Phys 322

Optics

Lecture 1

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EMERGENCY PREPAREDNESS A MESSAGE FROM PURDUE

Historical introduction

Lecture 1

See also Cosmos: A Spacetime OdysseyEpisode 5: Hiding in the Light Hiding in the Light

Optics in Ancient History

A mirror was discovered in workers' quarters nearthe tomb of PharaohSesostris II (1900 BCE).

Ancient Greeks (500-300 BCE)Burning glass mentioned by Aristophanes (424 BCE)Law of reflection: Catoptrics by Euclid (300 BCE)Refraction in water mentioned by Plato in The Republic But they thought that the eye emits rays that reflect off objects.

Pyramid of Sesostris II(also known as

Senusret II)

Ancient Greeks: Ancient light weapons

Early Greek and Roman historians report that

Archimedes equipped several hundred people

with metal mirrors to focus sunlight onto

Roman warships in the battle of Syracuse (213 -

211 BCE).

This story is probably apocryphal.

Optics in the Middle Ages: Alhazen

Arab scientist Alhazen (~1000 AD) studied spherical and parabolic mirrors.

Alhazen correctly proposed that the eyes passively receive light reflected from objects, rather than emanating light raysthemselves.

He also explained the lawsof reflection and refraction by the slower movement of light through denser substances.

Optics in early 17th-century Europe

Hans Lippershey applied for a patent on the Galilean telescope in 1608.

Galileo (1564-1642) used one to look at our moon, Jupiter and its moons, and the sun.

Two of Galileos telescopes

Galileos drawings of the moon

17th century: optics takes off

1621: Law of Refraction, Willebrord Snell 1664: interference: color in thin films, Robert Hooke

light is a rapid vibratory motion in media

1665: diffraction, Francesco Grimaldi 1677: wave theory, Christiaan Huygens 1704: particles, Isaac Newton

Newton about Newtons rings:"I forbore to treat of these Colors, because they seemed of a more difficult Consideration, and were not necessary for establishing the Properties of Light there discoursed of."

Progress in optical instrumentation:refracting telescope, compound microscope

Willibrord Snell

Willibrord Snell discovered the Law of Refraction, now named after him.

Willibrord Snell (1591-1626)

n1

n2

1

21 1 2 2sin( ) sin( )n n

ni is the refractive index of each medium.

17th-century Optics

Robert Hooke (1635-1703) studied colored interference between thin films and developed the first wave theory of light.

Rene Descartes (1596-1659)

Descartes reasoned that light must be like sound. So he modeled light as pressure variations in a medium (aether).

Christiaan Huygens

Huygens extended the wave theory ofoptics.

He realized that light slowed down onentering dense media.

He explained polarization and double refraction.

Huygens principlesays that a wave propagates as if

the wave-front werecomposed of an ar-ray of point sources

each emitting aspherical wave.Double refraction

Christiaan Huygens (1629-1695)

Isaac Newton

"I procured me a triangular glass prism to try therewith the celebrated phenomena of colours." (Newton, 1665)

Isaac Newton (1642-1727)

After remaining ambivalent for many years, he eventually concluded that it was evidence for a particle theory of light.

Particles or waves?

Christiaan Huygens1629 1695

Isaac Newton1643 1727

18th century

Corpuscular theory prevailsWave theory is forgotten

1801: interference, Thomas Youngfamous double-slit experimentcolor in thin filmsdiffraction of lightdiffraction grating:"These colors may be easily seen, in an irregular form, by looking at any metal, coarsely polished, in the sunshine; but they become more distinct and conspicuous, when a number of fine lines of equal strength are drawn parallel to each other, so as to conspire in their effects."

19th century

1814, Frensel rediscoversinterference and diffraction

James Clerk Maxwell

James Clerk Maxwell (1831-

1879)

Maxwell unified electricity and magnetism with his now famous equations and showed that light is an electromagnetic wave.

2

0

10

BE EtEB B

c t

where is the electric field, is the magnetic field, and cis the velocity of light.

E

B

19th century: Maxwell

0

qAEE

0ABBt

lE B ||E

tIlB E00||

~1864:Maxwell introduced four equations that described all known electro-magnetic phenomena and showed theoretically that electromagnetic pulse or wave moving in space could exist.

Surprisingly, he found that this EM-wave must move at a speed of 300,000 km/s - i.e. speed of light!

Maxwell suggested that light is electromagnetic wave

Maxwells equations

Light is an electromagnetic wave.

The electric (E) and magnetic (B) fields are in phase.

The electric field, the magnetic field, and the propagation direction are all perpendicular.

19th century: HertzIn 1886 Heinrich Hertz experimentally proved the electromagnetic wave nature of light

1857 - 1894

EM wave transmitter EM wave receiver

"It's of no use whatsoever,"This is just an experiment that proves Maestro Maxwell was right - we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.

Michelson & Morley

Michelson and Morley then attempted to measure the earth's velocity with respect to the aether and found it to be zero, effectively disproving the existence of the aether. Edward

Morley (1838-1923)

Albert Michelson

(1852-1931)

20th century

Difficulties:

Wave theory cannot explain:

- black body radiation spectrum- photoelectric effect- speed of light measured by two detectors moving in respect to each other is exactly the same

20th century: The birth of quantum theory

Max Karl Ernst Ludwig Planck

1900: To explain black body radiation, Planck suggested that energy of light consists of quanta

Ephoton = hPlancks constant

frequency

1905: Albert Einstein proposed that light consists of particles (photons) that have energy and momentum depending on frequency

Special theory of relativity: speed of light in vacuum is the same in all inertial reference systems

20th century: quantum theory

Quantum nature of light is pronounced at low light intensities

Light: dual nature, both wave and particle

In this course, we will mostly work in the frame of classical EM wave theory of light

Albert Einstein

Einstein showed that light:

Albert Einstein (1879-1955)

is a phenomenon of empty space;

has a velocity thats constant, independent of observer velocity;

is both a wave and a particle;

Excited medium

and undergoes stimulated emission, the basis of the laser.