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Chapter 29
Light Waves
In this chapter we will study
Huygens’ 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.
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 interference
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Demo - Single-color thin film interference

Demo - Optical flats and sodium lamp
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Demo - 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 pipe

Demo - Peacock feathers
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Demo – Thin film plates
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Example - Coated photographic lenses

Interferometers measure small distances.
Phase Reversal
upon reflection
Air
Thin Film
t
Air
Path Difference = 2t = nl for destructive interference
n = 1,2,3,4,……
You see the complement of
whichever color is
Thus t = n(l/2)
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
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Video - Polarizers and Stress

Demo - Polaroids and accessories
Polaroid sunglasses worn in a normal viewing
position will eliminate
(a) vertically
(b) horizontally
(c) all
polarized glare.
Two polaroids that have their polarization axes at
45o to each other will still allow light to pass
through.
(a) True
(b) False
Three-Dimensional Viewing
Stereoscopic viewers
Slide - Stereogram
5. HOLOGRAPHY
Hologram means whole message.

Demo - Reflection hologram
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Demo - 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 l of light
than was used in creating the hologram.