Chapter 29

Light Waves

In this chapter we will studyHuygens’ Principle

Diffraction

Interference

Polarization

Holography

1. HUYGENS' PRINCIPLE

Every point on a wave front can be regarded as a new source of wavelets, which combine to produce the next wave front, whose points are sources of further wavelets, and so on.

Huygen’s Principle can be used to explain the propagation of wave fronts involved in reflection, refraction, and diffraction.

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Consider several points on the wave front

to be sources of secondary wavelets.

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2. DIFFRACTION

The bending of light that passes around an obstacle or through a narrow slit, causing the light to spread and to produce light and dark fringes.

Demo - Laser, diffraction accessories, wire mesh screen, and rainbow disks

Water waves on the surface of water

If the wavelength of water waves are small compared to the

size of an ocean vessel, the vessel will cast a “shadow.”

For the same waves a post will not cast a “shadow.”

Long wavelengths bend a great deal around small objects.

Because of diffraction AM radio waves travel farther than FM radio waves.

Microscopy diffraction limit -

One cannot see details of objects that are approximately the same size as or smaller than the wavelength of the illuminating light.

3. INTERFERENCE

Slide - Interference Transparencies

3. INTERFERENCE

Slide - Interference Transparencies

Demo - Finger slit interferenceDemo - Finger slit interference

Demo - Single-color thin film interferenceDemo - Single-color thin film interference

Demo - Optical flats and sodium lampDemo - Optical flats and sodium lamp

Demo - Newton's rings and phase reversalDemo - Newton's rings and phase reversal

Interference Colors by Reflection from Thin Films

Iridescence - the phenomenon of seeing colors

by interference in thin films.

Demo - Soap bubbles and pipeDemo - Soap bubbles and pipe

Demo - Peacock feathersDemo - Peacock feathers

Demo – Thin film platesDemo – Thin film plates

Example - Coated photographic lenses

Interferometers measure small distances.

Thin Film t

Air

Air

Phase Reversal

upon reflection

Path Difference = 2t = n for destructive interference

Thus t = n(/2)

n = 1,2,3,4,…… You see the complement of whichever color is destructively interfered with.

4. POLARIZATION

Light waves are transverse. This is verified by polarization.

Most light you see is unpolarized meaning that it is composed of waves with vibrational planes in all directions.

Common sources of light are not polarized. Polaroid Crystals - one axis direction

absorbs, one transmits. Polarized glare occurs on reflection from

nonmetallic surfaces.

COLORS BY TRANSMISSION THROUGH POLARIZING MATERIALS

Video - Crossed Polaroids and Crystal Video - Crossed Polaroids and Crystal Video - Polarizers and StressVideo - Polarizers and Stress Demo - Polaroids and accessoriesDemo - Polaroids and accessories

Polaroid sunglasses worn in a normal viewing

position will eliminate polarized glare.

(a) vertically

(b) horizontally

(c) all

(b) horizontally

Two polaroids that have their polarization axes at 45o to each other will still allow light to pass through.

(a) True

(b) False

(a) True

Three-Dimensional Viewing

Stereoscopic viewersSlide - Stereogram

5. HOLOGRAPHY

Hologram means whole message.

Demo - Reflection hologramDemo - Reflection hologram

Demo - Transmission hologramDemo - Transmission hologram

No lenses are used for imaging in the

creation or the viewing of a hologram.

Object and source both illuminate all of the

photographic plate.

The light used to make the hologram

must be coherent.

A hologram is an interference pattern.

It is best to use coherent light when viewing a

hologram.

A hologram can be divided.

One gets a magnified holographic image by

viewing a hologram with a longer  of light

than was used in creating the hologram.