3. Waveparticle duality postulates that all particles exhibit
both wave and particle properties. A central concept of quantum
mechanics, this duality addresses the inability of classical
concepts like "particle" and "wave" to fully describe the behavior
of quantum-scale objects. Standard interpretations of quantum
mechanics explain this paradox as a fundamental property of the
Universe, while alternative interpretations explain the duality as
an emergent, second-order consequence of various limitations of the
observer. This treatment focuses on explaining the behavior from
the perspective of the widely used Copenhagen interpretation, in
which waveparticle duality is one aspect of the concept of
complementarity, that a phenomenon can be viewed in one way or in
another, but not both
4. The idea of duality originated in a debate over the nature
of light and matter that dates back to the 17th century, when
competing theories of light were proposed by Christiaan Huygens and
Isaac Newton: light was thought either to consist of waves
(Huygens) or of particles (Newton). Through the work of Max Planck,
Albert Einstein, Louis de Broglie,Arthur Compton, Niels Bohr, and
many others, current scientific theory holds that all particles
also have a wave nature (and vice versa).[1] This phenomenon has
been verified not only for elementary particles, but also for
compound particles like atoms and even molecules. For macroscopic
particles, because of their extremely small wavelengths, wave
properties usually cannot be detected
5. Aristotle was one of the first to publicly hypothesize about
the nature of light, proposing that light is a disturbance in the
element air (that is, it is a wave-like phenomenon). On the other
hand, Democritusthe original atomistargued that all things in the
universe, including light, are composed of indivisible sub-
components (light being some form of solar atom).[3] At the
beginning of the 11th Century, the Arabic scientist Alhazen wrote
the first comprehensive treatise on optics; describing refraction,
reflection, and the operation of a pinhole lens via rays of light
traveling from the point of emission to the eye. He asserted that
these rays were composed of particles of light. In 1630, Ren
Descartes popularized and accredited in the West the opposing wave
description in his treatise on light, showing that the behavior of
light could be re-created by modeling wave-like disturbances in a
universal medium ("plenum"). Beginning in 1670 and progressing over
three decades, Isaac Newton developed and championed his
corpuscular hypothesis, arguing that the perfectly straight lines
of reflection demonstrated lights particle nature; only particles
could travel in such straight lines. He explained refraction by
positing that particles of light accelerated laterally upon
entering a denser medium. Around the same time, Newtons
contemporaries Robert Hooke and Christian Huygensand later
Augustin-Jean Fresnelmathematically refined the wave viewpoint,
showing that if light traveled at different speeds in different
media (such as water and air), refraction could be easily explained
as the medium-dependent propagation of light waves. The resulting
HuygensFresnel principle was extremely successful at reproducing
lights behavior and, subsequently supported byThomas Youngs
discovery of double-slit interference, was the beginning of the end
for the particle light camp
6. ELECTRON EMISSION DIFFERENT METHODS OF ELECTRON EMISSION
PHOTOELECTRIC EFFECT EXPERIMENTAL STUDY OF PHOTOELECTRIC EFFECT
EINSTEINS PHOTOELECTRIC EFFECT OR ENERGY QUANTUM OF RADIATION
PARTICLE NATURE OF LIGHT DAVISSON AND GERMAR EXPERIMENT
7. The liberation of electrons from the surface of a metal is
known as Electron Emission. If a piece of metal is investigated at
room temperature, the random motion of the electrons will be shown
in Fig. However, these electrons are free to the extent that they
may transfer from one atom to another within the metal but they
cannot leave the metal surface to provide electron mission. It is
because the free electrons that start at the surface of metal find
behind them positive nuclei pulling them back and none pulling
forward. Thus at the surface of the metal , a free electron
encounters forces that prevent it to leave the metal. In other
words, the metallic surface offer a barrier to free electrons,
their kinetic energy increases and is known as surface barrier.
However, if sufficient energy is given to the free electrons, their
kinetic energy increases and thus the electrons will cross over the
surface barrier to leave the metal.
8. Work function (W0): The minimum energy required by an
electron to just escape (i.e. with zero velocity) from metals
surface is called Work function (W0) of the metal. The work
function of pure metals varies (roughly) from 2eV to 6eV. Its value
depends upon the nature of the metal, its purity and the conditions
of the surface. We selected those metals for electron emission
which have low work function.
9. The electron emission from the surface of a metal is
possible only if sufficient addition energy (equal to work function
of the sources such as heat energy, energy stored in electric
field, light energy or kinetic energy of the electric charges
bombarding the metal surface. Accordingly; there are following four
principal method of obtaining electron emission from (I) Thermionic
emission: In this method, the metal is heated to a sufficient
temperature (about 2500oC) to enable the free electrons to leave
the metal surface. The number of electrons emitted depends upon the
temperature. The higher the temperature, the greater is the
emission of electrons. This type of emission is employed in vacuu
(II) Field emission: In this method, a strong electric field (i.e.
a high positive voltage) is applied at the metal surface which
pulls the free electrons out of the metal because of the attraction
of positive field. The strong the electric field, the greater is
the electron emission.m tubes.the surface of a metal:
10. (III) Photoelectric emission: In this method, the energy of
light falling upon the metal surface is transferred to the free
electrons within the metal to enable them to leave the surface. The
greater the intensity of light beam falling on the metal surface,
the greater is the photoelectric emission. Photoelectric emission
is utilized in photo tubes which from the basis of television and
sound films. (IV) Secondary emission: In this method, a high
velocity beam of electrons or other out. The intensity of secondary
emission depends upon the emitter material, mass and energy of
bombarding particles.m the basis of television and sound
films.
11. In the photoelectric effect, electrons are emitted from
matter (metals and non-metallic solids, liquids or gases) as a
consequence of their absorption of energy from electromagnetic
radiation of very short wavelength and high frequency, such as
ultraviolet radiation. Electrons emitted in this manner may be
referred to as photoelectrons.[1][2] First observed by Heinrich
Hertz in 1887,[] the phenomenon is also known as the Hertz effect,[
although the latter term has fallen out of general use. Hertz
observed and then showed that electrodes illuminated with
ultraviolet light create electric sparks more easily. The
photoelectric effect requires photons with energies from a few
electronvolts to over 1 MeV in high atomic number elements. Study
of the photoelectric effect led to important steps in understanding
the quantum nature of light and electrons and influenced the
formation of the concept of waveparticle duality.[1] Other
phenomena where light affects the movement of electric charges
include the photoconductive effect (also known as photoconductivity
or photoresistivity), the photovoltaic effect, and the
photoelectrochemical effect. It also led to Max Plancks discovery
of quanta (e=hv) which links frequency with photon energy. Quanta
is also known asPlanck constant.
12. Emission mechanism The photons of a light beam have a
characteristic energy proportional to the frequency of the light.
In the photoemission process, if an electron within some material
absorbs the energy of one photon and acquires more energy than the
work function (the electron binding energy) of the material, it is
ejected. If the photon energy is too low, the electron is unable to
escape the material. Increasing the intensity of the light beam
increases the number of photons in the light beam, and thus
increases the number of electrons excited, but does not increase
the energy that each electron possesses. The energy of the emitted
electrons does not depend on the intensity of the incoming light,
but only on the energy or frequency of the individual photons. It
is an interaction between the incident photon and the outermost
electron. Electrons can absorb energy from photons when irradiated,
but they usually follow an "all or nothing" principle. All of the
energy from one photon must be absorbed and used to liberate one
electron from atomic binding, or else the energy is re-emitted. If
the photon energy is absorbed, some of the energy liberates the
electron from the atom, and the rest contributes to the electrons
kinetic energy as a free particle
13. The theory of the photoelectric effect must explain the
experimental observations of the emission of electrons from an
illuminated metal surface. For a given metal, there exists a
certain minimum frequency of incident radiation below which no
photoelectrons are emitted. This frequency is called the threshold
frequency. Increasing the frequency of the incident beam, keeping
the number of incident photons fixed (this would result in a
proportionate increase in energy) increases the maximum kinetic
energy of the photoelectrons emitted. Thus the stopping voltage
increases. The number of electrons also changes because the
probability that each photon results in an emitted electron is a
function of photon energy. If the intensity of the incident
radiation is increased, there is no effect on the kinetic energies
of the photoelectrons. Above the threshold frequency, the maximum
kinetic energy of the emitted photoelectron depends on the
frequency of the incident light, but is independent of the
intensity of the incident light so long as the latter is not too
high [9] For a given metal and frequency of incident radiation, the
rate at which photoelectrons are ejected is directly proportional
to the intensity of the incident light. Increase in intensity of
incident beam (keeping the frequency fixed) increases the magnitude
of the photoelectric current, though stopping voltage remains the
same. The time lag between the incidence of radiation and the
emission of a photoelectron is very small, less than 109 second.
The direction of distribution of emitted electrons peaks in the
direction of polarization (the direction of the electric field) of
the incident light, if it is linearly polarized
14. Stopping potential The relation between current and applied
voltage illustrates the nature of the photoelectric effect. For
discussion, a light source illuminates a plate P, and another plate
electrode Q collects any emitted electrons. We vary the potential
between P and Q and measure the current flowing in the external
circuit between the two plates. If the frequency and the intensity
of the incident radiation are fixed, the photoelectric current
increases gradually with an increase in positive potential on
collector electrode until all the photoelectrons emitted are
collected. The photoelectric current attains a saturation value and
does not increase further for any increase in the positive
potential. The saturation current depends on the intensity of
illumination, but not its wavelength. If we apply a negative
potential to plate Q with respect to plate P and gradually increase
it, the photoelectric current decreases until it is zero, at a
certain negative potential on plate Q. The minimum negative
potential given to plate Q at which the photoelectric current
becomes zero is called stopping potential or cut off potential.[13]
i. For the given frequency of incident radiation, the stopping
potential is independent of its intensity. ii. For a given
frequency of the incident radiation, the stopping potential Vo is
related to the maximum kinetic energy of the photoelectron that is
just stopped from reaching plate Q. If is the mass and is the
maximum velocity of photoelectron emitted
15. EINSTEINS PHOTOELECTRIC EQUATION According to Planks
quantum theory, light is emitted from a source in the forms of
bundles of energy called photons. Energy of each photon is .
Einstein made use of this theory to explain how photo electric
emission takes place. According to Einstein, when photons of energy
fall on a metal surface, they transfer their energy to the
electrons of metal. When the energy of photon is larger than the
minimum energy required by the electrons to leave the metal
surface, the emission of electrons take place instantaneously. He
proposed that an electron absorbs one whole photon or none. The
chance that an electron may absorb more then one electron is
negligible because the number of photons is much lower than the
electron. After absorbing the photon, an electron either leaves the
surface or dissipates its energy within the metal in such a short
interval that it has almost no chance to absorb second photon. An
increase in intensity of light source simply increases the number
of photon and the number of photo electrons but no increase in the
energy of photo electron. However, increase in frequency increases
the energy of photons and photo electrons.
16. Light as a particle The only thing that interferes with my
learning is my education. -- Albert Einstein Radioactivity is
random, but do the laws of physics exhibit randomness in other
contexts besides radioactivity? Yes. Radioactive decay was just a
good playpen to get us started with concepts of randomness, because
all atoms of a given isotope are identical. By stocking the playpen
with an unlimited supply of identical atom-toys, nature helped us
to realize that their future behavior could be different regardless
of their original identicality. We are now ready to leave the
playpen, and see how randomness fits into the structure of physics
at the most fundamental level. The laws of physics describe light
and matter, and the quantum revolution rewrote both descriptions.
Radioactivity was a good example of matters behaving in a way that
was inconsistent with classical physics, but if we want to get
under the hood and understand how nonclassical things happen, it
will be easier to focus on light rather than matter. A radioactive
atom such as uranium-235 is after all an extremely complex system,
consisting of 92 protons, 143 neutrons, and 92 electrons. Light,
however, can be a simple sine wave. However successful the
classical wave theory of light had been --- allowing
17. The DavissonGermer experiment was a physics experiment
conducted by American physicists Clinton Davisson and Lester Germer
in 1927, which confirmed the de Broglie hypothesis. This hypothesis
advanced by Louis de Broglie in 1924 says that particles of matter
such as electrons have wave like properties. The experiment not
only played a major role in verifying the de Broglie hypothesis and
demonstrated the wave-particle duality, but also was an important
historical development in the establishment of quantum mechanics
and of the Schrdinger equation
18. Davisson and Germers actual objective was to study the
surface of a piece of nickel by directing a beam of electrons at
the surface and observing how many electrons bounced off at various
angles. They expected that for electrons even the smoothest crystal
surface would be too rough and so the electron beam would
experience diffuse reflection.[5] The experiment consisted of
firing an electron beam from an electron gun directed to a piece of
nickel crystal at normal incidence (i.e. perpendicular to the
surface of the crystal). The experiment included an electron gun
consisting of a heated filament that released thermally excited
electrons, which were then accelerated through a potential
difference giving them a certain amount of kinetic energy towards
the nickel crystal. To avoid collisions of the electrons with other
molecules on their way towards the surface, the experiment was
conducted in a vacuum chamber. To measure the number of electrons
that were scattered at different angles, an electron detector that
could be moved on an arc path about the crystal was used. The
detector was designed to accept only elastically scattered
electrons. During the experiment an accident occurred and air
entered the chamber, producing an oxide film on the nickel surface.
To remove the oxide, Davisson and Germer heated the specimen in a
high temperature oven, not knowing that this affected the formerly
polycrystalline structure of the nickel to form large single
crystal areas with crystal planes continuous over the width of the
electron beam.[5] When they started the experiment again and the
electrons hit the surface, they were scattered by atoms which
originated from crystal planes inside the nickel crystal. As Max
von Laue proved in 1912 the crystal structure serves as a type of
three dimensional diffraction grating. The angles of maxim
