Properties of Light: Reflection, Refraction,

Dispersion, and Refractive Indices

Light
Light is electromagnetic radiation that has properties of waves. 
The electromagnetic spectrum can be divided into several bands
based on the wavelength of the light waves.  As we have
discussed before, visible light represents a narrow group of
wavelengths between about 380 nm (1 nm = 10-9 m) and 730
nm.  

Our eyes interpret these wavelengths as different colors.  If only

a single wavelength or limited range of wavelengths are present
and enter our eyes, they are interpreted as a certain color. 
If a single wavelength is present we say that we have
monochromatic light.  If all wavelengths of visible light are
present, our eyes interpret this as white light.  If no wavelengths
in the visible range are present, we interpret this as dark.
Interaction of Light with Matter
Velocity of Light and Refractive Index
We here define refractive index, n, of a material or substance as
the ratio of the speed of light in a vacuum, C,  to the speed of
light in a material through which it passes, Cm.
n = C/Cm  
Note that the value of refractive index will always be greater
than 1.0, since Cm can never be greater than C.  In general,  Cm
depends on the density of the material, with  Cm decreasing with
increasing density.  Thus, higher density materials will have
higher refractive indices.
The refractive index of any material depends on the wavelength
of light because different wavelengths are interfered with to
different extents by the atoms that make up the material. In
general refractive index varies linearly with wavelength.  
Materials can be divided into 2 classes based on how the
velocity of light of a particular wavelength varies in the
material.
1. Materials whose refractive index not depend on the
direction that the light travels are called isotropic
materials.  In these materials the velocity of light does not
depend on the direction that the light travels.  Isotropic
 materials have a single, constant refractive index for each
wavelength.  Minerals that crystallize in the isometric
system, by virtue of their symmetry, are isotropic. 
Similarly, glass, gases, most liquids and amorphous solids
are isotropic.
2. Materials whose refractive index does depend on the
direction that the light travels are called anisotropic
materials.  These types of materials will have a range of
refractive indices between two extreme values for each
wavelength.  Anisotropic materials can be further divided
into two subclasses, although the reasoning behind these
subdivisions will become clear in a later lecture.
a. Minerals that crystallize in the tetragonal and
hexagonal crystal systems (as well as some plastics)
are uniaxial and are characterized by 2 extreme
refractive indices for each wavelength. 
 
b. Minerals that crystallize in the triclinic, monoclinic,
and orthorhombic crystal systems are biaxial and are
characterized by 3 refractive indices, one of which is
intermediate between the other two.
Air, since it is a gas, is isotropic.  The refractive index of air is
usually taken as 1.0, although its true value is 1.0003.
 
Reflection and Refraction of Light
When light strikes an interface between two substances with
different refractive indices, two things occur. An incident ray of
light striking the interface at an angle, i, measured between a
line perpendicular to the interface and the propagation direction
of the incident ray, will be reflected off the interface at the same
angle, i.  In other words the angle of reflection is equal to the
angle of incidence.
If the second substance is transparent to
light, then a ray of light will enter the
substance with different refractive index,
and will be refracted, or bent, at an angle
r, the angle of refraction.  The angle of
refraction is dependent on the angle of
incidence and the refractive index of the
materials on either side of the interface
according to Snell's Law:
ni sin (i) =  nr sin (r)
Note that if the angle of incidence is 0o
(i.e. the light enters perpendicular to the
interface) that some of the light will be
reflected directly back, and the refracted
ray will continue along the same path. 
This can be seen from Snell's law, since
sin(0o) = 0, making sin (r) = 0, and
resulting in r = 0.

There is also an angle, ic, called the critical angle for total
internal reflection where the refracted ray travels along the
interface between the two substances.
This occurs when the angle r = 90o.  In
this case, applying Snell's law:
ni sin (ic) = nr sin (90o) = nr  
[since sin (90o) = 1]

sin (ic) = nr/ni

Dispersion of Light
The fact that refractive indices differ for each wavelength of
light produces an effect called dispersion.  This can be seen by
shining a beam of white light into a triangular prism made of
glass. White light entering such a prism will be refracted in the
prism by different angles depending on the wavelength of the
light. 
 The refractive index for longer
wavelengths (red) are lower than
those for shorter wavelengths
(violet).  This results in the a
greater angle of refraction for the
longer wavelengths than for the
shorter wavelengths.  (Shown
here are the paths taken for a
wavelength of 800 nm, angle r800
and for a wavelength of 300 nm,
angle r300 ). When the light exits
from the other side of the prism,
we see the different wavelengths
dispersed to show the different
colors of the spectrum.
Absorption of Light
When light enters a transparent material some of its energy is
dissipated as heat energy, and it thus looses some of its
intensity.  When this absorption of energy occurs selectively for
different wavelengths of light, they light that gets transmitted
through the material will show only those wavelengths of light
that are not absorbed.  The transmitted wavelengths will then be
seen as color, called the absorption color of the material.
For example, if we measure
the intensity of light, Io,  for
each wavelength before it is
transmitted through a
material, and measure the
intensity, I, for each
wavelength after it has passed
through the material, and plot
I/Io versus wavelength we
obtain the absorption curve
for that material as shown
here.   The absorption curve
(continuous line) for the
material in this example
shows that the light exiting
the material will have a
yellow-green color, called the
absorption color.  An opaque
substance would have an
absorption curve such as that
labeled "Dark", i.e. no
wavelengths would be
transmitted.  
Sunlight, on passing through the atmosphere has absorption
curve as shown, thus we see it as white light, since all
wavelengths are present.

Polarization of Light
Normal light vibrates equally in all direction perpendicular to
its path of propagation.  If the light is constrained to vibrate in
only on plane, however, we say that it is plane polarized light.
The direction that the light vibrates is called the vibration
direction, which for now will be perpendicular to the direction. 
There are two common ways that light can become polarized.
 The first involves reflection off of a
non-metallic surface, such as glass
or paint.  An unpolarized beam of
light, vibrating in all directions
perpendicular to its path strikes
such a surface and is reflected.  The
reflected beam will be polarized
with vibration directions parallel to
the reflecting surface
(perpendicular to the page as
indicated by the open circles on the
ray path).  If some of this light also
enters the material and is refracted
at an angle 90o to the path of the
reflected ray, it too will become
partially polarized, with vibration
directions again perpendicular to
 the path of the refracted ray, but in
the plane perpendicular to the
direction of vibration in the
reflected ray (the plane of the
paper, as shown in the drawing).

 Polarization can also be achieved by passing the light

through a substance that absorbs light vibrating in all
directions except one.  Anisotropic crystals have this
property in certain directions, called privileged directions,
and we will discuss these properties when we discuss
uniaxial and biaxial crystals.  Crystals were used to
produce polarized light in microscopes built before about
1950. The device used to make polarized light in modern
microscopes is a Polaroid, a trade name for a plastic film
made by the Polaroid Corporation.  A Polaroid consists of
long-chain organic molecules that are aligned in one
direction an placed in a plastic sheet.  They are placed
close enough to form a closely spaced linear grid, that
allows the passage of light vibrating only in the same
direction as the grid.  Light vibrating in all other directions
is absorbed.  Such a device is also called a polarizer.

If a beam on non-polarized light encounters a polarizer,

 only light vibrating parallel to the polarizing direction of
the polarizer will be allowed to pass.  The light coming out
on the other side will then be plane polarized, and will be
vibrating parallel to the polarizing direction of the
polarizer.  If another polarizer with its polarization
direction oriented perpendicular to the first polarizer is
placed in front of the beam of now polarized light, then no
light will penetrate the second polarizer.  In this case we
say that the light has been extinguished.
Polaroid sunglasses use these same principles.  For example,
incoming solar radiation is reflected off of the surface of the
ocean or the painted hood of your car.  Reflected light coming
off of either of these surfaces will be polarized such that the
vibration directions are parallel to the reflected surface, or
approximately horizontal (as in the first method of polarization
discussed above).  Polaroid sunglasses contain polarizers with
the polarization direction oriented vertically.  Wearing such
glasses will cut out all of the horizontally polarized light
reflecting off the water surface or hood of your car.
 
The Polarizing Microscope
In optical mineralogy we use a microscope called a polarizing
microscope. Such a microscope is equipped with two polarizers
that are normally oriented so that their polarization directions
are perpendicular to one another. 
Light from a light source
located below the tube
and stage of the
microscope is initially
unpolarized.  This light
first passes through the
lower polarizer (usually
just called the polarizer),
where it becomes
polarized such that the
light is vibrating from
the users right to left. 
These directions are
referred to as East (right
and West (left).  The
light then passes through
a hole in the rotatable
stage of the microscope
and enters the lower
lens, called the objective
lens.
Mounted within the
microscope tube is a
second polarizer, called
the analyzer, that can be
rotated or pushed so that
in can be in the light
path (inserted position)
or not in the light path
(analyzer out position). 
The analyzer has a
polarization direction
exactly perpendicular to
that of the lower
polarizer These
directions are usually
referred to as North -
South.  If the analyzer is
in, then the plane
polarized light coming
from the lower polarizer
will be blocked, and no
light will be transmitted
though the ocular lens
above.
If the analyzer is out, so
that it is not in the light
path, then the polarized
light will be transmitted
through the ocular lens.
Next time we will see how this microscope is used to examine
isotropic substances and determine their refractive indices.

Coherent light
Coherent light are light waves that are "in phase" with one
another.
For example, two waves are coherent if the crests of one wave
are aligned with the crests of the other and the troughs of one
wave are aligned with the troughs of the other. Otherwise, these
lightwaves are considered incoherent.

[ Two coherent waves ]

Light produced by lasers are coherent light. Light from light
bulbs or the sun, however, are incoherent light.

Scattering

Excited electrons emit light waves, and just so happens, the

opposite is true: light waves can excite electrons. When
electrons are excited by light waves, they jump to a higher
energy level. When they fall back to their original energy level,
the electrons reemit the light. This process is called scattering.
However, when the light is reemitted by scattering, not all of the
energy is given back to the light wave, but instead, some is lost
to the particle. This will result in a light wave of lower
frequency and wavelength as described by Compton's shift
formula:

When light is scattered on an object smaller than the wavelength

of light, the process is called Rayleigh scattering. Because of the
nature of Rayleigh scattering -- light waves scattered by objects
smaller than its wavelength -- it is very frequency dependent.
Higher frequency, shorter wavelength, light are scattered the
most while lower frequency, longer wavelength, light is
scattered the least by very small particles.
The color of the sky is the direct result of Rayleigh scattering of
the sunlight. Lower frequency light waves, such as red, are able
to pass though a network of air particles better than higher
frequency light waves, such as blue. During the day, the
particles in the atmosphere will scatter the sunlight and lower its
frequency to somewhere in the blue range. At sunset, the light
waves from the sun have to travel a greater distance to reach us.
Because of that, all of the light waves have been scattered so
much that it lowers the frequency to the other end of our visible
range: red.

Interference

Interference is the interaction between waves traveling in the

same medium. When two waves come into contact, depending
on the phase differences along the waves, constructive and
destructive interferences will occur.
In constructive interference, the amplitude of the wave is
amplified. This happens when the two waves are in phase -- if
the crests and troughs of the waves coincide with each other.
Consider two waves, one with a crest of +1 units and coinciding
with a wave of +2 units in amplitude at that point, traveling in
opposite directions on the same medium. When these troughs
come into contact, the resulting amplitude will be the sum of the
two waves, which is +3 in this case. Once the waves pass each
other, however, they will resume their original course with their
original amplitude -- as if they have not been disturbed at all.
Destructive interference is very much like constructive
interference except that the two waves cancels out each other.
This happens when the waves are out of phase -- when the crests
of one wave coincide with the troughs of the other. Consider two
waves, one with a crest of +1 coinciding with a wave of -2 units
in amplitude at that point, traveling in opposite directions on the
same medium. At the point of contact, the resulting amplitude
will be the difference of the two waves, which is -1 in this case.
Just like constructive interference, once the waves pass each
other, they will resume their original course with their original
amplitude -- as if they have not been disturbed at all.
The Doppler Effect

Suppose that there is a happy bug in the center of a circular

water puddle. The bug is periodically shaking
its legs in order to produce disturbances that
travel through the water. If these disturbances
originate at a point, then they would travel
outward from that point in all directions. Since
each disturbance is traveling in the same
medium, they would all travel in every direction
at the same speed. The pattern produced by the bug's shaking
would be a series of concentric circles as shown in the diagram
at the right. These circles would reach the edges of the water
puddle at the same frequency. An observer at point A (the left
edge of the puddle) would observe the disturbances to strike the
puddle's edge at the same frequency that would be observed by
an observer at point B (at the right edge of the puddle). In fact,
the frequency at which disturbances reach the edge of the puddle
would be the same as the frequency at which the bug produces
the disturbances. If the bug produces disturbances at a frequency
of 2 per second, then each observer would observe them
approaching at a frequency of 2 per second.
Now suppose that our bug is moving to the right across the
puddle of water and producing disturbances at
the same frequency of 2 disturbances per
second. Since the bug is moving towards the
right, each consecutive disturbance originates
from a position which is closer to observer B
and farther from observer A. Subsequently,
each consecutive disturbance has a shorter
distance to travel before reaching observer B and thus takes less
time to reach observer B. Thus, observer B observes that the
frequency of arrival of the disturbances is higher than the
frequency at which disturbances are produced. On the other
hand, each consecutive disturbance has a further distance to
travel before reaching observer A. For this reason, observer A
observes a frequency of arrival which is less than the frequency
at which the disturbances are produced. The net effect of the
motion of the bug (the source of waves) is that the observer
towards whom the bug is moving observes a frequency which is
higher than 2 disturbances/second; and the observer away from
whom the bug is moving observes a frequency which is less than
2 disturbances/second. This effect is known as the Doppler
effect.
The Doppler effect is observed whenever the source of waves is
moving with respect to an observer. The Doppler effect can be
described as the effect produced by a moving source of waves in
which there is an apparent upward shift in frequency for
observers towards whom the source is approaching and an
apparent downward shift in frequency for observers from whom
the source is receding. It is important to note that the effect does
not result because of an actual change in the frequency of the
source. Using the example above, the bug is still producing
disturbances at a rate of 2 disturbances per second; it just
appears to the observer whom the bug is approaching that the
disturbances are being produced at a frequency greater than 2
disturbances/second. The effect is only observed because the
distance between observer B and the bug is decreasing and the
distance between observer A and the bug is increasing.
The Doppler effect can be observed for any type of wave - water
wave, sound wave, light wave, etc. We are most familiar with
the Doppler effect because of our experiences with sound waves.
Perhaps you recall an instance in which a police car or
emergency vehicle was traveling towards you on the highway.
As the car approached with its siren blasting, the pitch of the
siren sound (a measure of the siren's frequency) was high; and
then suddenly after the car passed by, the pitch of the siren
sound was low. That was the Doppler effect - an apparent shift
in frequency for a sound wave produced by a moving source.

 
The Doppler effect is of intense interest to astronomers who use
the information about the shift in frequency of electromagnetic
waves produced by moving stars in our galaxy and beyond in
order to derive information about those stars and galaxies. The
belief that the universe is expanding is based in part upon
observations of electromagnetic waves emitted by stars in
distant galaxies. Furthermore, specific information about stars
within galaxies can be determined by application of the Doppler
effect. Galaxies are clusters of stars which typically rotate about
some center of mass point. Electromagnetic radiation emitted by
such stars in a distant galaxy would appear to be shifted
downward in frequency (a red shift) if the star is rotating in its
cluster in a direction which is away from the Earth. On the other
hand, there is an upward shift in frequency (a blue shift) of such
observed radiation if the star is rotating in a direction that is
towards the Earth.
Polarization

A light wave is an electromagnetic wave which travels through

the vacuum of outer space. Light waves are produced by
vibrating electric charges. The nature of such electromagnetic
waves is beyond the scope of The Physics Classroom Tutorial.
For our purposes, it is sufficient to merely say that an
electromagnetic wave is a transverse wave
which has both an electric and a magnetic
component.
The transverse nature of an electromagnetic
wave is quite different from any other type of
wave which has been discussed in The Physics
Classroom Tutorial. Let's suppose that we use
the customary slinky to model the behavior of an
electromagnetic wave. As an electromagnetic wave traveled
towards you, then you would observe the vibrations of the slinky
occurring in more than one plane of vibration. This is quite
different than what you might notice if you were to look along a
slinky and observe a slinky wave traveling towards you. Indeed,
the coils of the slinky would be vibrating back and forth as the
slinky approached; yet these vibrations would occur in a single
plane of space. That is, the coils of the slinky might vibrate up
and down or left and right. Yet regardless of their direction of
vibration, they would be moving along the same linear direction
as you sighted along the slinky. If a slinky wave were an
electromagnetic wave, then the vibrations of the slinky would
occur in multiple planes. Unlike a usual slinky wave, the electric
and magnetic vibrations of an electromagnetic wave occur in
numerous planes. A light wave which is vibrating in more than
one plane is referred to as unpolarized light. Light emitted by
the sun, by a lamp in the classroom, or by a candle flame is
unpolarized light. Such light waves are created by electric
charges which vibrate in a variety of directions, thus creating an
electromagnetic wave which vibrates in a variety of directions.
This concept of unpolarized light is rather difficult to visualize.
In general, it is helpful to picture unpolarized light as a wave
which has an average of half its vibrations in a horizontal plane
and half of its vibrations in a vertical plane.
It is possible to transform unpolarized light into polarized light.
Polarized light waves are light waves in which the vibrations
occur in a single plane. The process of transforming unpolarized
light into polarized light is known as polarization. There are a
variety of methods of polarizing light. The four methods
discussed on this page are :

 Polarization by Transmission
 Polarization by Reflection
 Polarization by Refraction
 Polarization by Scattering
 
 
Polarization by Use of a Polaroid Filter
The most common method of polarization involves the use of a
Polaroid filter. Polaroid filters are made of a special material
which is capable of blocking one of the two planes of vibration
of an electromagnetic wave. (Remember, the notion of two
planes or directions of vibration is merely a simplification which
helps us to visualize the wavelike nature of the electromagnetic
wave.) In this sense, a Polaroid serves as a device which filters
out one-half of the vibrations upon transmission of the light
through the filter. When unpolarized light is transmitted through
a Polaroid filter, it emerges with one-half the intensity and with
vibrations in a single plane; it emerges as polarized light.
 
A Polaroid filter is able to polarize light because of the chemical
composition of the filter material. The filter can be thought of as
having long-chain molecules that are aligned within the filter in
the same direction. During the fabrication of the filter, the long-
chain molecules are stretched across the filter so that each
molecule is (as much as possible) aligned in say the vertical
direction. As unpolarized light strikes the filter, the portion of
the waves vibrating in the vertical direction are absorbed by the
filter. The general rule is that the electromagnetic vibrations
which are in a direction parallel to the alignment of the
molecules are absorbed.
The alignment of these molecules gives the filter a polarization
axis. This polarization axis extends across the length of the filter
and only allows vibrations of the electromagnetic wave that are
parallel to the axis to pass through. Any vibrations which are
perpendicular to the polarization axis are blocked by the filter.
Thus, a Polaroid filter with its long-chain molecules aligned
horizontally will have a polarization axis aligned vertically.
Such a filter will block all horizontal vibrations and allow the
vertical vibrations to be transmitted (see diagram above). On the
other hand, a Polaroid filter with its long-chain molecules
aligned vertically will have a polarization axis aligned
horizontally; this filter will block all vertical vibrations and
allow the horizontal vibrations to be transmitted.
Polarization of light by use of a Polaroid filter was is often
demonstrated in a Physics class through a variety of
demonstrations. Filters are used to look through an view objects.
The filter does not distort the shape or dimensions of the object;
it merely serves to produce a dimmer image of the object since
one-half of the light is blocked as it passed through the filter. A
pair of filters are often placed back to back in order to view
objects looking through two filters. By slowly rotating the
second filter, an orientation can be found in which all the light
from an object is blocked and the object can no longer be seen
when viewed through two filters. What happened? In this
demonstration, the light was polarized upon passage through the
first filter; perhaps only vertical vibrations were able to pass
through. These vertical vibrations were then blocked by the
second filter since its polarization filter is aligned in a horizontal
direction. While you are unable to see the axes on the filter, you
will know when the axes are aligned perpendicular to each other
because with this orientation, all light is blocked. So by use of
two filters, one can completely block all of the light which is
incident upon the set; this will only occur if the polarization axes
are rotated such that they are perpendicular to each other.
A picket-fence analogy is often used to explain how this dual-
filter demonstration works. A picket fence can act as a polarizer
by transforming an unpolarized wave in a rope into a wave
which vibrates in a single plane. The spaces between the pickets
of the fence will allow vibrations which are parallel to the
spacings to pass through while blocking any vibrations which
are perpendicular to the spacings. Obviously, a vertical vibration
would not have the room to make it through a horizontal
spacing. If two picket fences are oriented such that the pickets
are both aligned vertically, then vertical vibrations will pass
through both fences. On the other hand, if the pickets of the
second fence are aligned horizontally, then the vertical
vibrations which pass through the first fence will be blocked by
the second fence. This is depicted in the diagram below.
In the same manner, two Polaroid filters oriented with their
polarization axes perpendicular to each other will block all the
light. Now that's a pretty cool observation which could never be
explained by a particle view of light.
 
 
Polarization by Reflection
Unpolarized light can also undergo polarization by reflection off
of nonmetallic surfaces. The extent to which polarization occurs
is dependent upon the angle at which the light approaches the
surface and upon the material which the surface is made of.
Metallic surfaces reflect light with a variety of vibrational
directions; such reflected light is unpolarized. However,
nonmetallic surfaces such as asphalt roadways, snow fields and
water reflect light such that there is a large concentration of
vibrations in a plane parallel to the reflecting surface. A person
viewing objects by means of light reflected off of nonmetallic
surfaces will often perceive a glare if the extent of polarization
is large. Fisherman are familiar with this glare since it prevents
them from seeing fish which lie below the water. Light reflected
off a lake is partially polarized in a direction parallel to the
water's surface. Fisherman know that the use of glare-reducing
sunglasses with the proper polarization axis allows for the
blocking of this partially polarized light. By blocking the plane-
polarized light, the glare is reduced and the fisherman can more
easily see fish located under the water.

 
 
Polarization by Refraction
Polarization can also occur by the refraction of light. Refraction
occurs when a beam of light passes from one material into
another material. At the surface of the
two materials, the path of the beam
changes its direction. The refracted
beam acquires some degree of
polarization. Most often, the
polarization occurs in a plane
perpendicular to the surface. The
polarization of refracted light is often demonstrated in a Physics
class using a unique crystal which serves as a double-refracting
crystal. Iceland Spar, a rather rare form of the mineral calcite,
refracts incident light into two different paths. The light is split
into two beams upon entering the crystal. Subsequently, if an
object is viewed by looking through an Iceland Spar crystal, two
images will be seen. The two images are the result of the double
refraction of light. Both refracted light beams are polarized - one
in a direction parallel to the surface and the other in a direction
perpendicular to the surface. Since these two refracted rays are
polarized with a perpendicular orientation, a polarizing filter can
be used to completely block one of the images. If the
polarization axis of the filter is aligned perpendicular to the
plane of polarized light, the light is completely blocked by the
filter; meanwhile the second image is as bright as can be. And if
the filter is then turned 90-degrees in either direction, the second
image reappears and the first image disappears. Now that's
pretty neat observation that could never be observed if light did
not exhibit any wavelike behavior.
 
 
Polarization by Scattering
Polarization also occurs when light is scattered while traveling
through a medium. When light strikes the atoms of a material, it
will often set the electrons of those atoms into vibration. The
vibrating electrons then produce their own electromagnetic wave
which is radiated outward in all directions. This newly generated
wave strikes neighboring atoms, forcing their electrons into
vibrations at the same original frequency. These vibrating
electrons produce another electromagnetic wave which is once
more radiated outward in all directions. This absorption and
reemission of light waves causes the light to be scattered about
the medium. (This process of scattering contributes to the
blueness of our skies, a topic to be discussed later.) This
scattered light is partially polarized. Polarization by scattering is
observed as light passes through our atmosphere. The scattered
light often produces a glare in the skies. Photographers know
that this partial polarization of scattered light leads to
photographs characterized by a washed-out sky. The problem
can easily be corrected by the use of a Polaroid filter. As the
filter is rotated, the partially polarized light is blocked and the
glare is reduced. The photographic secret of capturing a vivid
blue sky as the backdrop of a beautiful foreground lies in the
physics of polarization and Polaroid filters.
 
Applications of Polarization
Polarization has a wealth of other applications besides their use
in glare-reducing sunglasses. In industry, Polaroid filters are
used to perform stress analysis tests on transparent plastics. As
light passes through a plastic, each color of visible light is
polarized with its own orientation. If such a plastic is placed
between two polarizing plates, a colorful pattern is revealed. As
the top plate is turned, the color pattern changes as new colors
become blocked and the formerly blocked colors are
transmitted. A common Physics demonstration involves placing
a plastic protractor between two Polaroid plates and placing
them on top of an overhead projector. It is known that structural
stress in plastic is signified at locations where there is a large
concentration of colored bands. This location of stress is usually
the location where structural failure will most likely occur.
Perhaps you wish that a more careful stress analysis was
performed on the plastic case of the CD which you recently
purchased.
Polarization is also used in the entertainment industry to produce
and show 3-D movies. Three-dimensional movies are actually
two movies being shown at the same time through two
projectors. The two movies are filmed from two slightly
different camera locations. Each individual movie is then
projected from different sides of the audience onto a metal
screen. The movies are projected through a polarizing filter. The
polarizing filter used for the projector on the left may have its
polarization axis aligned horizontally while the polarizing filter
used for the projector on the right would have its polarization
axis aligned vertically. Consequently, there are two slightly
different movies being projected onto a screen. Each movie is
cast by light which is polarized with an orientation
perpendicular to the other movie. The audience then wears
glasses which have two Polaroid filters. Each filter has a
different polarization axis - one is horizontal and the other is
vertical. The result of this arrangement of projectors and filters
is that the left eye sees the movie which is projected from the
right projector while the right eye sees the movie which is
projected from the left projector. This gives the viewer a
perception of depth.
 
Our model of the polarization of light provides some substantial
support for the wavelike nature of light. It would be extremely
difficult to explain polarization phenomenon using a particle
view of light. Polarization would only occur with a transverse
wave. For this reason, polarization is one more reason why
scientists believe that light exhibits wavelike behavior.