to the in-finite number of wavelengths present, e.g., in atrue white light source. One of the most com-mon sources of continuous spectra is the emis-sion spectrum for a heated solid. The standardfor such spectra is the black body which absorbsand emits all wavelengths completely.

spectra, ghost Artifacts of periodic errors inthe spacing of the lines of a grating, that give riseto ghost lines accompanying the principal max-ima. ForRowland ghosts,the error has a singleperiod and the ghosts are symmetrical about theprincipal maxima.Lyman ghosts,involving two


incommensurate periods or a single error of veryshort period, are much harder to identify.

spectra, line Correspond to images of a spec-trometer slit illuminated by one or several distin-guishable wavelengths. Since only individualatoms give true line spectra, emission spectralsources are gaseous in nature and may be ex-cited by arcs, sparking, flames, etc. For someelements, spectral lines are arranged in series,being related to the distribution of the atomicenergy levels. The simplest and most famous isthe Balmer series for hydrogen.

spectral luminous efficacy The ratio of theradiant flux at a given wavelength in the visiblespectrum divided by the radiant power at thatwavelength. It rises to a maximum value for astandard observer of 683 lumens per watt nearthe middle of the visible spectrum. The param-eter gives a measure of the luminous flux that isproduced by unit radiant power.

spectra, vibrational Spectra lines or bandscorresponding to transitions between molecularvibrational levels. For diatomic molecules, thevibrational levels can be treated in the harmonicoscillator approximation

En = hv

(n+

12

),

with vibrational quantum numbern = 0, 1, 2.The frequencyv is a measure of the spring con-stant for interatomic forces. Transitions be-tween vibrational energy levels can be observedby infrared absorption or by Raman scattering,with selection rule∆n = ±1. These selec-tion rules are modified by anharmonicity, whichchanges the form of the potential function andallows overtone transitions∆n = ±2,±3, . . .Stacks of rotational levels are associated witheach vibrational level and transitions betweenthese levels are governed by∆J = ±1 whereJis the rotational quantum number. Such transi-tions give rise to vibration rotation band spectra.

For polyatomic molecules, one must considerthe normal modes of vibration and stretching.The selection rules have additional requirementsthat there be a change in dipole moment for in-frared and a change in the induced dipole mo-ment for Raman.

The selection rules are determined by thesymmetry of the molecule, which can in turn beinferred from infrared and Raman spectra. Thedifferent vibrations can be represented by po-tential energy surfaces and anharmonic effectsin the potential mix the different modes. Use ofmodel potential functions allows interpretationof the vibrational spectra and hence a completedescription of the vibrational modes.

spectrometer An instrument to measure thewavelengths or radiant intensities of the spec-trum of a light source, the design depending onthe wavelength of radiation. For example, oneuses glass prisms and transmission gratings inthe visible region, rock salt prisms for the in-frared region, and reflection gratings for ultra-violet and X-ray regions.

spectroscope An instrument or system thatenables us to see the spectrum of a light source,usually consisting of a slit, a collimating lens, aprism or grating and a telescope.

spectroscopy The study of the absorption,emission or scattering of electromagnetic radia-tion by matter.

spectroscopy, electron The collection oftechniques involving the spectral analysis ofelectrons from a sample taken from a beam ofincident photons or electrons. There are manydifferent types, principally photoelectron spec-troscopy (PES, UPS or MPS for ultraviolet exci-tation, and ESCA or XPS for X-ray excitation),Auger electron spectroscopy (AES), and others.

The applications in chemistry and surfacestudies are numerous, where the techniques havebecome standard analytical laboratory tools.PES is a powerful technique for determiningelectronic binding energies in various speciesleading to determination of electronic structureof molecules and solids. XPS and ESCA arecomplementary techniques to PES for studyinginner core energy levels, and all are importanttechniques for studying surfaces and surface re-actions. AES is an extremely useful techniquefor element detection on surfaces at extremelylow coverage.


spectroscopy, interference Spectroscopybased on examination of light by an interferome-ter. A light beam with a given wavelength is splitinto two or more components which are thenrecombined coherently such that their phase ispreserved, giving rise to a set of interferencefringes. Since each wavelength has its own setof fringes, spectroscopy can be carried out.

In a two-beam interferometer the incidentbeam is split so that the light follows two sep-arate paths. The most well known two-beaminstrument is theMichelsonwhich can also beused for accurate measurement of displacement.In a similar instrument, theMach-Zender inter-ferometer,a sample to be studied is put into oneof the light paths and changes in the interfero-gram are observed.

The most important interference spectrome-ter based on multiple beam interference is theFabry-Perot,where interference takes place ina thin etalon composed of two glass plates. Forhigh reflectance plates, the fringes are extremelysharp so that the instrument is useful for study-ing the fine structure of spectral lines.

spectroscopy, laser The use of a laser sourcein a spectrometer to replace traditional sourcessuch as the mercury arc lamp. The main ad-vantage of the laser is that it is an intense, al-most perfectly monochromatic source. The highintensity allows studying weak lines that areunobservable with other sources. It also per-mits development of special techniques such asnon-linear optical spectroscopy and numerousvariations of Raman spectroscopy. The highmonochromaticity of the laser permits higherresolution in the spectrometer.

spectroscopy, Mossbauer Based on theMossbauer effect, which is the resonant absorp-tion of nuclear gamma radiation in a condensedmedium. The Mossbauer spectrometer is basedon the measurement of the transmission of reso-nant gamma rays through the sample as a func-tion of energy. This is accomplished by sweep-ing the Doppler velocity of the sample with re-spect to the source. Since the effect is char-acterized by the absence of recoil or thermalbroadening due to transfer of recoil energy di-rectly to the lattice, the spectrum is best mea-sured for strongly bound nuclei at low temper-

atures. Mossbauer spectra may be measured intransmission and also by scattering, the latterallowing study of surface effects.

There are many applications of Mossbauerspectroscopy in physics, technology, chemistryand biology. These include study of surfacefilms, metal layer interfaces, amorphous ribbonsand wires, diffusion in intermetallic alloys, in-dustrial glasses, high pressure effects, chemicalisomer shift, chemical bonds, proteins and manymore.

spectroscopy, NMR The phenomenon of ab-sorption of electromagnetic energy by magneticnuclei of the nucleus, particularly protons, inthe presence of an external applied field. Tran-sitions between the proton levels at a given fieldcan be obtained by applying an appropriateRFfield at the proton resonant frequency. The ex-periment is generally done at a fixedRF fre-quency and by sweeping the magnetic field.

Applications include: (a) determination ofthe “chemical shift” in the resonance due to thechemical environment; (b) determination of finestructure, due to spin-spin coupling, used forthe identification of unknown molecules; and(c) line width determination, which gives infor-mation on the molecular motion.

Magnetic resonance imaging in magneticfield gradients for medical diagnostics is one ofthe most important commercial applications.

spectrum, absorption Obtained by passinglight from a source with a continuous emissionspectrum through a target, absorbing materialand thence into a spectrometer.

spectrum analyzer A device used to de-termine the spectral components of radiationpresent in a designated range of the electromag-netic spectrum. The technology used dependson the specified frequency range; for radio fre-quencies, an electronic device is used while forthe visible range, a diffraction grating spectrom-eter is appropriate.

An optical spectrum analyzer uses theacousto-optic effect to analyze the frequencycontent of an electrical signal. The latter is con-verted into an acoustic signal by a piezoelec-tric transducer and the acoustic wave diffracts amonochromatic beam of light incident upon it.


The form of the resulting diffraction pattern de-pends on the frequency content of the acousticwave from which the latter can be deduced.

spectrum, angular The energy spectrum ofa given phenomenon as a function of angle withrespect to a reference direction.

spectrum, color Seespectrum, visible.

spectrum, electromagnetic The entire rangeof electromagnetic radiation. At low frequen-cies (long wavelengths), there is no limit to thewavelength; values up to 50 million km havebeen observed in radio astronomy. Increasingfrequency covers radio waves, microwaves, vis-ible spectrum, ultraviolet, X-rays andγ-rays.

spectrum, emission The spectrum obtainedby observing the radiation emitted from an op-tical source.

spectrum, normal In the development ofthe classification of atomic spectra by Kayserand Runge in the nineteenth century, the spec-trum of iron was chosen as a standard or normalspectrum that could be used as a reference forthe spectral series of other elements. This workled to the establishment of a series of empiricalrules for the positions of the spectral lines of theelements.

spectrum, secondary The focal length of anoptical imaging system can be achromatized in acertain region of the visible spectrum such thatthe rate of change of focal length with wave-length is zero for that part of the spectrum. Notall wavelengths in the spectrum will have this fo-cal point, leading to a colored zone around theimage point, known as thesecondary spectrum.

spectrum, spark The emission spectrum ob-tained by an electrical arc or spark in a gas.When a condenser is connected across the gap,a very rich spectrum characteristic of the metalof the electrodes is obtained. Most emissionspectra are currently produced by an inductivelycoupled plasma source.

spectrum, visible Corresponds to thosewavelengths to which the eye is sensitive. This

corresponds roughly to the wavelength range400 to 700 nm, with violet (shorter than 450nm), blue (450 to 490 nm), green (490 to 560nm), yellow (560 to 590 nm), orange (590 to630 nm) and red (longer than 630 nm).

spectrum, X-ray Produced when a metalis bombarded with high energy electrons. Itconsists of a series of sharp lines superposedon a continuum. The continuum, calledBrem-strahlung, is classical in origin, and is causedby radiation from decelerating electrons in themetal. The discrete spectral lines are due to ejec-tion of electrons from the inner atomic shellsand the falling of another electron into the holeleft behind, accompanied by the emission of anX-ray. Hard X-rays are associated with tran-sitions to theK atomic shell. Softer X-raysof longer wavelength are associated with transi-tions to theL atomic shell.

speech, analysis of Microphones connectedto an amplifier can be used to determine the dis-tribution of frequencies in speech.

spontaneous ordering This refers to the or-dering of magnetic spins at the Curie tempera-ture Tc as a result of the exchange interactionbeing able to overcome the thermal randomiza-tion. The figure shows examples of disorderedand ordered states respectively.

Disordered and ordered states.

sputtering Charged radicals split out. Thisprocess is usually employed for depositing ma-terial on films. The charged radicals bombardthe target material and produce certain radicalsthat deposit on the substrate. A film is formedon this substrate.


square wave A periodic wave whose ampli-tude has two values. The shape of the wave issquare.

Square wave.

squid Refers tosuperconducting quantuminterference device.It is made up of a ring ofsuperconducting material with a weak link. Themagnetic flux through this ring is quantized inunits ofh/2e whereh is Planck’s constant ande is the electron charge. Can be used to measuremagnetic flux very accurately and forms the ba-sis of a magnetometer that is able to measuremagnetic moments extremely accurately.

squid, medical use of Superconductingquantum interferometric devices (SQUID) workby detecting minute magnetic fields. Their useis widely spread since magnetic fields appearanywhere there is an electrical current.

Squids are usually circular or square-shapeddevices of less than a millimeter in height. Asquid consists of a superconducting ring orsquare interrupted in two places by Josephsonjunctions (insulating links connecting two su-perconductors). When sufficient electrical cur-rent is applied to the squid, a voltage is generatedacross its body. In the presence of a magneticfield, this voltage will rise or fall in proportionto the magnetic field.

Magnetometer squids are coupled to a “fluxtransformer” that functions to amplify the mag-netic field input. Gradiometer squids consist oftwo loops that give signals from two differentpoints in space. In the latter, a flux in the squidonly appears if the field is not the same at thetwo points. This apparatus is thus sensitive tonon-uniform fields.

Modern squids can be approximated as closeas 20µm to samples. In this way, squids can beused as “microscopes” on living organisms and

other specimens that cannot be frozen. It hasuses in the study of biomagnetic phenomena.Namely, migration patterns of bacteria and mi-crobes that possess magnetite particles in theirbodies can be tracked down through the environ-ment. These studies play a role in bioremedia-tion, which proposes using bacteria to converthazardous waste into inert byproducts. In ad-dition, coupling of low-Tc squids to NMR helpunderstand the chemical environments of atomsand molecules.

Squids are widely used to detect magneticfields from the beating heart (magnetocardiogra-phy). In practical terms, they can be used in themeasurement of irregularities in the heartbeat ofan unborn child. Squid-based apparatuses serveas diagnostic tools in the early detection, diag-nosis and follow-up of ischemic heart disease.Also, they serve as clinical tools for arrhythmialocalization. They also serve to detect and ana-lyze brain signals in magneto-encephalography.

stability The quality of being stable.

stabilization Making stable.

stabilized power supply A power source thatoutputs a constant power.

stable states Firm states that do not changewith time under certain conditions.

standard cell A primary cell used for in-strument calibration because of its very accu-rately known and constant potential difference.A standard cell differs from other secondarycells in that the electrochemical reactions arenot reversible for a standard cell.

standard illuminant A blackbody radiatorworking at the freezing point of platinum withthe luminous intensity per square centimeter de-fined as 60 candelas. To measure colors of nonself-luminous samples, three standard luminantsA,B andC are used. The sourceA is a tungstenfilament lamp of specified characteristics oper-ating at a color temperature of 2575 C. Thestandard illuminantsB andC are obtained byusing suitable filters withA so as to mimic noonsunlight and normal overcast daylight, respec-tively.


t

standing waves Boundary conditions in anyscenario, e.g., for a string with two fixed ends,points of zero displacement cause the string tovibrate with nodes at the ends. The fundamentalor the first harmonic consists of two nodes and anantinode. The points of maximum amplitude ofvibration are calledantinodesand spaced evenlybetween the nodes. The distance between nodalpoints for the nth harmonic mode of vibration isseen to bel/n.

star connection Another commonly usedthree-phase circuit for an AC motor. Here, fourwires are employed. The star-connection is alsocommonly used in a two-phase, four-wire mo-tor as shown in the figure below. It has the sameadvantages as in the delta connection.

Star connection.

stark effect The splitting of spectral linesin the presence of a strong electric field. It isanalogous to theZeeman effectwhere lines aresplit by a magnetic field. The strong electricfield can be produced by an electrical dischargein a gas at low pressures.

state attribute The state attribute refers to thecharacteristics of a particular state of matter. Forexample, simple solids possess crystalline sym-metry that indicates short- as well as long-rangecorrelations in their positions; i.e., given the po-sition of a point, subsequent positions of the rest

of the crystal can be generated starting from thelattice constant. This is due to the regularity ofthe crystal. In addition, the solid has discrete ro-tational symmetries, depending on the geomet-rical configuration of the unit cells. On the otherhand, simple liquids and gases have symmetriesthat correspond to fluids in the sense that theyposses only short-range positional correlationsthat do not continue to long distances.

The thermodynamic attributes are given bythe value of the state variables of the substance,and the different phases can be separated intodomains, as described by their state diagram.Seestate variables; state diagram.

state diagram The state diagram of a ther-modynamic system is a diagram that denotes thedifferent equilibrium states of matter as a func-tion of a thermodynamic state variable. Two-dimensional representations of state diagramshave to be drawn by having one dependent andone independent state variable, with all the restfixed at particular values.

The different states (solid, liquid, gas) of thesubstance whose state diagram is represented aredelimited by lines that indicate where the tran-sition from one state to the other occurs. Thelines are calledlines of first order transitionbe-cause they involve non-analytic behavior in thestate variable, as opposed tolines of second or-der that involve non-analytic behavior in the firstor higher derivatives of the state variables.

As an example, the P-V state diagram for asimple liquid is given in the figure, where thelines separate the different equilibrium states ofthe system. The diagram is drawn for a particu-lar fixed volume of the system. The gas and liq-uid transition line terminates at a point called acritical point. The tricritical point is the point atwhich all of the phases can coexist. Note that atsufficiently high temperature, a continuous pathcan be drawn from the gas to the liquid (and viceversa) because there are no fundamental differ-ences in the symmetry between each other, onlyin their density.

state variables In the study of thermody-namics, the systems that are considered are inequilibrium. The equilibrium state is character-ized by being a state stable against internal fluc-tuations in temperature, pressure, chemical po-


State diagrams.

tential, and composition among other variables.The equilibrium requisite implies that all of thevariables have definite values at the equilibriumand thus they are called state variables. Ther-modynamics provides a series of relations be-tween these state variables that make it possibleto calculate the value of any other variable atequilibrium.

State variables are classified as eitherexten-siveor intensive.The value of an extensive vari-able depends on the size of the system (e.g., vol-ume). In contrast, intensive variables are notdependent on the system size or number of par-ticles (e.g., temperature). Each state variablehas a complementary or conjugate variable ofthe other type. The variable complementing thevolume is the pressure, while the variable com-plementing the composition of a component isits chemical potential.

stationary channel A stationary channelrefers to a communication channel whose fre-quency response does not vary with time. Sucha channel may be represented as a time-invariantfilter.

step function A function that only has twovalues. The change from a low value to a highvalue is abrupt.

Step function.

stereophony Necessitates the coinciding ofacoustic image with visual image regarding thedirection of sound and the production of stereosound. Systems should possess a frequencyrange that includes all the audible componentsof the sounds being reproduced, a distortion-free intensity range that embraces the intensityrange associated with the recording sounds, andshould be able to preserve the spatial sound pat-tern of the original sound as well.See alsoquadraphonic sound.

stereopsis Seeacuity, stereoscopic.

stereoscope An instrument that separates thefield of view of the two eyes (usually by op-tical methods) such that only certain portionsof the stereogram targets viewed through it areseen by one eye and other portions by the othereye. Together, they give rise to a binocular pre-cept of depth. Common stereoscopes includetheWheatstoneandBrewster stereoscopes.

stereoscopic effect The principle of estab-lishing a three-dimensional image by an opti-cal system using a binocular eyepiece for botheyes. The effect is of great practical importancein several devices, such as prism binoculars andbinocular microscopes. The effect is based ontwo slightly differing images seen by the twoeyes; based on normal experience, the brain isable to construct a mental three-dimensional im-age and obtain depth information.

Stern-Volmer kinetics Stern–Volmer kinet-ics describe how the fluorescence lifetime ofphotochemical systems decay in the presence ofan interacting environment. The fluorescencedescribed here comes from a chemical reactionwhose driving energy is light. Lifetimes of thedecay determine the reaction mechanism as wellas the electronic structure of the excited statesof organic and inorganic molecules.


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The simplest way to determine the fluores-cent lifetime of a compound is to excite it witha short pulse of light and measure how the fluo-rescence intensity decays with time. For simplemolecules, the fluorescent intensity decays asI(t) = Io exp(−t/τ), whereI(t) is the inten-sity, Io, the intensity at time 0,t is time, andτis the lifetime.

When the initial sample is analyzed in thepresence of other molecules, they might in-teract. In typical photochemical experiments,the sample is “quenched” by the surroundingmolecules that decrease the lifetime of the fluo-rescent molecule. If the interaction between thetwo types of molecules is of collisional nature,the lifetime dependence on the quencher con-centration is given by the Stern–Volmer equa-tion asτo/τ = 1 + kqτo[Q] whereτ is the newlifetime in the presence of the quencher,kq is thebimolecular quenching rate constant, and[Q] isthe quencher concentration.

Applications of fluorescence lifetimes findtheir way into areas of biomedical researchwhere fluorescent probes are used to studybiomembranes, enzymes, photosynthetic sys-tems, nucleic acids, and malignant tissues. Ithas also found applications in the study of semi-conductors, laser dyes, polymers, and other ma-terials.

Stokes parameters Used to give a com-plete, quantitative representation of polarizedlight. For a monochromatic wave propagatingin the z direction

E(t) =

Ex(t) +

Ey(t)

Ex(t) =i Eox(t) cos [(kz − wt) + δx(t)]

Ey(t) =j Eoy(t) cos [(kz − wt) + δy(t)] ,

then the Stokes parameters are defined as:

S0 =⟨E2

ox

⟩+⟨E2

oy

⟩S1 =

⟨E2

ox

⟩−⟨E2

oy

⟩S2 = 〈2EoxEoy cos δ〉S3 = 〈2EoxEoy sin δ〉 ,

whereδ = δx − δy.It follows thatS2

0 = S21 + S2

2 + S23 so that

S0 is proportional to the intensity of the wave.

A convenient geometrical representation is thatS1, S2 andS3 can be regarded as Cartesian co-ordinates on the surface of a sphereP of radiusS0, called thePoincaré Sphere.Then any possi-ble polarization of the wave can be representedby a point on the surface ofP .

stop Any kind of diaphragm to restrict theamount or angle of light passing through anypart of the optical system.

stop, aperture Any diaphragm or rim of thelens or anything else that restricts the amount oflight reaching the image. In addition to limit-ing the light gathering power, it also limits theresolving power of the system. The aperturestop could be located anywhere inside or out-side the optical system. Its image in the objectspace is called the “entrance pupil”. For an opti-cal system with many components, the effectiveaperture stop for the whole system is the stopwhose entrance pupil subtends the smallest an-gle as seen from the object point.

stop, field Any device that limits the lateralsize or the angular breadth of the object that canbe imaged by the system. One usually employsa diaphragm in the image plane to act as a fieldstop (e.g., the opening in the film holder of acamera). Thus, the field stop is conjugate to theobject. Often, to retain only the good qualitypart of the image (by avoiding the contributionsfrom the far off-axis rays), an aperture is insertedas a field stop in an optical system.

storage vessels, liquefied gas The efficiencyof storage of any liquefied gas is related to itslatent heat of vaporization, which controls theevaporation rate as a function of extraneous heatinflow and the normal boiling point which gov-erns the extraneous heat inflow from the sur-roundings. Glass, usually pyrex, dewar flasksare used for liquid air. Metal dewars have the dif-ficulty of outgassing them completely and there-fore require an internal adsorbent trap. Powderand foam insulation are useful. It should alsobe noted that metal dewars have a larger heatleak with liquid hydrogen in them than with he-lium. It should be noted that helium gas diffusesthrough glass.


stored energy in capacitor When a batteryis connected to a capacitor, the positive chargesflow into one terminal of the capacitor while thenegative charges flow into the other, until thepotential difference between the plates reachesthe EMF of the battery. In the process, energyis converted from the chemical energy in thebattery to the electric energy stored inside thecapacitor,E = 1/2QV = 1/2CV 2.

stray capacitance Stray capacitance is theundesirable capacitance that exists between con-ductors carrying currents or charges. In generalstray capacitance is quite small and can be ig-nored in most cases, but for critical applications,it should be minimized.

stress waves Seeshear waves.

striking note Seesubharmonics.

stringed instruments Examples of pluckedstring instruments are the harp, guitar, mandolin,and banjo. A plucked string has the full har-monic series of overtones. The stiffness of anactual string and damping due to internal fric-tion causes a slight departure from the harmonicseries.

strings, vibrations in A string can be setinto vibration by plucking, striking, or bowing.Nodes are set up at the fixed ends, separated bylengthL, and the antinodes in between. Thereis the fundamental, or the first harmonic, whichsatisfies wavelengthλ = 2L, then second har-monicλ = L, third harmonicλ = 2/3L, andso forth.

stroboscope An instrument for studying pe-riodic or varying motion by illuminating movingbodies with a rapidly flashing electric light, thusmaking the rotating or vibrating bodies look al-most stationary.

subharmonics The presence of low fre-quency harmonics in the sound of bells, tuningforks and vibrating piezoelectric crystals, for ex-ample, that are not produced by a resonator orfrom resonances. It is also known as astrikingnoteand is possibly due to intermittent contact

between the bell and clapper or the tuning forkand surface, respectively.

subscriber A subscriber station is a tele-phone station that has access to the public tele-phone network. A telephone station is a tele-phone set connected to a telephone system.

subsonics Speeds less than the speed of soundin air or other medium. Frequencies lower thanthe auditory capacity of human ear.

subsonic whistle Whistles that blow at a fre-quency that is inaudible to human ear but can bepicked up by animals such as dogs.

subtracter A device that has two inputs andone output, the output being equal to the differ-ence of the two inputs.

sufficient (logical) A condition needed tofinish a logic operation.

SUM An operation that adds all input signals.

sunburn Reddening of the skin caused bythe exposure of the skin to the ultraviolet (UV)light (wavelengths between 290 and 320 nm)coming from the sun. Its effects range from amild redness to more severe reactions with addi-tional tenderness, pain, swelling and occasionalblistering formations. The effects are felt usu-ally within the first 6 to 12 hours, and peak inintensity within 24 hours. After a period of 3 to5 days, a tan develops that may last a few moredays. If the sunburned area is extensive, symp-toms such as nausea, headaches, fever, chills,and delirium may occur.

UV light can also reach the skin by reflectionfrom snow, sand, water, sidewalks, and grass.Even on a bright cloudy day with a thin cloudcover, it is possible to receive 60 to 80% of theUV light from a clear day.

Creams to protect against sunburn and theUV rays are calledsunscreens.They act as bar-riers, filtering out the transmission of particularwavelength ranges of light (e.g., zinc oxide oint-ment). Sunscreens are classified according totheir sun protective factor (SPF). An SPF ratingof 4 provides only limited protection, while a rat-ing of 8 provides maximum sunburn protection.


A rating of 15 gives ultra protection (absorbsburning as well as tanning rays). A disadvan-tage of most sunscreens is that they need to beapplied frequently during the day for protectionto be constant.Seesun exposure and skin can-cer.

sundial An instrument that indicates time ofday by the position of the shadow of a pointeror gnomon cast by the sun on the face of a cali-brated dial.

sun exposure and pigmentation Pigmen-tation, coming from the darkening of the skinafter sun exposure, is due to an increase in theproduction of melanin in the skin. The melanin(which absorbs UV light) is produced by the skinas a reactive and protective response to protectagainst further UV rays.

There are three major ranges in wavelengthfor the classification of UV rays: ultraviolet-A(UVA), ultraviolet-B (UVB), and ultraviolet-C(UVC). UVA has the longest wavelength withUVC the shortest. Because UVC gets absorbedby the ozone layer, only UVA and UVB are ofconcern to sun exposure. UVA stimulates ingreater degree the production of melanin due toits ability to penetrate into deep layers of theskin. Therefore, it is predominantly responsi-ble for tanning, although in sufficient exposurequantity may result in sunburn. UVB probablycauses most cases of sunburn and most skin can-cers, because it gets absorbed in the outer layersof the skin, burning without stimulating muchtanning.

sun exposure and skin cancer There is evi-dence that links excessive sun exposure to can-cer. This evidence even relates sunburns andcancer occurrences separated by years, meaningthat a sunburn may have repercussions later inlife. Not all of the types of skin cancer turn out tobe melanoma that can spread through the body,and may be surgically removed. Another con-nection that may lead to tumor formation comesfrom studies of the exposure to ultraviolet-B(UVB, shorter wavelengths than UVA) light thatshows interference with the immune system ofthe skin.

There is also evidence that a risk factor forsome skin cancers is light skin color. Cumu-

lative, long-term exposure, is associated withhigher rates of skin cancer, particularly in lightskinned people. Similarly, whites have a higherproportion of skin cancer in the body areas thatare routinely exposed to higher levels of sun(e.g., face, shoulders, noses, arms). People witha large number of moles, freckling, or a fam-ily history of melanoma have a higher risk ofskin cancers. People with dark skin color aresomewhat protected by the amount of melaninin their skin because their skin filters twice asmuch UVB as the skin of whites, although theamount of UV that gets transmitted to the dermallayers of the skin is still significant.

It is estimated that one person in five willdevelop skin cancer during their lifetimes, inwhich 90% of all skin cancers are due to sunexposure.Seesunburn.

superconducting circuits (1) Electrical con-nections to samples that one wishes to isolatethermally use niobium wires. Such wires are su-perconducting with a high critical field through-out the entire liquid helium temperature range.They combine low thermal transport with per-fect electrical conductivity. Lead wires as heatswitches at temperatures below 0.1 K are alsocommonly used in circuits. Superconductorsare used as switching device and memory stor-age elements in electronic computers.

(2) If a current is flowing in a circuit contain-ing a resistanceR and an inductanceL and anEMF is suddenly removed, the current falls to1/e of the original value with normal conduc-tors in about 10−5s. However, due to the al-most negligible resistance in a superconductor,the current persists long after the EMF has beenremoved, and in some cases a persistent currenthas been observed for a period of several years.

superconducting electrons This refers tothe electrons in a superconductor that are in theground state. In a superconductor at any finitetemperature, a dynamic equilibrium exists, thatis, normal electrons recombine to create pairs— the Cooper pair — continuously created bythe break up of pairs. Electron tunneling is acommon phenomenon when at least one of theelectrodes is a superconductor at a junction.


superconducting thin films These materialsoffer virtually zero resistance to electrical cur-rent and are extremely useful in microelectron-ics. They have the capacity to carry large cur-rents at significantly higher temperatures thanthe superconducting material. Thin films of nor-mal metals and superconductors in contact canform superconductive electronic devices, whichreplace transistors in some applications.

superconducting tunneling It is possible forelectrons to tunnel between a superconductingfilm and a normal one across a thin insulatingbarrier. Quantum mechanically, an electron hasa finite probability of tunneling through the bar-rier if there is an allowed state of equal or smallerenergy available for it on the other side. In thismanner, a direct measurement of the energy gapcan be made.

superconduction, Heisenberg’s microscopictheory of Explains how the influence ofthe strong intermolecular magnetic field causesspontaneous magnetization of ferromagneticsubstances. Heisenberg showed that the fieldoriginates in the quantum mechanical exchangeintegral. There is no classical counterpart of thisand it is associated with the difference in theCoulomb interaction energy of electrons whenthe spins are parallel or antiparallel. This modelhas also been successful at explaining the spinwaves at low temperatures.

superconductive alloys Examples of suchalloys include two parts gold and one part bis-muth, and rhenium with molybdenum which, asalloys, display supercondcuting properties.

superconductive compounds Seesupercon-ductors.

superconductive cylinder For the case ofa long thin cylindrical sample in a longitudinalmagnetic field, the magnetization curve for anideal superconductor is as follows: at the criticalfield, the flux penetrates the superconductor andthe normal state is restored.

superconductive elements Examples of su-perconducting elements are aluminum, cad-mium, gallium, indium, lead. Superconductiv-

Superconducting cylinder.

ity has been observed in metallic substances forwhich the number of valence electronsZ lies be-tween about 2 and 8 and the critical temperatureTc shows a sharp maxima for transition metalswith Z = 3, 5, and 7.

superconductive sphere The magnetizationcurve for a sphere is as seen below. Betweenthe statesHp, the penetration field, andHc, thecritical field, the superconductor is in the inter-mediate state. This is a geometric effect. Sincethe sample has broken into alternately normaland superconducting states, the magnetic flux isable to pass through the normal state.

Superconductive sphere.

superconductivity A phenomenon shown bycertain metals, alloys, and other compounds ofhaving negligible resistance to the flow of elec-tric current at temperatures approaching abso-lute zero. Each material has a critical temper-atureTc, which generally is under 10 K, abovewhich it is a normal conductor and below which,a superconductor. Recently, some materialshave been shown to be superconductive at tem-peratures hundreds of degrees above absolutezero.

superconductivity, BCS theory A theoryof superconductivity proposed by the Ameri-can physicists John Bardeen, Leon N. Cooperand John R. Schrieffer in 1957 for which the


three were awarded the 1972 Nobel Prize inphysics. The theory describes superconductiv-ity as a quantum phenomenon, in which the con-duction electrons move in pairs and thus showno electrical resistance.

superconductivity, destruction by currentsHigh current densities cause a superconductorto be restored to its normal or resistive state.When a transport current flows in a supercon-ductor that is in the mixed state, a Lorentz forceacts on the flux lines so as to move them at rightangles to their axes and to the direction of cur-rent flow. Movement of magnetic flux causes avoltage to be induced across the sample in thesame direction as the current flow. This causesthe superconductor to become resistive, and heatis dissipated. The motion can be reduced if theflux lines are pinned by defects such as disloca-tions, grain boundaries, vacancies and clustersof impurities in the crystal structure. This canbe achieved by work hardening.Seework hard-ening.

superconductivity, electrodynamics of Fora mathematical description of the response of asuperconductor to an applied DC magnetic field,one needs to take into account perfect conduc-tivity, the Meissner effect. The equations usedin conjunction with Maxwell’s equations predictthat the magnetic flux is excluded from all ex-cept a surface region of the bulk superconductor.The decay of the field at the surface has a simpleexponential form with a characteristic length. Ina normal metal the eddy currents produced byan induced current are quickly reduced by thescattering process. However, these currents onan atomic scale persist and give rise to a weakdiamagnetic susceptibility in all materials.

superconductivity, transition of The transi-tion to superconductivity occurs in several met-als, alloys and compounds when they are cooledto below the transition temperatureTc. There isa drastic change in electrical and thermal proper-ties. The DC electrical resistance disappears inthe new phase. The transition to superconduc-tivity is a function of temperature and appliedmagnetic field.

superconductivity, transition, pressure effectIt has been found that the transition temperatureTc below which a substance will behave as a su-perconductor is a function of pressure and stressgenerally. However, pressure is not a dominat-ing factor, and the density of the electron statesat the Fermi surface is more important as wellas the interaction between electrons which arisefrom coupling with the lattice vibrations.

superconductivity transition, resistive Inthe transition to superconductivity, the most sig-nificant feature is the disappearance of resis-tance to DC current.

superconductivity, two-fluid model Thismodel is based on the postulate that a supercon-ductor possesses an ordered ground state thatis characterized by an order parameter, and thatthe entire entropy of the system resides in the ex-cited states at energies above that of the groundstate. Superconducting properties are associ-ated with the superfluid fractionf of the con-duction electrons in the ground state, while theremaining “normal” fraction (1− f ) retains theproperties of electrons in the normal state atT > Tc. The two components are totally inter-penetrating and non-interacting. This model isuseful in that it provides a simple physical pic-ture providing a semi-quantitative understand-ing of, and the interrelation between, thermaland magnetic properties.

superconductor, Gibb’s free energy in Thetransition to a superconducting state is a func-tion of temperature and applied magnetic field.In pure samples, the transition is reversible andcan be described by equilibrium thermodynam-ics. The condition for equilibrium is found byminimizing the magnetic Gibb’s free energy.

superconductors Materials that may be met-als, alloys or compounds that display the phe-nomenon of no resistance to the flow of anelectric current below critical temperaturesTc.Superconductors also exhibit strong diamag-netism, meaning that they are repelled by mag-netic fields. Superconductivity is manifestedonly below a certain critical temperatureTc anda critical magnetic fieldHc, which vary with thematerial used.


superconductors, critical field of Below thecritical temperatureTc, the superconducting be-havior can be quenched and normal conductivityrestored by the application of an external mag-netic field — the critical fieldHc.

Hc ∼Ho

(1− (T/Tc)

2)

where

Ho = Hc at T = 0K .

Critical field of superconductors.

superconductors, energy gaps in The ther-mal and electrical properties of the supercon-ducting state differ from those of metals in thenormal state, and there exists an energy gap be-tween the superfluid ground state and the statesof the normal electrons. The energy gap is ac-tually temperature-dependent, falling from itszero temperature value to zero at the critical tem-perature.

superconductors, high field A high mag-netic field applied to a superconductor will re-store a superconductor to its normal or resistivestate.Seesuperconductors.

superconductors, infrared absorption andtransmission The transmission and absorp-tion effects in superconductors are generallythe same in normal and superconducting statesfor different frequencies (e.g., optical frequen-cies) except in the microwave and infrared fre-quencies. This is determined by observingfrequency-dependent conductivity. The absorp-tion becomes very small as the temperature ap-proaches 0 K. This indicates a quantum effect ofthe excitation of electrons across an energy gap.

superconductors, intermediate state Seesuperconductive sphere.

superconductors, London Deals with thetheoretical treatment of the skin effect in super-conductors. It is defined as those superconduc-tors that deal with a penetration depth greaterthan the intrinsic coherence length. Local elec-trodynamics is used to treat this type of super-conductor.Seesuperconductors, Pippard.

superconductors, penetration depth Cur-rents in a superconductor are not strictly super-ficial but occupy a layer of finite depth below thesurface. The magnetic field also penetrates to acertain depth. This causes the susceptibility forsmall objects to be reduced, and the observedcritical magnetic field for a superconductor be-comes greater for one of small dimensions thana larger one.Seeskin effect, anomalous.

superconductors, phase diagram The tran-sition to superconductivity by substances is akinto a second order phase transition. This meansthat there is no latent heat involved and a sharpfinite discontinuity in the specific heat is seen.

superconductors, Pippard Deals with thetheoretical treatment of the skin effect in super-conductors. It is defined as those superconduc-tors that deal with penetration depths less thanthe intrinsic coherence length but greater thanthe London penetration depth. Non-local elec-trodynamics is used to treat this type of super-conductor.Seesuperconductor.

superconductors, resistance of, high frequen-cy effect Resistance disappears in supercon-ductivity for the DC case. In the anomalous skineffect, when the skin layer involved in electri-cal conduction at high frequencies is very thin,if this becomes less than the electron mean freepath, the classical theory of electrical conduc-tion breaks down and resistance is proportionalto the cube root of the frequency. High fre-quency resistance occurs in superconductors be-low the transition point by several factors thanthat observed at higher temperatures. This hasbeen observed for frequencies of the order of1200 Hz to 23,000 Hz.


superconductors, specific heat The specificheat displays a strong discontinuity at the tran-sition point for superconducting substances. Asthe temperature drops below the transition tem-perature in a zero magnetic field, the specificheat increases substantially and then decreasesslowly.

superconductors, transition temperatures inThis refers to the temperature at which an ele-ment or alloy will display superconducting prop-erties. The values for elements generally rangefrom about 0.105 K for iridium to higher valuessuch as 7.199K for lead and 9.25K for niobium.

superconductors, type I The magnetic fluxis expelled thereby producing magnetizationthat increases with the magnetic field until a crit-ical value is reached, at which it falls to zeroas for a normal conductor. These types exhibitMeissner diamagnetism.

Superconductors, type I.

superconductors, type II In this type themagnetic field begins to penetrate the specimenat a lower critical field but the superconductiv-ity is completely destroyed only at a higher field.This type does not exhibit the full Meissner ef-fect.

supercooling This refers to the cooling ofa substance below the temperature at which aphase change is expected to take place withoutthe phase change occurring, causing it to be-come metastable; i.e., it is possible to cool aliquid below its freezing point without its freez-ing.

Superconductors, type II.

superfluidity This is shown by particles thatobey Bose–Einstein statistics and are in the low-est allowed energy state. The particles thereforehave zero resistance to motion and zero entropy.Substances that exhibit superfluidity are HeliumII, which can flow through tiny holes impervi-ous to any other liquid, and a pair of electronsin superconductors.

superleak This is used, for example, ina refrigerator. Stainless steel tubes packedtightly with jeweller’s rouge which prevents liq-uid other than superfluid from circulating.

superposition of light waves The principleof superposition for a linear optical system statesthat, if there are different individual solutions tothe wave equation, then a linear combination ofthem is also a solution. In practical applicationsto interference and diffraction, a scalar theoryis used: e.g., for superposing two waves of thesame frequency traveling in the same direction.The superposition can be carried out by direct al-gebraic addition. Alternatively, a complex rep-resentation can be used and the superposition isthen carried out by the addition of phasors whichincorporate the amplitude and the phase of eachwave. This method forms the basis for analy-sis of interference and diffraction phenomena inoptics.

superposition of waves When two wavestraverse the same medium, the waves add. Theamplitude of the resultant wave at a particularpoint is the algebraic sum of the two componentamplitudes.

supersonic vibrations Speeds greater thanthat of sound or frequencies above those thatcan be heard by the ear.


surface charge The charge that resides on thesurface of an object or at the interface betweentwo substances is calledsurface charge.In a per-fect conductor, the surface charge results fromthe fact that the field inside a perfect conduc-tor is zero. For a dielectric material, the surfacecharge results from the polarization of the ma-terial and equals the product of polarization anda unit vector normal to the surface,σ = P·n.

surface charge density The amount of elec-tric charge,Q, that is spread over the surfacearea,A, of an object as a thin layer. The surfacecharge density,σ, is given by

σ = Q/A .

It has the SI units of coulombs per square me-ter, Cm−2. The charge distribution may not beuniform on insulators and can therefore have dif-ferent local surface charge densities.

surface tension, cell membrane Becausethe cell membrane is the boundary between twodissimilar liquids, but at the same pressure, anykink or deformation in the topology of the mem-brane will be met by an equivalent imbalance inpressure from one of the liquids. The directionof the restoring forces will be such that the mem-brane will minimize its surface area, as a directresult of its surface tension.

The importance of the surface tension of thecell membrane has been linked to olfactory stim-ulation. In this process, receptor cells may trig-ger an olfactory response based on chemical re-actions that occur at the surface of the cell andto a solution of odorant molecules that may al-ter the surface tension at the surface of the cellmembrane.Seeturgor pressure.

surface waves SeeRayleigh waves.

surge protection To protect from a abruptchange in signals.

susceptance, electric Unit: siemens. Theimaginary part of the admittance,Y , which isgiven by

Y = G+ iS ,

whereS is the susceptance,G is the conduc-tance, andi =

√−1. For a circuit containing a

resistanceR, and reactanceX, the susceptanceis given by

S =−X

R2 +X2.

susceptibility, diamagnetic A negative mag-netic susceptibility due to an induced magneticmoment in a system of electrons by an appliedmagnetic field. The susceptibility is usuallysmall with a magnitude 10−5 to 10−6. The neg-ative value indicates that the induced magneti-zation is opposite the applied field. Important inmaterials that have no permanent magnetic mo-ment on their atoms, which include Cu, Bi, B,Si, and many inorganic and organic molecules.Superconductors have the largest diamagneticsusceptibility, with type I superconductors hav-ing a value of -1. They are often referred to asperfect diamagnetssince the induced magneticmoment is large enough that it exactly cancelsthe applied magnetic flux within the supercon-ductor and so there is no penetration of magneticflux into the interior of a superconductor.Seesusceptibility, magnetic.

susceptibility, electric A dimensionlessquantity,χe, which relates the electric polariza-tion P of a material to the applied electric field,E, by

χe =Pε0E

,

whereε0 is the permitivity of free space. Elec-tric susceptibility is a measure of the ease withwhich a dielectric can be polarized. The electricsusceptibility is also given by

χe = εr − 1 ,

whereεr is the relative permittivity of a material.

susceptibility, magnetic Measures how eas-ily a material is magnetized by a (usually) smallapplied magnetic field. The susceptibilityχ of amaterial is the proportionality constant betweenthe applied magnetic fieldH and magnetizationM of a material and is defined by

M = χH .

If H andM are parallel to each other, thenχis a scalar quantity.χ is related to the relative


permeability of the material byµr = 1+χ. Seepermeability, magnetic.

susceptibility, measurement by Sucksmith’smethod Method of measuring magnetic sus-ceptibility in which a sample is suspended froma deformable ring in a non-uniform magneticfield. The force on the sample in the non-uniform field is proportional to the magneticmoment of the sample, which in turn dependson the susceptibility of the sample. The force it-self is measured by measuring the deformationof the ring and leads to a determination of thesusceptibility of the sample.

sweep generator An electric circuit that pro-duces a signal to examine other periodic signals.

switch A mechanical or solid state device foropening and closing a circuit, such as a circuitbreaker (mechanical) or a transistor (solid state).Switches are also used to select one of severalcomponents to be included in a circuit for a de-sired mode of operation. Other switches, suchas flip-flops, cause the operating condition of acircuit to change between two discrete levels.

switching, time division A time divisionswitch separates data paths in time; a crossbarswitch separates data paths in space. In timedivision switching, then inputs are stored in atemporary buffer. The switch reads from thebuffer n times faster than the input and writesthe data to the outputs in the proper order.

switch, timer A switch operated by clock-work, electric motor or resistor-capacitor circuitto open or close a circuit at a predetermined time.

synchrocyclotron A cyclotron in which themagnetic field is held constant, and the fre-quency of reversal of voltage used to acceleratecharged particles is adjusted so that the particlesstay in step as they speed up. This allows forsuch effects as the relativistic increase in massthat occurs at speeds close to the speed of lightfor the accelerating particles.Seecyclotron.

synchronization Any circuit or device thatis operated by means of clock pulses is syn-chronous.

synchronous capacitor A synchronous mo-tor running without mechanical load and draw-ing a large leading current, like a capacitor; usedto improve the power factor and the voltage reg-ulation of an alternating-current power system.

synchronous motors Such a motor is onein which the rotor normally rotates at the samespeed as the revolving field in the machine. Thestator is similar to that of an induction machineconsisting of a cylindrical iron frame with wind-ings located in slots around the inner periph-ery. A revolving field can be produced in syn-chronous motors by use of the same method asfor induction motors. With the main stator wind-ing connected directly to the supply, an auxiliarywinding may be connected through a capacitor.

synchroscope An instrument used to com-pare both the phase and frequency of two dif-ferent AC sources. Used mainly to determinewhen a synchronous motor has been brought tosynchronous speed; this occurs when the EMFinduced in its armature windings have a phasedifference of about 180 with the AC power sup-ply line potential. The motor can then be con-nected directly to the line.See alsomotor, syn-chronous.

synchrotron Device used to acceleratecharged particles to high speeds so that exper-iments to determine their structure and proper-ties may be performed. Uses a magnetic field tomake particles move in a circle and an electricfield whose polarity changes periodically to ac-celerate the particles. The strength of the mag-netic field and the frequency of the electric fieldare changed as the particles speed up. Very highspeeds are possible with protons reaching speedsof more than 99% of the speed of light. Alsoused to generate intense electromagnetic radi-ation (synchrotron radiation) in the infrared tohard X-ray range.

synthetic sound This is produced by sinu-soidal alternating currents of various definitefrequencies that are generated continuously andcan imitate the quality of various musical instru-ments.


system (in communication) A communica-tions system is the complete assembly of ap-paratus and circuits required to effect a desiredtransfer of information. In telecommunications,

a system is a set of equipment or apparatusesthat is combined to perform a function within acarrier’s telecommunications network. An ex-ample is a switching system.


TTalbot’s Law A slowly flickering light sourcecan be detected by the eye. However, due to thefinite response time of the eye, this flicker can-not be detected above a critical frequency, Thisprinciple is known asTalbot’s Lawand can bedemonstrated quantitatively by a rotating sectorwheel. Above the critical frequency of rotationof the wheel, the intensity of transmitted lightvaries as the proportion of sectors cut from thewheel. Talbot’s Law finds application in motionpictures, where for practical purposes blurring isprevented by projection at 24 frames per second.

tangent law Assuming that there is no spher-ical aberration of the principal rays from theobject points, the condition to be satisfied forelimination of distortion for a Gaussian imageis

ny tanu = n′y′ tanu′ ,

whereu andu′ are the angles made by the prin-cipal rays with the optical axis,y andy′ are thesizes of the object and image, andn andn′ arethe refractive indices of the object and the imagespaces, respectively.

tangent plane The plane containing the opti-cal axis and the off-axis object point. The princi-pal ray always lies in the tangent (or meridional)plane.

telecentricity A lens is said to be telecen-tric if the chief rays are parallel to one another.Usually, they are also parallel to the lens axis andperpendicular to the object and/or image planethat are also perpendicular to the axis. A lensis said to be telecentric in object space and/ortelecentric in image space. A focal lens can benon-telecentric or telecentric on either side, butcannot be doubly telecentric. An afocal lens canbe non-telecentric or doubly telecentric but nottelecentric on one side.

telecommunication Telecommunication isthe study or practice of the transfer of informa-

tion over a distance by electromagnetic means,such as wire in cable telegraph and telephone,or radio waves in broadcasting.

telegram A message sent by telegraph andthen delivered in written form.

telegraph A system or device for transmit-ting messages or signals to a distant receiver,usually by making and breaking an electricalconnection.

telegraph, polarential A direct-current tele-graph system employing polar transmission inone direction and a form of differential duplextransmission in the other. There are two typesof polarential systems, known as types A and B.In half-duplex operation of a type A polarentialsystem, the direct-current balance is indepen-dent of line resistance. In half-duplex operationof a type B polarential system, the direct currentis substantially independent of the line leakage.Type A is better for cable loops where leakageis negligible but resistance varies with tempera-ture. Type B is better for open wire where vari-able line leakage is frequent.

telegraphy, radio Communication by meansof a telecommunication system that transmitsdocumentary matter, such as written or printedmatter or fixed images, and reproduces it at adistance. The matter is transmitted as a suitablesignal code, such as international Morse code,either by means of wire or by radio (radio teleg-raphy).

telemetry (1) Measurement at a distance.

(2) A measuring instrument that measures aquantity and transmits the measured data as anelectrical signal to a distant recording point isknown as atelemeter. Space exploration andphysiological monitoring in hospitals both re-quire the use of telemetry.

telephone An apparatus for transmittingsound (especially speech) over a distance bywire, cord or radio. It is an assembly of appa-ratus that includes a suitable handset containingthe transmitter and receiver, and usually a switchhook and the immediate associated wiring.


telephone, analog A plain old telephone ser-vice connection with no advanced features.

telephone, radio Normally, communicationbetween two points takes place along suitablecables (telephone lines) except where this is in-appropriate, such as ship-to-shore telephony. Aradio telephone is used when a radio link isneeded to connect a particular access point tothe main system.

teleprinter A device for transmitting tele-graph messages as they are keyed, and for print-ing messages received. It is a form of a start-stoptypewriter that comprises a keyboard transmit-ter, which converts keyboard information intoelectrical signals, and a printer receiver, whichreverses the process. Teleprinters are used intelex systems and in some older computing sys-tems.

telescope, astronomical Any telescope usedto study astronomical objects. The telescopefirst used by Galileo for this purpose consistedof two convex lenses, separated by the sum oftheir focal lengths, with the focal length of theobjective larger than that of the eyepiece. Inan astronomical telescope, the rays that enter itparallel leave it parallel; the ratio of their angleswith the telescope axis give the angular magni-fication. Instead of a lens system, mirrors canbe used to collect light from the stars. (Seetele-scope, reflecting). Outside the visible region(for example for radiotelescopes), one uses spe-cial radio-dishes and antennas.

telescope, electron Telescopes that are un-like optical telescopes in that the radiation fallson the photocathode surfaces or on charge-coupled devices (CCDs). By accelerating theresulting electrons and making them incident ona fluorescent screen, direct image of objects inthe ultraviolet and infrared regions can be ob-served. In the optical region also, the intensityof faint star images can be enhanced.

telescope, reflecting Astronomical tele-scopes using a (front polished paraboloidal) mir-ror to collect light from the stars. Dependingon various popular arrangements, the star is ob-served at the primary focus of the mirror (for a

large mirror), or the collected light is broughtout at right angles to the beam by use of a planemirror or prism (Newtonian), or through a holein the primary by reflection from a concave ellip-soidal secondary mirror (Gregorian), or by theuse of a convex hyperboloidal secondary mirror(cassegranian). The reflecting-type telescopescan be made very large because, unlike the dif-ficulties involved in making a large bubble- andstrain-free lens for a refracting telescope (withchromatic aberration problems), one can polisha very large geometrically well-defined surface.The mirror can be supported on the back also,in contrast to the lens, which can be supportedfrom the rim alone. The chromatic aberration isabsent because refraction is not involved.

teletype (1) A kind of teleprinter.(2) To operate a teleprinter or send by means

of a teleprinter.

teletypewriter (Also known asteleprinter.)A device for transmitting telegraph messages asthey are keyed and for printing messages re-ceived.

television A display instrument that convertsa received electromagnetic signal into a visibleimage.

telex An international system of telegraphy inwhich printed messages are transmitted and re-ceived by teleprinters using the public telecom-munications network. The wordtelex comesfrom a combination of the wordsteleprinterandexchange.

temperament Certain adjustment of tones orintervals of the scale of fixed tone instrumentslike organs or pipes, so that a larger variety ofmelodious combinations are possible.

temperature, Bloch The electrical con-ductivity or resistivity is extremely sample-dependent at low temperature. The BlochT 5

temperature law is observed in many metals atlow temperatures. The residual resistivity isvery sensitive to the presence of impurities andstructural defects and, for a dirty specimen, maynot be much smaller than that at room tempera-ture.


Bloch temperature.

temperature changes during passage ofsound Compressions and rarefactions takeplace so rapidly that the gas does not have suf-ficient time to lose or absorb heat from the sur-roundings. The process is adiabatic. In generalthe speed of sound is proportional to the squareroot of the absolute temperature in general.

temperature, critical This refers to the tran-sition temperature in superconductors belowwhich drastically altered thermodynamic andelectrical properties are observed in the sample.

temperature, Debye Dealing with the heatcapacity that results from lattice vibrations, theDebyeT 3 law predicts that the lattice heat ca-pacity is a universal function scaling for allsolids through the parameterθd, the Debyetemperature. Materials with strong interatomicforces and light atoms such as diamond and sap-phire have relatively high Debye temperature,and soft materials with low acoustic velocitieshave smaller values.

temperature, degeneracy The number ofelectrons per unit volume in the conduction bandand the number of holes per unit volume inthe valence band are a function of temperature.However, these depend strongly on the presenceof impurities in the semiconductor. Introductionof additional levels of energies due to the impu-rities can introduce degeneracy.

temperature, inversion According to theJoule-Kelvin effect, when gas is allowed to ex-pand through a porous plug or orifice, it maybecome warmer or cooler. The temperaturechange can be determined from isenthalps forthe gas. Temperatures increase on one side of

the isenthalp and reduce on the other side of it.Inversion temperatures are along the isenthalp.

Inversion temperature.

temperature, lowest helium Experimentalparameters for helium have been found for tem-peratures down to about 0.1 K.

temperature, magnetic For paramagneticsalts there is an observable magnetic property,the susceptibility of which is temperature sensi-tive and provides a thermometric parameter. Formost magnetic cooling salts at temperatureT ∼1K the magnetic susceptibility varies inverselywith the absolute temperature; at higher temper-atures, there is a complicating influence due tomagnetic interactions. There may be discrep-ancies between the thermodynamic temperatureand the magnetic temperature below 1 K.

temperature measurements, clinical Thetemperature of the body is a direct result of thebalance between heat production and heat loss.In humans, the metabolism maintains the bodytemperature within a narrow range (36.5 C to37.5 C) despite wide variations in heat produc-tion or environmental temperature. The part ofthe human brain in charge of thermo-regulatingthe body is the hypothalamus, where elevationin the temperature of blood going to that part ofthe brain initiates heat loss by causing dilationof blood vessels and sweating.

Because of the narrow range of values, tem-perature measurements are key in preventionand diagnosis of diseases. In particular, fever


can easily be detected, being a result of a distur-bance in the regulation of temperature. Becausethe hypothalamus tries to balance the body tem-perature, during fever, the temperature that thehypothalamus tries to achieve has been shiftedfrom its normal position. This may be due tochemicals calledendogenous pyrogens,whichare derived from white blood cells.

Clinical measurements of temperature haveto be taken with care. For example, pressure ap-plication to the skin whose temperature is beingmeasured is undesirable due to possible changesin the vascular tissue around it. Also, localsweating may influence the final reading.

temperature, Neel This is with respect tomagnetic behavior at different temperatures.The spins become ordered below the Curie tem-peratureTc; however, the ordered state is not al-ways ferromagnetic. In some cases the adjacentspins may be antiparallel. In this antiferromag-netic state, there is no net spontaneous magneti-zation but hysteresis is present and the suscepti-bility shows a sharp maximum at this transitiontemperature. Known also as Neel point.Seespontaneous ordering.

temperature, negative absolute The abso-lute temperature scale refers to the Kelvin scalewhich extends upwards from 0 K representing−273 C. There are no negative values on thisscale.

Tesla The SI unit of magnetic flux density,one tesla (1 T) is equal to 1 Vs/A (or equivalently1 J/A2). The average magnetic flux density fromthe geomagnetic field on the surface of the earthis around4× 10−5 T.

Named after Nikola Tesla (1856–1943)whose investigations into time varying electro-magnetic fields led to the utilization of AC elec-tric power distribution and the development ofradio.

test charge A minute amount of charge usedto evaluate the electric field at a particular po-sition is called atest charge.The electric fieldat a point in space is defined as the force expe-rienced by the test charge divided by the chargeof the test charge. The test charge needs to be

as small as possible to avoid disturbing the fieldto be measured.

thaw rigor Upon death, muscles of animalsand humans undergo what is usually calledrigormortis. This means that muscles contract afterdeath due to Ca2+ leaks from the sarcoplasmicreticulum, artificially initiating the muscle con-traction cycle without neuronal signal. After thecontraction consumes all of the remnant ATP,the muscles remain contracted because there isno ATP to pump back the calcium or at least torelease the myosin from the actin.Seemuscle,mechanics.

A similar process occurs during thaw rigor inwhich meat shortens considerably after thawing.This happens when the meat has been frozenbefore the completion of rigor mortis and theice crystals have slashed open the sarcoplasmicreticulum. Then on thawing, the meat is finallywarm enough to respond to the accumulationsof Ca2+ that causes extreme contraction.

This concept is of importance to the meat in-dustry since the final tenderness of their productdepends on the technical steps in freezing themeat.

theorem, the relationship becomes in differ-ential form ∇.E = ρ/ε0.

thermal drift A fluctuation caused by thetemperature change.

thermionic emission (1) Electrons liberatedfrom the surface of a metal as a result of thermalenergy supplied to them by heating the metalto high temperatures. The energy supplied perelectron must be greater than the potential bar-rier at the surface that ordinarily keeps it in.

(2) Term describing the emission of elec-trons or positively charged ions from the sur-face of a conducting material when it is heldat high temperatures. It is caused by some ofthe electrons/ions gaining sufficient kinetic en-ergy through random fluctuations to overcomethe binding energy of the conductor’s surface.This effect is the basis for the operation of vac-uum tubes.


thermistor A thermal meter whose readoutis responsive to the temperature, such as a ther-mometer.

thermocouple A device that converts ther-mal energy to electrical energy in order to mea-sure the temperature of an object. It consistsof two wires of dissimilar metals connected to-gether in a circuit. A current will flow if twosuch junctions are connected together and main-tained at different temperatures. A temperaturemeasurement is usually made by maintainingone of the junctions at a known temperature(e.g.,0 C) while the other junction is broughtinto contact with the object. The magnitude ofthe potential difference between the two junc-tions is a measure of the temperature. Tabu-lated data of potential differences and their cor-responding temperatures are readily availablefor the different types of thermocouples.SeealsoSeebeck effect.

thermodynamics, third law of The entropyof all systems and of all states of a system is zeroat absolute zero. In other words, it is impossibleto reach the absolute zero of temperature by anyfinite number of processes.

thermoelectricity Conversion of heat energydirectly to electrical energy and vice versa.SeePeltier effect, Seebeck effect, Thomson effect.

thermography (1) Technique in which an ap-paratus records temperatures in sequential mea-surements for diagnostic purposes. The ther-mometer used for the temperature readings iscalled athermograph.

(2) Diagnostic technique in which an infraredcamera is used to measure temperature varia-tions on the surface of the body. Images arethen produced that may reveal sites of abnormaltissue growth.

thermometer, acoustic Thermometers thatoperate by sensing sound waves. This is done bymeasuring the velocity of sound in a gas at verysmall pressures since this is directly related totemperature. The velocity of sound is measuredby observing acoustic resonances.

thermometer, carbon radio resistor Themost commonly used and cheapest thermome-ter to work below 4 K. Its characteristics varywidely from manufacturer to manufacturer anddifferent ones are required for different temper-ature ranges. It is usually made from graphitecomposite inside a ceramic coating and does notexhibit simple semiconducting behavior. Car-bon resistors undergo a change in calibrationover a period of time but the change betweensuccessive runs gradually decreases after the re-sistor has been “trained” by repeated cyclingsbetween room temperature and 4 K. The carbonresistor is less sensitive to magnetic fields thangermanium.

thermometer, germanium Semiconductormaterial thermometer that gives very repro-ducible results even after many cycles from 300K to less than 1 K. Recalibration is not usu-ally needed. The resistivity of a semiconductorincreases as the temperature is reduced. Thesemiconductor needs to be doped with suitableimpurities to prevent the low temperature resis-tances from becoming very high. Self heatingcan sometimes be a problem and the power dis-sipation during measurement must therefore bekept low.

thermometer, hyperfine This is a ther-mometer employed in measurements in the mi-crodegree region by means of nuclear cool-ing, i.e., hyperfine enhanced nuclear cooling.This relies on the temperature-dependent para-magnetic susceptibility that results from nuclearspins. It involves adiabatic demagnetization toquench the hyperfine fields.

thermometer, Matsushita resistors Type ofthermomenter that employs the resistance prop-erty of metals.Seethermometer, resistance.

thermometer, resistance Pure metals havea linear resistivity-temperature relationship atroom temperature, but the resistivity generallydoes not follow a simple relationship at low tem-peratures when it may also be a function of themagnetic field. The cryogenic behavior is afunction of sample purity, since residual resistiv-ity arises from impurity scattering of conductionelectrons. Generally, the usefulness of metallic


resistance thermometers below 10 K is limited.The resistance of certain alloys like constantanis nearly independent at high temperatures, butdecreases with decreasing temperature below 5K. These are used as resistance thermometersbut their sensitivity is not high. Sensitivity canbe increased by adding lead to bronze or brass.

thermometers, clinical A clinical ther-mometer is a thermometer used to measure bodytemperature. Typically, it is made of glass in theshape of a tube of uniform bore with a narrowingabove the mercury bulb in its lower part. Themercury is in a vacuum inside the tube, and atemperature scale is usually etched on its front.When measuring temperatures, the mercury ex-pands or contracts, thus changing the height ofthe mercury column inside the tube. The nar-rowing above the mercury bulb permits the mer-cury column to remain in position when the in-strument is removed from the body or positionedin any way other than vertical.Seetemperaturemeasurements, clinical.

thermometers, semiconducting These typeof resistance thermometers maintain their highsensitivity down to the lowest attainable tem-peratures, where metallic elements become tooinsensitive although it is difficult to calibrate be-yond the lowest point on the3He vapor pressurescale.Seethermometer, resistance.

thermometers, vapor pressure Providesquite accurate measurements of temperatureover an extended range, however they usuallyare not commercially made devices. A ther-mostat is usually used to achieve and maintainsteady temperatures throughout the desiredrange. The thermometers are based on the prin-ciple that the saturation vapor pressure of a puresubstance is a monotic and sensitive functionof temperature. The following components areneeded: supply of working substance in highlypure form, a sensitive, accurate and wide-rangepressure measuring device, and a cell to be con-nected to measuring and supply systems.

thermometry, gamma ray anisotropy Seethermometry, nuclear orientation.

thermometry, NMR Thermometers that em-ploy the principle of nuclear magnetic resonance(NMR) for increased sensitivity. They are suit-able for temperature measurements in the mKrange. The pulsed NMR thermometer has theadvantage of being self-calibrating. It is sub-jected to an applied field for a short time. Thedecay of the induced voltage signal is propor-tional to the nuclear magnetization and hence in-versely proportional to temperature. Very pureplatinum is the material used in such thermome-ters.

thermometry, noise This deals with the factthat a measurable but variable voltage arises inan n electrical conductor of resistanceR as aresult of random thermal excitation of the con-duction electrons. In 1928, it was shown that thenoise voltage detected across the resistor variesas the square root of the product ofR andT , thethermodynamic temperature.

thermometry, nuclear orientation Appli-cation of quantum level population differencesin thermometry. It is applicable in very lowtemperature thermometry. The basic idea thisoperates on is that the spatial distribution of nu-clear radiation arising in the decay of the 60-Conuclei emitted from a radioactive source can berelated to an identifiable laboratory axis, i.e., thesymmetry of a single crystal of cobalt metal andto the temperature of the source atoms. It canobtain values of temperature below 2 K. Nu-clear orientation thermometry depends on theanisotropic emission of gamma rays from po-larized radioactive nuclei. The scale of theanisotropy in the intensity pattern is determinedby the degree of polarization, which in turn de-pends on the absolute temperature. In practicethe polarization is achieved by substituting theradioactive nuclei in a ferromagnetic host lat-tice so that the nuclear dipoles are aligned bythe very large∼ 10T internal field.

thermometry, osmotic pressure This usesthe principle of osmosis as a measure of temper-ature. In osmosis, certain molecules are able tobe transmitted through a semi-permeable mem-brane connected to a tube. A hydrostatic pres-sure is therefore built up in the tube. This causesdiffusion of water downward until equilibrium is


reached. The pressure balancing osmosis obeysthe ideal gas law and proportional to the temper-ature of the gas and is equal to the pressure thesolute molecules would exert if they were a gasat that volume. Osmotic pressure can thereforebe used as a measure of temperature.

thermophone Device used for the produc-tion of sound by heating a thin conductingwire by an alternating current; the temperaturechanges cause changes in length. This sets thewire into resonant vibration, which producessound.

thermopile Thermocouples connected in se-ries, where every alternate junction is exposedto radiant heat or brought into thermal contactwith an object. This arrangement results in theadding together of the EMFs due to pairs ofjunctions, thus providing greater sensitivity thana single thermocouple to temperature measure-ment.

Thevenin equivalent A theory suggested byThevenin to use an equivalent circuit instead ofa real circuit to simplify physical analysis.

Thomson coefficient Used in measurementof Thomson heat which is evolved or absorbedwhen a current flows through a conductor acrossthe ends of which a temperature difference ismaintained. The rate at which Thomson heatis transferred into a small region of a wire car-rying currentI with temperature differencedTis equalσIdT whereσ is the Thomson coeffi-cient. The coefficient depends on the materialof the wire and on the temperature of the smallregion under consideration.

Thomson effect The EMF generated in a sin-gle electrical conductor by maintaining a ther-mal gradient in it; a heating and cooling effect inthe conductor is then produced by current flowalong the thermal gradient. This effect is closelyrelated to thePeltier andSeebeck effects. SeealsoPeltier effect; Seebeck effect.

three-phonon process An Umklapp processdealing with three phonon scattering, it is de-fined as one in which the total crystal momentumis not conserved, a process more likely to oc-

cur at high temperature than at low temperature.At higher temperatures, the mean free path ofthe phonons is ultimately limited by interatomicspacing thus reducing the spread in thermal con-ductivity of different crystalline solids.

threshold of hearing The amplitude of theweakest sound wave that can be detected by theear varies with the frequency of the wave withthe normal ear being most sensitive at 3500 Hz.The minimum audible amplitude is of the or-der of 10−9 cm. The lowest frequency that thenormal ear can distinguish is about 30 Hz. Thehighest audible frequency diminishes with agefrom about 30,000 Hz, down to 10,000 Hz oreven lower.

threshold voltage A voltage at which anelectronic device begins to conduct a current.

thyristor A semiconductor switch device thatchanges the current direction in an electric cir-cuit.

timbre The subjective characteristics thatmake it possible for us to distinguish be-tween two tones having the same intensity leveland fundamental frequency but different waveforms; i.e., it expresses our ability to recognizesound of a violin as different from that of a trum-pet even when the instruments are sounding thesame note with equal loudness. It is primarilydependent on the waveform of the tone beingheard and to a lesser extent on the intensity andfrequency.

time base A line produced by sweep circuitoperation on a display screen.

time constant The length of time requiredfor the amplitude of an exponentially chang-ing quantity (current or voltage) to change by63.2%. For example, the decreasing currenti(t)through a series resistance,R, and capacitance,C, is given by

i(t) =V

Re−t/RC ,

whereV is the applied potential difference. Thetime constant is the value of time that reduces theexponential factor toe−1; i.e., the time constant


isRC. Similarly, the time constant for a seriesinductance,L, and resistance,R, isL/R.

timer circuits Electric circuits that periodi-cally supply time pulse.

tomography, emission Any of several tech-niques for making detailed imaging of only apredetermined plane section of a solid object(planar imaging) while blurring out the imagesof other planes. The signal usually comes fromhigh energy photons (X-rays, gamma rays). Acomputer is usually used to assist in forming acomposite image.

The computerized axial tomography (CAT)is an example where a CAT scanner producescross-sectional views of previously inaccessi-ble internal body structure. Another exampleis neutron tomography where neutrons are usedto visualize a sample and detect substances con-taining water, organic substances, plastics, andlubricants.Seetomography, positron emission.

tomography, positron emission Techniquefor measuring the concentrations of positron-emitting radioisotopes within the tissue of liv-ing patients. Inpositron emission tomography(PET), the distribution of positron-emitting ra-dionuclides are imaged in the patient who hasbeen administered with the tracer after the in-troduction of the compound usually either byinjection or inhalation. Like other imaging tech-niques (CT, MRI, SPECT), PET relies on com-puterized reconstruction procedures to producetomographic images.

Radionuclides used in PET include Car-bon11, Nitrogen13, Oxygen15, and Fluorine18,with (relatively short) half-lives of 20 min, 10min, 2 min, and 110 min, respectively. Becauseof their short half-lives, facilities equipped forPET that use these radionuclides are usually e-quipped with a particle accelerator (cyclotron).

In PET, the tracer decays by emitting apositron from the nucleus that, after combin-ing with an outer electron, annihilate each otheremitting two high-energy photons (gamma rays)at 180 from each other. The emitted radiation isthen detected inside the detector as coming fromthe tracer. Both photons have to be detectedsimultaneously on opposite sides of the detec-tor, otherwise the signal is discarded as coming

from the outside. The detection places the sig-nal along the line connecting the two sections ofthe detector that recorded the signals.

PET is widely used in areas of neurologi-cal diseases, including cerebrovascular disease,epilepsy, and cerebral tumors. In general, PEThas the capability to image parts of the bodywhere abnormal biochemical changes are oc-curring. For that same reason, PET is usefulin drug research, pharmacokinetics, and phar-macodynamics.

tone arm The pivoted pickup bar of a recordplayer with a head consisting of a needle setinto a cartridge, that follows the grooves of therecord, converting the oscillation into the elec-trical impulses.

tone control Output at different frequenciescan be controlled by variable amounts depend-ing on whether the frequency is high or low.Tone control is easily obtainable for electric gui-tars by the use of capacitors and resistors in cir-cuits; however for pianos, whether tone controlis possible at all is still controversial.

tone deaf The inability to express or to dis-criminate distinctions in musical pitch.

tones Sounds that impress the ear with theirindividual character, especially pitch, quality ofsound or timbre.

tones, combination When two or more notesof different frequencies are played simultane-ously, the sound produced is a composite of thetones. There is thedifference tone,thesumma-tion tones,and other combinations too.

tone, warble A continuous trilling tone as op-posed to steady tones particularly used in alarmsand sirens.

tooth rigidity The nature of the rigidity ofa tooth comes from its composition. The toothis a bonelike calcified structure, composed ofa core of soft pulp-like tissue containing bloodvessels and nerves that is surrounded by a layerof hard dentin. The dentin is coated in turn withcementum or enamel at the crown (the visible


part in the mouth). The enamel constitutes thehardest substance in the body.

The dentin in humans is composed of het-erogenous material of a solid (circumpulpal)phase surrounding a network of tubules. Thesetubules, measuring about 1 to 3µm in diam-eter, contain elongated cell bodies that radiatefrom the dental pulp organ throughout the entiredentin.

toroid A coil of wire wrapped as a solenoid.The solenoid is curved so that the ends join to-gether forming a donut or toroid shape. For acurrent-carrying toroid with a small width and alarge radius, the magnetic field is uniform withinthe toroid and no magnetic field lines emergefrom the toroid.Seesolenoid.

traffic The message, signal, etc. that is trans-mitted through a communications system. It isthe flow or volume of such information.

transducers Electrical device that picks upsound vibrations, e.g., a microphone. A devicethat transforms a signal of one type into a signalof another type. An acoustic signal is a mechan-ical signal; on being incident on a microphone,it is transformed into an electrical signal.

transducers, medical applications In gen-eral, a transducer is a device that converts a formof input energy into another form of output en-ergy. The relationship between the input andoutput is usually fixed and well known.

Common examples of transducers are theones that convert sound and other mechanicalinputs into electrical signals, like microphones,cassette recorders, piezoelectric crystals, ultra-sound; or light into electricity, like photoelec-tric cells; or take electricity as input and out-put in several forms, like loudspeakers, lightbulbs, and solenoids. Medical applications oftransducers include all sorts of general probesthat convert temperature, sound waves, and me-chanical force into electrical and electromag-netic output suitable for imaging, and surgicalprobes.

There are two classifications for transducers:those that require a source of energy in additionto the input signal (active), and those that do not(passive).

transfer function The ratio of the measuredoutput divided by the applied input of an elec-tronic device or circuit. The transfer function,H(jω), is usually expressed as the ratio of theoutput to input voltage as a function of frequency,ω.

transformer An electrical device for increas-ing or decreasing the voltage of an alternatingcurrent source by electromagnetic induction be-tween two or more coils that are not connectedelectrically. The coils are usually arranged sothat the magnetic flux associated with one wind-ing also threads the others either by placing themin close proximity or by winding them on thesame ferromagnetic core. The simplest trans-former consists of two sets of windings: the pri-mary and secondary windings. The primary isthe winding that receives the AC voltage fromthe supply circuit, while an AC voltage is in-duced in the secondary by the primary.

The primary voltage,V1, and secondary volt-age,V2, are related by the following:

V1

N1=V2

N2,

whereN1 andN2 are the number of turns in theprimary and secondary, respectively. If therewere no losses in the transformer, its power out-put would be the same as its power input so that

I1 × V1 = I2 × V2 ,

whereI1 andI2 are the currents in the primaryand secondary, respectively.

The secondary may have several electricalconnections (known as taps) on different turnsto enable the selection of different output volt-ages. Transformers are used in power suppliesof many electronic devices that plug into themain power supply. They are also widely usedin transmitting electrical energy over long dis-tances by raising the secondary voltage to highvalues in order to reduce line losses.

transformer, acoustic A device or mate-rial that couples two different media of differingcharacteristics, e.g., acoustic impedance, so asto allow continuity of transmission of acousticwaves across both medium.


Circuit diagram symbol of a transformer.

transformer, auto Transformer that consistsof a single winding so that the primary is formedby the whole winding while the secondary isformed by a part of this winding that is tappedto give the desired voltage.

Auto-transformer.

This arrangement acts as a step-down trans-former. Such transformers are used in startingdevices for induction motors or as a means forvarying the AC voltage to be applied to a device.See alsoVariac.

transformer, auto- A transformer typifiedby having only one common coil onto whichboth the input and output terminals are con-nected. The output terminal has an adjustablebrush which can slide along the common coil,thus adjusting the output voltage to values lessthan or greater than the input voltage.

transformer, current (1) A device that en-ables the alternating current in a circuit to bemeasured. There are basically two types:

1. An auto-transformer configurationwherethe primary is connected in series with the circuitin which the current is to be measured, and thesecondary is connected directly to the terminalsof an ammeter. The current through the ammeteris proportional to, but much less than, the maincurrent and the ammeter will not be subjected tothe high potentials of the main circuit.

2. A toroidally wound coil, usually on a fer-romagnetic core. The conductor carrying thecurrent to be measured is passed through thecenter of the toroid, thus forming the primarywinding and consequently, the toroidal wind-

ings form the secondary which steps-down thecurrent in a known ratio. An ammeter can thenbe connected directly to the secondary.

(2) A transformer device commonly used tomeasure currents in a circuit. The primary coilis placed in series with the circuit in which cur-rent is to be measured, while the secondary coil,which has more turns than the primary, is con-nected to a current measuring device.

transformer, input One used to match theimpedance of an AC signal source to the in-put impedance of a circuit for maximum powertransfer. They are also used to exclude DC volt-age from the input of the circuit.See alsotrans-former.

transformer, output One that matches theoutput impedance of a circuit to the impedanceof a load connected to the output. For example,the output impedance of an audio amplifier hasto match the impedance of a speaker for max-imum power transfer and therefore maximumsound output.

transformer, step-down One in which theoutput voltage from the secondary is lower thanthe input voltage to the primary. This is achievedby having a secondary with a smaller number ofturns than the primary.Seetransformer.

transformer, step-up One in which the out-put voltage from the secondary is higher than theinput voltage from the primary. This is the oppo-site to a step-down transformer and is achievedby having a secondary with a larger number ofturns than the primary.Seetransformer.

transformer, voltage One used to connecthigh tension lines to an instrument in order tomeasure voltage. The primary winding is con-nected in parallel to the main circuit and the sec-ondary winding is connected to a suitable instru-ment such as a voltmeter.

transient, behavior Opposite to steady be-havior, an instant state.

transistor, mn-mp-mn A transistor consistsof n-type, p-type, andn-type semiconductorsthat form a sandwich configuration.


primary secondary

transistor, mp-mn-mp A transistor consistsof p-type, n-type, andp-type semiconductorsthat form a sandwich configuration.

transistor, complementary Two transistorswhose geometry is the same, but one isn-channel in which the electrons are charge carri-ers and the other isp-channel in which the holesare charge carriers.

transistor, drift A transistor in which thebase region has a variable conductivity to reducethe carrier transition time.

transistor, field effect A transistor whoseconductance in the current path is controlled byapplying an electric field perpendicular to thecurrent.

transistor, IMPATT A transistor that em-ploys an IMPATT (impact avalanche transittime) and is used in high frequency regions.

transistor, junction An n-type and ap-typesemiconductor contact each other and form ajunction. A transistor contains two junctions.

transistor, mesa A transistor in which twoterminals are positioned at the platform and theemitter is made by the vapor-deposition metal.

transistor, photo A transistor whose outputis controlled by external light.

transistor, power A transistor that can beused in high electric power conditions. Its de-sign is different from normal transistors.

transistor, surface barrier A transistor is asemiconductor device. The semiconductor con-tacts the other materials, which forms a contact-ing barrier between the surface of the semicon-ductor and the material.

transistor, unijunction A transistor that con-sists of a semiconductor bar with two ohmic con-tacts and a single, small area emitterp-n junctionpositioned between them.

transistor, unipolar A transistor whose cur-rent flow is due to one type of carrier, electronor hole.

transition, order-disorder Deals with thesituation of spins in magnetic ordering in alloys.There is a disordered phase and a sharp transi-tion temperature above which the ordering dis-appears. For example, in an alloyβ-brass madeup of copper and zinc, copper at a site corre-sponds to spin-up and zinc at a site to spin-down.Seespontaneous ordering.

transit time A time for a carrier transportingfrom one terminal to the other.

transmission, analog Analog transmissionin telephony is a method of conveying voice,data, image, or video information by a signal thatvaries continuously in amplitude or frequencywith the information being transmitted.

transmission, asynchronous Asynchronoustransmission is the transfer of data in which theintervals between the transmitted data packetsare of unequal length. The transmission is usu-ally controlled by start and stop signals.

transmission, biternary Biternary transmis-sion is the transfer of two binary pulse trains overa single channel by combining the pulse trainsand using a single communications channel inwhich the available bandwidth is sufficient forthe transmission of only one of the two pulsetrains at a time.

transmission, blind A form of data trans-fer that does not require an acknowledge signalfrom the receiver. Blind transmission may oc-cur or be necessary when security constraints,such as radio silence, are imposed, when tech-nical difficulties with a sender’s receiver or a re-ceiver’s transmitter occur, or when lack of timeprecludes the delay caused by waiting for re-ceipts.

transmission, bursty (1) The operation of adata network in which the data transmission isinterrupted at intervals. It is a form of trans-mission that combines a very high data sig-naling rate with very short transmission times.


Burst transmission allows communication be-tween data terminal equipment and a data net-work operating at dissimilar data signal rates.

(2) A burst in data communication is a se-quence of signals, noise, or interference countedas a unit in accordance with some specific crite-rion or measure.

transmission coefficient, acoustic This isthe ratio of the transmitted sound energy to theincident flow of sound energy when a progres-sive plane wave in one medium impinges uponthe boundary of a second medium and is trans-mitted. The sound transmission coefficient isindependent of the direction of the wave mo-tion; i.e., it is the same from water into air asvice versa.

transmission control protocol (TCP) Theprotocol used in the Internet to provide connec-tions. Both ends of a TCP connection can simul-taneously read and write packets. The sourceadjusts its transmission rate to the rate currentlysupportable in the network. TCP uses timeoutsand retransmission to ensure that a destinationreceives a transmitted packet.

transmission, diversity A diversity trans-mission system is a communication system thathas two or more signal paths or channels. Theoutputs of these channels are combined to give asingle received signal and thus reduce the effectsof fading.

transmission, full-carrier Full-carrier trans-mission is a telecommunication system that am-plitude modulates a carrier signal and transmitsthe modulated signal along with the carrier. Theother approach, calledcarrier-suppression,does not transmit the carrier signal.

transmission, intercellular (1) Transmis-sion of a signal from one cell to another. Anexample is in the transmission of an action po-tential from one cell to another either throughsynapse or through gap junctions. In thesynapse, the signal is propagated by the releaseof chemicals (neurotransmitters) from the sig-naling cell to the other end of the synapse, wherethe signal is received. In a gap junction the sig-nal is transmitted by an ionic current flow that

flows from one cell’s intracellular space directlyto the other cell via ion channels that connectboth cells directly.

(2) Transmission or transport of moleculesacross a tissue boundary, where the moleculesare transported through a passage between cells(paracellular transport).Seetransport, transcel-lular.

transmission line Generally any conductorused to transmit electric or electromagnetic en-ergy. In particular it refers to:

1. the power lines that carry electrical powerto residential and industrial areas,

2. the electrical cables and waveguides usedin telecommunication to transmit electrical sig-nals,

3. the cable that connects an arial to a trans-mitter or receiver. Some of the common typesused are coaxial cable and two-wire line.

All transmission lines can be described by anetwork of discrete parameters such as induc-tors, capacitors, and resistors, distributed uni-formly along its length. The transmission lineis referred to as being balanced if the conduc-tors have identical properties (e.g., resistanceand impedance).

transmission loss The decrease of power inthe signal being transmitted from one point toanother of a telecommunication system. It isgiven by the ratioP1/P2 whereP1 is the mea-sured power closer to the signal source than themeasured powerP2. This is usually expressedin decibels or nepers.

transmission, multipath Multipath trans-mission is when radio signals reach the receivingstation by more than one path. Some causes forthe signals propagating over more than one pathare atmospheric ducting, ionospheric reflectionand refraction, and reflection from terrestrial ob-jects, such as mountains and buildings. Multi-path transmission causes constructive and de-structive interference, and phase shifting of thesignal.

transmission, serial Serial transmission isa communication system in which the bits in aword are transmitted sequentially along a singleline. This is in contrast to parallel transmission


in which the bits in a word are transmitted at thesame time over several lines.

transmission, synchronous Synchronoustransmission is a form of digital transmissionin which the time interval between any two sim-ilar significant instants in the overall bit streamis always an integral number of unit intervals.

transmittance We consider a beam of lightincident at angleθi to the normal of the bound-ary between two media of different refractiveindices. The radiant flux density or irradianceof the incident beam isI = CEo

2 E2ow/m

2. Thisbeam will be partially reflected at angleθi andpartially transmitted at angleθt.

The transmittance of the interface is

T =It cos θt

Ii cos θi=nt cos θt

ni cos θi

(Eot

Eoi

)2

.

Similarly, the reflectance is

T =Ir cos θr

Ii cos θi=IrIi

=(Eor

Eoi

)2

.

Application of the principle of conservation ofenergy yields the important result

R+ T = 1 .

Transmittance varies from zero (no transmis-sion) to one (transparent interface). This param-eter is of fundamental importance in describingthe performance of optical components and de-vices.

transmitter A device, circuit or apparatusused in a telecommunication system to generateand transmit an electrical signal to the receivingpart of the system.

transport, coupled solute and solvent (cell)In transport of molecules across the cell mem-brane sometimes the problem of forcing amolecule (substrate) opposite to its concentra-tion gradient is solved by coupling the move-ment to the downward flow of another substrate.In this way, the favorable energy balance thatcomes from the diffusion of the substrate downits concentration gradient is used to drive theother in the energy-absorbing motion from low

to high concentration. This transport is some-times called “secondary” active transport be-cause the energy used to achieve it does notcome directly from the energy released from cellmetabolism.

Two kinds of coupling can occur in this trans-port, one in which the two crossing species crossthe membrane in different directions (antiport),and the other in which they cross it in the samedirection (symport). Because osmotic pressurewould drive a solvent to cross in favor of itsconcentration gradient, coupled solute and sol-vent would occur in symport, where both speciescross in the same direction. In both types of thecoupled transport the two species of moleculeshave to be available either simultaneously orsequentially on the corresponding sides fromwhich they are transported.

Many essential nutrients are transported bysymport systems coupled to Na+ or proton gra-dients. The system of uptake of neurotransmit-ters is a similarly coupled symporter process.

transport, electrical (cell) Refers to the abil-ity of cells to transport electricity along theircell membrane. An example is nerve cells whentransporting an electrical signal across the ner-vous system. This is accomplished by sequen-tial opening and closing of ion channels alongthe membrane that permit the propagation of thenon-equilibrium membrane potential (action po-tential) for long distances. The strength of theelectrical signal is proportional to the perme-ability of the membrane to the ions that cross,and this is dependent on the action of the ionchannels across the membranes.

Another example of electrical transport isseen in the heart, where an electrical transportcycle is established between the Sinoatrial node,the Atrioventricular node, the Bundle of His,and the Purkinjie fibers that are responsible forkeeping the heart beating in rhythm. These cellswork like conductive wiring in the heart.

transport, ion current (cell) Changes in thecell membrane polarization may cause flow ofions across the membrane. This is because eachion has an “equilibrium potential” that keepsboth sides of the concentration of the ion sta-ble and without flowing; once perturbed, an ioncurrent is established that will stop when the


equilibrium potential for that particular ion isobtained.

Ion current across the cell membrane is con-ducted via “ion channels” or ion pumps (seeos-motic equilibrium (cell)). Ion pumps requirethe expenditure of energy in their functioningand form the basis for active transport that regu-lates the osmotic balance in the cell.SeeNernstequilibrium potential; nerve impulses, propaga-tion of; potential, membrane.

transport, transcellular Transport ofmolecules from one region to another separatedby cells. The transport is carried out by trans-porting the molecules across the plasma mem-brane of the blocking cells.

An example of this transport is the transportof molecules across the intestinal epithelium.Transport of molecules via a transcellular routehappens when they cross the plasma membraneof the epithelial cells. If the transport occursacross the junctions between epithelial cells, itis calledparacellular transport.

In the intestinal epithelium, water, for exam-ple, can be transported both trans- and paracel-lularly. Large organic molecules (e.g., aminoacids and glucose) cannot pass in between cells,so they are transported via the transcellular routewith the help of transporter molecules.Seetransport, uncoupled.

transport, uncoupled Transport that occurswhen particular molecules or ions are trans-ported selectively through membranes withoutany coupling to any other substrate. It is alsocalledfacilitative transport.

Molecules that fall into this category aremany water-soluble molecules, like sugars andamino acids, that cannot penetrate the mem-brane because they are too large to fit throughopen channels. Included in this group are alsosome ions that do not diffuse through channels.

The molecules and ions that undergo uncou-pled transport penetrate or leave the cell throughthe action of membrane transporters. Thesetransmembrane transporters offer a highly spe-cific binding site to particular molecules that,once attached to the binding site, the moleculeis transported either out or into the cell. The pro-cess by which the molecule crosses is not fullyunderstood, but it is known that the transporters

do not offer just a hole for the molecule to gothrough, like typical channels do.

Because the molecules that get transportedmove down their concentration gradient, uncou-pled transport is considered a type of diffusion.Two examples of transporters are glucose andthe bicarbonate ion.

transverse waves The direction of propaga-tion of the wave is at right angles to the direc-tion of propagation of the particles. This type ofwave can be propagated on a stretched string.

traveling wave A wave pulse will movetransporting energy as it does by the vibration ofneighboring particles in the media. The mediumitself does not move, but energy is thereforetransported in this manner.

traveling wave tube A tube in which inter-action of electrons produces a wave.

triad A chord of three tones, one consistingof a given tone with its major or minor aug-mented or diminished.

triboluminescence Phenomena involving lu-minescence when intense ultrasonic waves ex-ist in materials; e.g., discharge occurs in crystalplanes separated by a small distance on whicha high voltage gradient exists.See alsosonolu-miniscence.

trichromatic coefficient Gives a quantita-tive assessment of color in terms of three stan-dard primary colors: red, green and blue vio-let for example. In the analysis of an unknowncolor, a spectrometer is used to measure thespectral reflectance (or transmittance if appro-priate) of the three components above, and tri-stimulus valuesx, y and z are determined atthe given wavelengths. The trichromatic coeffi-cients are then given byx = x/(x+ y+ z), y =y/(x+ y+ z), andz = z/(x+ y+ z). See alsocolor match.

triggering Releasing, emitting, and creatingby an external pulse.

triggering, ramp An electric circuit that gen-erates a wave when receiving an external pulse.


triplet A spectral line that can be split intothree component lines upon removal of degen-eracy by an appropriate applied field.

Trouton’s rule Involves the heat of vapor-ization at the normal boiling point for liquids.The ratio giving entropy change due to vapor-ization at the normal boiling point is not con-stant but increases with temperature. A roughapproximation is about 9, which is useful whenthe critical temperature is not known.

trumpets, sound from The blown frequen-cies with the valves open are in general multiplesof 115 Hz with the fundamental frequency of115 Hz missing. It has a frequency range fromabout 200 Hz to about 1000 Hz, significantlygreater than other orchestral brass instruments.

trunk call A telephone call on a trunk linein which the charges are calculated according tothe distance of the call.

trunk, line A transmission line that is usedto interconnect two electric power stations ortwo electric power distribution networks. It isconsidered a main telephone line in the system.

truth table A table that lists the value of oneor zero for each input and output terminals.

tune Succession of notes or chords formingthe characteristic music of a song or other piece.

tuned circuit The circuit has been adjustedto produce a resonant wave.

tuning forks Instruments of great purity oftone and constancy of frequency. It is used asa means of indicating and preserving standardpitches.

tunnel diode A diode in which carriers passthrough a sharp barrier by a quantum effect.

turbulence, acoustic Sound levels of soundwaves are strongly affected by random and tur-bulent fluctuations in wind and temperature inparticular in shadow zones. It can also cause thedirection of a source to be difficult to identify,and a degradation in signal coherence.

turgor pressure Turgor pressure gives thenormal fullness or tension found in animal tis-sue and plants. The turgor pressure originatesfrom the pressure that comes from the internalfluids of cells from plants and animals, and thefluid content of blood vessels and capillaries inanimals, for example.

turmalin A mineral with electric propertiesthat is also used as a gem.

turns ratio The number of active turns in thesecondary windings of a transformer divided bythe number of turns in the primary windings.See alsotransformer.

tweeter A small loudspeaker that reproduceshigh frequency sounds in high fidelity audioequipment for high frequencies 3000 to 20,000Hz.

twin cable A transmission line that has twoparallel conductors separated by insulating ma-terial. The line impedance of a twin cable isdetermined by the diameter and spacing of theconductors.

twinning, crystal The mode of plastic de-formation of a crystal (particularly hexagonalclosed pack or body centered cubic crystals) re-sulting in a partial displacement successivelyon each of many neighboring crystallographicplanes such that the deformed part of the crystalis a mirror image of the undeformed part.


Uultra high frequencies (UHF) The shortwave band containing radio frequencies fromabout 300 MHz to 3000 MHz. This band ismainly used for the transmission of televisionsignals, radar, and airplane navigation.

ultrasonics Inaudible sound waves in therange greater than 20,000 Hz. The ability to fo-cus and direct these waves in beams of vibrationat high powers in a small area makes them verysuitable for applications such as the productionof heating effects, destruction of bacteria, un-derwater signaling, depth sounding, testing ofmaterials, and seismic exploration.

ultrasound therapy Ultrasound therapy in-volves the use of sound waves in healing softtissue ailments and pain, and speeding up re-covery. The apparatus consists of a soundheadwithin which a crystal vibrates generally in thefrequency range of 1 MHz to 3 MHz.

In ultrasound therapy, it is important to trans-mit the signal with the least loss from the sound-head to the treated tissue. Because air is not agood sound conductor, a gel or lotion is usu-ally put between the soundhead and the skin. Inthis way the gel acts as a “coupling” betweenthe two parts hampering air from getting in be-tween. Underwater therapy is also another op-tion to couple the soundhead and the tissue.

Ultrasound has been used in the treatment ofarteries clogged by cholesterol plaque and bloodclots. Application of the focused sound to theclot in some cases has led to the reduction ofthe blocking material, thus proving beneficial inthe prevention of heart attack. Also, it has beenused in the treatment of breast cancer where theultrasound is used to produce localized heating.

Umklapp process This is the processby which thermal resistance occurs in non-conducting materials. It means “flop over” inGerman and refers to the interaction of three ormore lattice or electron waves in a solid. The

sum of the wave vectors is equal to a vector inthe reciprocal lattice.Seeresistance, Umklapp.

unary operation An operation that containsone variable.

undercooling The most important factor forthe survival of biological cells during cooling isthe rate of cooling. This is of particular interestin areas where cells in suspensions are exposedto low temperatures.

If the cells are cooled too slowly, osmoticeffects may cause harm to the cell since the os-motic equilibrium between intracellular and ex-tracellular fluid is temperature dependent. If thecooling rate is too fast, formation of ice crystalsin the intracellular space may harm the internalstructure.

Optimal values for the rate of cooling yield-ing a high survival curve are related to the hy-draulic membrane permeability that is the lim-iting factor in the cell volume shrinkage.

underwater acoustics Of use primarily tonautical and naval personnel in detecting otherships.See alsoultrasonics.

unidirectional current A direct current flow-ing in one direction only is called aunidirec-tional current.

unison Two notes estimated by ear to havethe same pitch, possessing the same frequency.The harmonic frequencies of each tone matchthe other exactly; the sound is pleasant to theear and is considered a consonant.

unit gain buffer A circuit whose output sig-nal is the same as the input signal and the outputresistor is very large.

unit gain op amp An operation amplifier inwhich the gain is one.

unit planes/points The conjugate planes foran optical system for which the transverse mag-nification is+1.

unit pole A magnetic pole whose strengthis such that when it is exposed to a magneticfield of 1 Oe a force of 1 dyne is exerted on it.


In nature it is impossible to obtain an isolatedpole (magnetic monopole) but in practice, if abar magnet is made very long, a north or a southpole may be approximately isolated to performthis measurement.See alsomagnetic poles.

unstable A state that is changed with a slightvariation of outside conditions.

up-converter A transmitter that changes alow state to a high state.

up-down waveform generators An electricdevice that produces a step wave.


Vvan’t Hoff’s law This refers either to the lawrelating to chemical kinetics or the law govern-ing the osmotic pressure of solutions.

Regarding osmotic pressure,van’t Hoff ’s lawdictates the fundamental law that relates the os-motic pressure in dilute solutions to other pa-rameters as

PV = iRT ,

whereP is the osmotic pressure,V is the vol-ume,T the absolute temperature,R the univer-sal gas constant, andi a measure of the “ab-normality” of the substance. Large values ofiare due to the dissociation of the dissolved sub-stance into ions.

In chemical kinetics,van’t Hoff ’s lawrelatesthe equilibrium constant of the reaction to theparticular concentration of the reacting species.Seeosmometer, and law of mass action.

varactor A device whose reactance can bechanged by external bias voltage.

variable-length code A variable-length coderepresents an efficient source coding methodwhen the source symbols are not equally prob-able. A variable-length code allows more fre-quently occurring data symbols to be repre-sented by shorter code words and more rarelyoccurring data symbols to be represented bylonger code words, thereby increasing informa-tion transmission rate. A variable-length codemapsk input symbols inton output symbols,where eitherk or n (or bothk andn) may vary,depending on the value of the particular symbolin question. The Huffman code and the Morsecode exemplify variable-length coding.

Variac The tradename of a variable auto-transformer with windings on a toroidal core.The output voltage is varied by a rotating brushcontact on the windings.See alsotransformer,auto.

varistor A non-linearly variable resistor.

vector potential A vector quantity,A, usedin electromagnetic field theory to deduce themagnetic induction,B, at position (x, y, z) andtime t by applying the following:

B = ∇×A .

The vector potential,A, is expressed in we-bers/meter. This is in contrast to the potentialV used to determine the electric field strengthEby

E = −∇V ,

whereV is a scalar potential.

vector potential, magnetic A potential thatcan be used to describe the magnetic field. Themagnetic fieldB may be determined from thevector potentialA by

B = curl A .

velocity of sound, Laplace equation Thisvaries in different media, and is 330 m/s in air.Pressure, temperature, density and humidity canaffect the velocity of sound in a gas. Laplaceintroduced a correction to the original formulafor velocity of soundV , in a medium given by√E/ρ, whereE is modulus of elasticity and

ρ the density of the medium. The correctioninvolved recognized that the compression andrarefaction took place so rapidly that the gas didnot have sufficient time to lose or take heat fromsurrounding air; i.e., it took place adiabaticallyrather than isothermally as thought earlier.

velocity selector A device that selects parti-cles of a certain velocity from a stream of other-wise identical particles (same mass and charge).Operates on the principle that, in an appliedmagnetic field, the magnetic deflection force ona moving particle depends on the particle’s ve-locity. The principle of operation is similar tothat of a mass spectrometer.Seemagnetic forceon moving charge, mass spectrometer.

ventriculography, radionuclide A tech-nique using an intravascular radioactive tracerthat gives images of the heart ventricles that ul-timately are interpreted for examination of thestructure and functioning of the ventricles. Data


from several hundred cardiac cycles may be col-lected to present a single composite cardiac cy-cle.

When compared with other competing tech-niques:

1. It assesses the functioning of the ventri-cles (systolic and diastolic functions) in a morereliable and precise way, such as with echocar-diograms.

2. It gives information about regional andglobal wall motion.

3. It gives the heart chamber size and mor-phology.

4. It gives left and right ventricular ejectionfractions (proportion of blood present in the leftventricle at the end of ventricular diastole that ispumped through the aortic valve during systole).

On the down side, besides being more costly,it requires the puncture of veins and radiationexposure, and it yields less accurate left ventricledata, as with echocardiograms.

ventriloquism The act of speaking or utter-ing sounds with barely visible lip movement.

vertical scanning Movements of the scan-ning beam with the change of a vertical variable.

very high frequencies (VHF) The band ofradio signals in the frequency range from 30MHz to 300 MHz. This band is used for FMand amateur radio broadcasting as well as fortelevision transmission.

very low frequencies (VLF) The band ofradio frequencies lower than 30 KHz, some-times employed in underwater communicationbetween submarines.

vibration of magnet Refers to the oscillationof a bar magnet when suspended in a magneticfield. The frequency of oscillationf is givenby f = (1/2π)(MH/I)1/2, whereM is themagnetic moment of the bar magnet,H is themagnetic field intensity, andI is the moment ofinertia of the bar magnet. The measurement off allows determination of the magnetic momentM of the bar magnet.

vibrations The to and fro motion of someparticles, either freely or acted upon by external

periodic forces and friction in a medium. Whena vibrating body is immersed in a solid, liquid orgas, sound waves are set up since the vibratorydisplacements are in the same direction as thepropagation of the wave.

vibrators, bone-conduction Conventionalhearing aids make use of amplification of thesound waves and require that the ear still hassome perception (to some degree) of the air con-duction hearing mechanisms. There are people,however, for whom this poses a problem, andother methods have to be used.

By attaching a special vibrator directly to thecranial bones, sound can be transferred directlyto the inner ear, bypassing the air conductionparts (ear canal, ear drum, and ossicles) of themiddle ear. This allows pure tones to still betransmitted by bone conduction, as in air con-duction hearing. Depending on the apparatusused, the attachment is generally performed asa minor surgical procedure.

vibrometer Instruments used for monitoringvibrations in systems often using optical tech-niques.

video frequencies Frequencies in the band ofGHz that allow for the operation of equipmentsuch as video systems, transmitters, receivers,antennas, and power supplies.

videophone A telephone device that trans-mits a visual image and sound. The receivercan receive and display visible images simulta-neously with the telephone signals.

vidicon A camera tube that shows an imageafter the emitting electron beam hits the insidesurface of the tube.

viewing distance The distance between thefilm or plate and the second nodal point (thepoint where the extended outgoing ray meets theoptical axis). For distant objects, it is simply thefocal length of the camera lens, which is usuallyless than the least distance of vision. Therefore,to get proper perspective, one can use a convexlens with the same focal length as the cameralens and well-accommodated eye. Alternately,one can enlarge the photograph until the view-


ing distance is equal to or greater than the leastdistance of distinct vision. In the case of a tele-photo lens, to get proper perspective, one has touse a viewing distance equal to the focal length,which is quite large, thus losing the advantageof the large image size.

vignetting The practice of cutting off or ob-structing the rays in the outer fringes of theimage-forming pencil, thus avoiding the asso-ciated aberrations and improving the quality ofthe image. In a camera, the finite size of thephotographic plate effectively performs the vi-gnetting of the image. The term is also usedto describe the reduction in the effective beamarea with increasing obliquity due to mechan-ical obstructions in the optical system such asapertures, lens holders, etc.

violins, sound from The origin of the soundfrom the vibrating strings. The vibrations of thestring are transmitted to the instrument via thebridge; the sound post and the entire body ofthe violin is involved in the sound productionprocess.

virtual spaces The virtual extension of the(real) space on the left side of a refracting sur-face to the right of the surface, and vice versa.In detail, a refracting surface can be consideredto divide the space into two parts, known as theleft- and right-hand spaces. Both spaces can beconceptually extended to infinity in both direc-tions. The left-hand space is considered real onthe left side and virtual on the right-hand side ofthe refracting surface. Similarly, the right-handspace is considered to be real on the right-handside and virtual on the left-hand side of the re-fracting surface.

viscoelasticity, surface A viscous substanceis such that when stress is applied the substancewill flow continuously opposing the stress witha constant opposing force. The flow is the resultof the movement of molecules past one another,and the resistance is due to intermolecular fric-tion. The deformation does not involve chem-ical bond stretching or bond rotations, and ide-ally all of the mechanical energy is dissipated asheat. At the moment that the stress is removed,the substance will remain in the deformed state

because there has been no internal energy storedfor the substance to return to its initial (or anyother) state. In this sense, viscous deformationis irreversible.

Elasticity describes a substance that, upondeformation, returns quickly and completely toits initial conformation. Contrary to viscosity,the deformation is entirely by chemical bondstretching and rotating. The deformation is re-versible.

Viscoelasticity is a fundamental property ofpolymers, in which both the viscous and the elas-tic properties are exhibited in some degree thatdepends on the chemical composition, temper-ature, and time and strength over which the de-formation stress exists. Viscoelastic responseis found typically when a polymer undergoesdeformation and, upon release, reverses the de-formation in a process that requires substantialperiods of time. The same polymer, under dif-ferent conditions of stress, may exhibit eitherviscous or elastic behavior. For a typical poly-mer, at higher temperatures, the response shiftsfrom elastic towards viscous behavior. At lowertemperatures, the response is reversed, resultingin elastic rather than viscous behavior.

Polymers deposited on surfaces or the surfaceof bulk polymers may exhibit a slightly modi-fied behavior than in bulk since the layers at thesurface only interact with the bulk at one side;thus the elastic behavior could be modified.

viscoelastometer A viscoelastometeris aninstrument for the measurement of viscoelasticproperties. The materials under study are usu-ally polymers, prototypes of viscoelastic sub-stances. Results from trying to determine thematerial’s deformational response to mechani-cal force and flow under stress, give informationregarding the rheology of these materials.

Techniques vary, but the general method is tomeasure the resilience (elasticity) of the materialand flow properties when subjected to mechan-ical stress. This has application, for example,in measuring the change in flow of a resin as afunction of time during the hardening process.Seeviscoelasticity, surface.

vision, binocular Vision in which both eyesare used to give rise to a single, fused precept.This occurs as a coordinated function of both


eyes. The fusion of two distinct images fromthe two eyes by the brain into a single imagewhereby one can judge depth or portion of ob-jects accurately in a three-dimensional field canbe defined as binocular vision. To have goodbinocular vision without diplopia (double vi-sion), the images of the object must fall on whatare called corresponding points on the two reti-nas of the two eyes.

vision, color Vision in which color sense ispresent.Seevision, photopic.

vision, defects Any ametropia of the eye (i.e.,myopia, hyperopia, hypermetropia).

vision, photopic Vision due to cone photore-ceptor function. This is normally at high lev-els of illumination (>10 candelas/meter2) andis characterized by the ability to discriminatecolors and small details. Color is perceived be-cause of the trichromatic nature of the three conetypes.

vision, scotopic Vision due to rod photore-ceptor function. Rods are active at low levels ofillumination (<0.001candelas/meter2). This ischaracterized by a lack of ability to discriminatecolors and small details. Scotopic vision (nightvision) is characterized by high sensitivity at lowlight levels and for detection of movement.

vocoder A synthesizer that produces soundsfrom an analysis of speech input. The wordvocoder comes from the combination of thewordsvoiceandcode.

voice coil That part of a loudspeaker thatconnects to the vibrating diaphragm. It is ca-pable of moving to and fro in a radial magneticfield whose direction is perpendicular to the coilwinding. The driving force applied to the di-aphragm is directly proportional to the currentflowing through the driving coil.

voice, human The sounds of the human voiceare produced when a current of air from the lungsis forced through theglottis or narrow slit be-tween the vocal chords. These two membranousreeds are situated just above the junction of thewindpipe with the larynx and are coupled to a

series of air cavities formed by the larynx, thefront and back parts of the mouth, and the noseand its associated cavities.

Voigt effect Discovered by Voigt in 1902.When a strong magnetic field is applied to a va-por through which light is passing, the mediumbecomes birefringent; i.e., double refractiontakes place. The effect is analogous to the Fara-day effect of optical activity induced by a mag-netic field.

If a vapor has a response frequencyvo, sup-pose that a normal Zeeman triplet is formed byapplication of a magnetic field:

v = vo ±∆v .

When white light is incident that spectral rangeatvo will be absorbed and components at±∆vwill have 50% absorption compared to the cen-tral peak. When plane polarized light is inci-dent, it will be split into components paralleland perpendicular to the magnetic field, whichhave different refractive indices. Induced bire-fringence and a change to elliptical polarizationresult.

volt Symbol: V . The SI unit of electric po-tential, potential difference, and electromotiveforce. One volt potential difference is definedas the ratio of 1 watt of power dissipated by 1ampere current between two points in a circuit.Alternatively, 1 volt is 1 joule of energy requiredto transfer 1 coulomb from one point in a circuitto another. An electric potential of 1 volt ata point is 1 joule of energy used to transfer 1coulomb from infinity to that point. The stan-dard for potential difference is obtained from aspecial form of electrolytic cell, e.g., a Westonstandard cell. The volt is named after countAlessandro Volta (1745–1827) who developedthe first rudimentary battery.See alsopotential,electric; potential difference.

voltage The value of potential difference be-tween two points and electromotive force.Seealsoreactive voltage; volt; potential difference;potential, electric.

voltage clamp, ionic current in cell In thevoltage clamp technique it is possible to con-trol the voltage potential across a cell membrane


while measuring the current that is directly re-lated to the ionic movement across the mem-brane. When the voltage clamp establishes apotential across the membrane, the induced cur-rent will be a result of the ionic current as wellas the capacitive contribution, given by

Ic = C(dv/dt) .

Once the charge distribution on each side of themembrane has been established, the capacitivecontribution goes to zero, and for long periodsthe current will be the result only of the ionicmovement across the membrane.

If the clamped voltage is that of a particularequilibrium potential of a particular ion, then thecurrent will be the result of the flow of the otherions in solution.Seepotential, resting.

voltage drop The decrease in potential alonga conductor or across the terminals of a resistiveelectrical component as a result of the flow ofcurrent through them. It is given by the potentialdifference between two points of the conductoror the two sides of the component.Seepotentialdifference.

voltmeter A device that measures potentialdifference. Generally it can be used to measureboth AC and DC voltage. The input impedanceof a voltmeter is very high so that minimal cur-rent is drawn from the circuit being measured.Moving coil analog voltmeter, digital voltmeter,and cathode ray oscilloscopesare some com-monly used devices for measuring voltages.

vortex sound If a steady flow of fluid passesan obstacle, eddies are usually formed behindthe obstacle. At the edges of the stream, swirlingwhirlpools are formed. The rotational motionof the vortices causes the superposition of manyfrequencies and sound is heard. Examples area person whistling, or sound of wind through acrack.

vowel sound Characteristic frequencies as aresult of the natural vibrations of the oral cavitiesexcited impulsively by the more or less periodicpuffs of air from the glottis.


Wwater, undercooled Also known assub-cooled water,this refers to water that continuesto exist in a liquid state at a temperature belowthe freezing point of 0 C. It is in a metastablephase.Seesupercooling.

water, unfreezable Seewater, undercooled;supercooling.

watt Symbol: W. The unit of power. It isdefined as the dissipation of one joule of energyin one second. The power,P , dissipated by aresistor is given by

P = IV ,

whereI is the current through the resistor inamperes andV is the potential difference acrossit in volts. See alsoOhm’s law.

wattmeter An instrument that indicates theinstantaneous value of the power expended in acircuit to which it is connected. Generally, it canbe used in both AC and DC. Its operating prin-ciple is based on the product of current througha circuit with the potential difference across it.Consequently, there are four terminals on theinstrument, i.e., two for the current to be con-nected in series with the circuit, and two for thevoltage to be connected in parallel.

wave analyzer Spectrometers that displaysimultaneously the frequency and amplitude ofthe more important component of complexsound.

waveform Refers to the different types ofwaves that can be produced, e.g., the sine wave,square wave, and sawtooth waveforms.

wavefront For a three-dimensional wavepulse, the surface in its path of propagation allof whose points are in the same phase of motion.

Sine wave, square wave, and sawtooth wave.

wave front or wave surface This is the lo-cus of points of equal phase of a wave. For aharmonic wave

ψ(

r , t)

= A(

r)ei(

k ,r−ox) ,

where

k is the propagation vector.The equation of the wave front is

k • r = constant.

If the amplitudeA(r ) is not constant over the

wave front, the wave is calledinhomogenous.The concept of wave front is important for

wave propagation in general in that it allows def-

inition of the phase velocityvp ≡ ω/|

k |. It isthe basis for the description of all wave phenom-ena.

wave function Of a system of particles, thisis the solution to the Schrodinger equation. Itcontains all of the dynamic information of thesystem. Its main physical interpretation is as aprobability density amplitude used to determinethe spatial probability function for the particles.From the wave function and the Schrodingerequation one can determine the quantum num-bers and energy levels of the system — of primeimportance in statistical physics.

waveguide A tube along which electromag-netic waves are transported.


wavelength For a harmonic plane wave prop-agating in a direction of unit vector

s .

ψ(

r , t)

=(A/

r)

exp i

[ω

(t−

r .

s

v

)].

The wavelength is the spatial period of the mo-tion such thatψ(

r , t) is the same when

r •

s

is replaced byr •

s + λ.This givesλ = vp

2πω .

wavelets, secondary The laws of propaga-tion of light waves are based onHuygen’s princi-ple,which states that every point on a wave frontacts as a source of secondary wavelets that havea velocity and frequency corresponding to theprimary wave. The envelope of the secondarywavelets corresponds to the new primary wave-front a short time later. This intuitive notion waslater put on a firm theoretical basis by Fresneland Kirchoff.

wave number This is defined as2π/λwhereλ is the wavelength of the wave in question. It isa convenient way of expressing the wavelengthin the wave equation.

wave propagation A wave is transmittedthrough a medium by the vibrations of parti-cles. It moves forward by the vibrations be-ing transmitted to adjacent particles. Regionsof compressions and rarefactions are set up as itpropagates.See alsotraveling wave.

wave pulse This can be produced in astretched string by giving it a sideways tug. Thiswill produce a pulse that will travel down thestring, each particle in string remaining at restuntil the pulse reaches it; then it moves for shorttime and then returns to rest.See alsowavetrain.

waves, intensity from point source In athree-dimensional wave, such as a sound wave,spherical waves travel outward from the pointsource, and energy may be absorbed as theytravel through space or a medium. The intensityof the space wave is defined as the power trans-mitted across a unit area normal to the directionin which the wave is traveling and is proportionalto the square of the amplitude.

wave speed Mechanical waves need to travelin a medium and the properties of the medium,such as inertia and elasticity, determine thespeed of the wave through it. The speedV isalso related to the wavelengthλ and frequencyf by the following relation.

V = fλ .

wavetrain A wavetrain is a monochromaticwave of finite length that contains a certain num-ber of cycles. Light emitted from atoms is emit-ted as wavetrains approximately10−8 to 10−10

secs, as a series of random bursts. The widthof the wavetrains is a measure of the coher-ence time; from this, one can define a coher-ence length over which the phase is well defined.Several wave pulses in succession will producea train of waves.

weber International System unit of magneticflux, equal to Tesla.meter2. One weber is de-fined as the flux linking a one turn circuit that,when reduced uniformly to zero in one second,induces an electromotive potential of one voltin the circuit. Named after German physicistWilhelm E. Weber (1804–1891). Equal to 108

maxwells (or gauss.cm2) in CGS units.

Wheatstone bridge A very useful elec-tric circuit developed and advocated by CharlesWheatstone in 1843. It is widely used to de-termine the unknown resistance of a resistor. Itconsists of two known resistors, a variable resis-tor, an unknown resistorRx, as in the followingfigure. A voltage source is connected to pointsA andB. Adjust the variable resistor until thecurrent i of the galvanometer is zero. Underthis balance condition, the unknown resistanceis given by

Rx =R1

R2Rv .

whistle To make a clear musical sound by theexpulsion of breath; an instrument for producinga whistling sound. Sounds may be generated bypassing a gas or liquid through an orifice or overan edge. The passage generates vortices, spacedperiodically, which propagate as a sound wave.


Wheatstone bridge.

whistle, Galton One of the first type of whis-tles, the Galton whistle consists of a jet thatsends out a stream of gas against a small cavity.A miniature organ pipe is used to determine theupper limit of audibility, generally for controland signaling purposes. The small, adjustableclosed pipe is about 1 in. in diameter; the reso-nant length of the pipe is altered by means of apiston controlled by a screw.

whistler waves A type of wave that seeksapplication in many branches of physics, e.g.,plasma physics, the study of magnetic fields, andastrophysics.

whole tone The interval on the musical scalebetween any two notes such asC andD.

Wiedemann-Franz law This gives the rela-tionship between the electrical and the thermalconduction in a metal, since the flow of electronsis responsible for both these quantities. Accord-ing to the law, for all metals at temperaturesnot too far below 300 K, the ratio of thermalto electrical conductivity is approximately con-stant and, when divided by the temperature, isknown as theLorenz number.However, the lawis not obeyed at all temperatures, andL eventu-ally starts to fall as the temperature is decreased;at the lowest temperatures, it starts to rise againto its original value. This implies that the meanfree path of the electrons in electrical conduc-

tion is not always the same as that for thermalconduction.

Wimshurst machine An electrostatic highvoltage generator that consists of two counter-rotating glass disks that have a large numberof metal plates at their perimeters. The disksusually face each other since their axes coin-cide, and the metal plates are situated on theouter faces of the disks. Metal combs, placed onthe outer sides of the disks, collect charge fromthe metal plates of opposite polarities. Metalgrounding brushes are placed at the top and bot-tom of each disk. Wimshurst machines can varyin size and method of rotating the disks. Somedemonstration models are spun by hand whileothers used for generating X-rays are motor-driven.

wind effect, on sound Sound travels bet-ter with the wind than against it. The velocityof wind increases from the earth’s surface up-wards. If the wave is traveling against the windthen its upper portion will be retarded more thanthe lower; the opposite occurs when the soundtravelswith the wind. In this case the upperportion moves with a greater velocity than thelower, the direction of motion of the wavefront isgradually brought down toward the ground, andthe observer may experience a concentration ofsound.

winding, primary The coil winding in atransformer that receives the energy from thesupply circuit.Seetransformer.

winding, secondary The coil winding in atransformer that receives energy from the pri-mary winding.Seetransformer.

wind instruments Any instrument that canbe played by breath or air, e.g., flute, horn ororgan.

wolf note On stringed instruments, a verydifficult note to find, which on being producedcauses the whole body to vibrate to an unusualdegree.


woofer A loudspeaker in a sound-reproducing apparatus that picks up low soundfrequencies up to 500 Hz.

work hardening Repeated bendings of a barof soft metal, until the bar eventually refuses tobe bent and breaks. With every bending, moreand more dislocations flow into the metal untilthere are so many dislocations that they impedeeach other’s flow. The crystal is then incapableof further plastic deformation and breaks undersubsequent stress.

work of electrical force The energy ex-pended by an electric field,E, in moving acharge,q, a displacement,l. For a uniformelectric field this is given by

W = qE · l ,

whereW is the work done andqE is the electric

force on the charge. Equivalently, it is given by

W = qV ,

whereV is the potential difference between theinitial and final position of the charge. In a non-uniform electric field, the work is defined as

W = q

∫ B

A

E · dl ,

wheredl is an infinitesimally small displace-ment, andA andB are the initial and final posi-tions respectively. Generally, one deals with thework done by an external force in moving thecharge from A to B in the electric field. This isthe negative of the work done by the field and isthus given by

W = −q∫ B

A

E · dl .


Xxerography An electrostatic process forreproducing documents. Many technical ap-proaches have been used but the most commonis to use a photoconductive layer. The opticalimage to be copied is formed on the surface ofthe layer which is then electrostatically chargedby corona discharge. The charge leaks off at theoptically exposed regions, forming an electro-static image; colored particles (toner) are thendeposited at the charged regions and the finalimage is formed.

xeroradiography Technique in radiologysimilar to X-rays where instead of an X-ray film,a positively charged selenium plate is used. Ex-posure to radiation reduces the positive chargeof the plate in different regions, depending onthe level of exposure received by each region.Developing of the impressed image follows byapplying toner powder to the plate.

The xeroradiography imaging process pro-duces an image with higher resolution on theedges of bones and better visualization between

different soft tissue structures in the patients’body. It is widely used in mammography.

X-ray fluorescence Absorption of an inci-dent X-ray and subsequent re-emission of anX-ray by fluorescence from a higher atomic coreenergy level. The selection rules for the pro-cess in terms of atomic quantum numbers are∆n ≥ 1,∆` = ±1,∆j = 0,±1.

X-ray fluorescence is carried out with anX-ray source (Coolidge tube) used to irradiate asolid or liquid sample. The emitted X-rays areanalyzed by an X-ray spectrometer.

X-rays Electromagnetic waves in the rangeof about 0.1 nm wavelength. They are gener-ated by electronic transitions due to the bom-bardment of materials of high atomic weight byhigh energy electrons.

X-rays have many applications in science andtechnology. Since their discovery, elastic X-ray diffraction has been the principal means forthe determination of crystal structure and ori-entation. Diverse spectroscopic tools (seespec-troscopy, electron) involving X-rays are used forthe characterization of ultra-clean surfaces. Inboth medicine and industry, X-rays have longbeen used for the non-destructive detection ofdefects in the interior of opaque objects.


Yyoke Completes a magnetic circuit. Usuallya soft ferromagnetic material. Can be in contactwith a core that is also made of a soft ferromag-netic material and is surrounded by a current-carrying coil. The yoke transmits the magnetic

flux from the core through the yoke to anotherpart of the magnetic circuit.

Young-Helmholtz law When any point of astring is plucked, struck or bowed, all the over-tones requiring that point for a node will be ab-sent from the vibration. Therefore, the pointof plucking determines the quality of the noteemitted.

Y-parameters Variables that are in responseto other parameters and plotted in they-axis.


ZZeeman effect Discovered by P. Zeeman in1896 who observed the broadening of spectrallines in the presence of a magnetic field. It waslater shown that the lines were split into dou-blets, triplets and higher order. The effect wasexplained by Lorentz in his classical theory ofthe electron: Ifvo is the electronic frequency atzero field, then in a magnetic fieldH, it is splitinto componentsvo ± ∆v, where∆v = eB

m ,wheree = electronic charge,m = electronicmass andc = velocity of light.

The Lorentz theory predicts the following be-havior:

(a) Transverse Zeeman effect(light beam per-

pendicular to

H): splitting of the line into atriplet with plane polarized components. This iscalled thenormal triplet.

(b) Longitudinal Zeeman effect(light beam

parallel to

H): splitting of the line into a doubletwith components circularly polarized in oppos-ing senses.

In fact, only the normal triplet can be ex-plained by the classical theory. More complexobserved behavior such as multi-component split-ting higher than three (anomalous Zeeman ef-fect), asymmetric splitting, etc. can all be ex-plained by a detailed quantum mechanical treat-ment.

Zeeman effect, inverse The Zeeman effectas observed in absorption. It is produced bysending white light through a vapor subject toa magnetic field. Since the light is not com-pletely absorbed, the transmitted componentscorrespond to those observed in emission butthey are circularly polarized in the opposite di-rection.

zener breakdown Avalanche breakdownin which electrons in a diode rapidly increasethrough ionization collisions with atoms.

zener diode A pn junction that forms a diodewhose reverse current rapidly increases at someparticular reverse bias voltage.

zero sound The effect that occurs at thecharacteristic temperature in a Fermi gas whenthe frequency between collisions becomes equalto the applied frequency. The propagation ofsound is impossible since the collisions cannotoccur fast enough. For the interacting Fermi liq-uid, a collisionless sound propagation may beexcited. In this situation an increase in propa-gation velocity and a maximum in attenuation isobserved as the sample is cooled through ordi-nary region where ordinary sound gives way tozero sound.

zone of silence With highly intense soundssuch as explosions, these are regions where thesound is not audible.

zone plates A circular grating that acts as acondensing lens causing the intensity of soundat the center of the grating to be intensified.

zone refining A melting region in a crystalgrowth apparatus. The melting region is smallerthan the ingot length. Using this technique amore purifying crystal can be made.

zoom lens A telephoto lens whose focallength can be varied from about 80 mm to 1000mm or more without changing the sharpness ofthe image. This is achieved by employing a sys-tem of converging and diverging elements, oneor more of which can be moved. To keep thef -number unchanged, one usually has a basicimaging system and a variable focus arrange-ment.


Documents

Dictonary of Pure and Applied Mathematics