PHYSICS 111 LASER & LIGHT

CHAPTER 2

CLASSICAL THEORIES OF LIGHT

KEY WORDS

Newtonian mechanics, Rectilinear motion, Refraction, Diffraction, Huygen's principle, Wave front, Longitudinal and Transverse waves, Polarization of light, Wave length, Frequency, Amplitude, Principle of superposition, Interference, Aberration and, Electromagnetic waves.

FORMULA: c = lf

OUTLINES OF CHAPTER 2

Sir Isaac Newton (1642-1727) is the father of Classical Mechanics, that is a very important branch of physics. He established a valid model of particle motion backed by his laws. Predictions of his laws are quite accurate except when the particle speed is extremely high and particle size extremely small. He was a strong advocate of the "Corpuscular theory of light." In his book Opticks (1705 C.E.) he considers light to be a stream of tiny particles. He understood light to move rapidly in a straight-line motion that produces sharply defined shadows of opaque objects. Grimaldi’s observation of some illumination inside the sharp boundary of the shadow was not seriously taken. Grimaldi called it diffraction. This effect was certainly not as pronounced as water waves or sound waves. Newton thought it to be a kind of extended refraction.

Dutch scientist Christian Huygens (1629-1695), another great name in the history of optics, however believed in the “Wave theory of light”. He argued that a beam of light crossing another beam has no effect on each other. Each beam moves forward after meeting without being affected. One observes the same effect in water waves.

The question of whether light is a wave or a particle has never been completely settled even today. Light has a dual nature. Sometimes it behaves as a particle and sometimes it behaves as a wave. We now say that light is, what it likes to be. It does not have to fit into our notion of particle or wave structure.

WAVE DESCRIPTION: Waves have amplitude, defined as the maximum displacement from the zero line to the crest or trough. Wavelength is the distance between two adjacent crests or troughs. Its unit is meter, being the dimension of length.

Time period of a wave is the time it takes to complete one cycle of motion. Its unit is second.

Frequency is defined as number of waves produced in one second. Its unit is cycles per second or Hertz (Hz) named after the scientist who made a significant contribution to the knowledge of electricity and magnetism.

Waves are generally produced by some kind disturbance in the medium. Transverse waves are those that travel in a direction perpendicular to its displacement (e.g., amplitude). Examples are water waves and light waves.

Longitudinal waves are those that travel in the same direction as its displacement. Example is sound wave.

Wave front: In the wave theory of light, light pulses starting out from a point source, can be imagined as expanding spherical surfaces. If the light source is extended in a plane, light pulses are like traveling plane surfaces. These imaginary surfaces are called Wave fronts. Any point on the wave front has the same physical property as another point on the same wave front. Wave fronts are nearly plane surfaces when the light source is at a long distance from where it is observed.

Principle of Superposition: If two or more waves can combine or interfere with each other to produce a resultant wave, the displacement of the resultant wave is the algebraic sum of the component waves. Let us consider two identical waves. If the two waves combine in phase, the resultant wave is enhanced (grow larger). If the two waves combine out of phase, the resultant wave is reduced (become smaller).

HUYGEN'S PRINCIPLE: Christian Huygens published “Treatise on Light” in 1960. Any point on the wave front of a light wave is the source of and has the potential of producing, new secondary waves or wavelets. It can be easily explained by geometric construction of the secondary waves from each point of the primary wave to show how light travels.

Young's double slit experiment showed that a parallel beam of light passing through two close adjacent slits on an opaque wall produces bright and dark bands on a screen held behind the wall.

Young's experiment can be explained by using Huygen's principle. The parallel beam of light is like a plane wave front. When this passes through the two slits, it is disturbed and hence secondary expanding spherical waves are produced at each slit. The superposition of the two secondary waves when they combine, results in constructive or destructive interference at the screen, resulting in bright or dark bands or fringes.

SPEED OF LIGHT MEASUREMENTS

1. Danish Astronomer Olaf Roemer (1644-1710) used one of the moons of the Jupiter to measure the speed of light.

2. English Astronomer James Bradley (1692-1762) used Stellar Aberration to measure the speed of light.

3. French scientist Armand Fizeau (1819-1896) in 1849 A.D. measured the speed of light with a toothed wheel.

4. French scientist Jean Foucault (1819-1868) measured speed of light in 1850 A.D., using a rotating mirror.

5. The US scientist Albert Michelson (1852-1931) measured the speed of light in an experiment with his newly built Interferometer.

ELECTROMAGNETIC WAVES: James Clerk Maxwell (1831-1879) an English physicist produced the most successful theory of light. He is credited for combining light, electricity and magnetism into one theory. Light is the visible part of a very long electromagnetic wave spectrum.

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Shadows: Umbra and Pneumbra:

http://webphysics.ph.msstate.edu/javamirror/ntnujava/shadow/shadow.html

WATER WAVES: http://www.colorado.edu/physics/2000/index.pl?Page=waves_particles/waves.html

Stadium of people making WAVES: http://www.colorado.edu/physics/2000/index.pl?Page=waves_particles/stadium_wave.html

WAVE disturbance TRANSVERSE WAVES:

http://www.colorado.edu/physics/2000/index.pl?Page=microwaves/water_rotates3.html

Superposition of two waves: http://webphysics.ph.msstate.edu/javamirror/ntnujava/waveSuperposition/waveSuperposition.html

HUYGEN'S Principle: http://webphysics.ph.msstate.edu/jc/library/24-2/huygens.htm

Constructive and Destructive Interference: http://www.vislab.usyd.edu.au/photonics/fibres/fizzz/diffraction3.html

Interference of light: http://members.tripod.com/~vsg/interfer.htm

http://webphysics.ph.msstate.edu/javamirror/interf/interference.html