Cool Ray and the Polaroids
Teacher Notes
Students are familiar with the phrase of polarized sunglasses although most will only know what
the advertisement states. Polarizing glasses block glare, but students do not know what it means
to have polarized light or how light can be polarized. The light waves reaching our eyes are
randomly oriented. Electromagnetic waves, light, vibrate in many planes at once. In polarized
light the light waves vibrate in one plane only. Nature polarizes light by reflection, refraction and
scattering. Manmade polarized light can be made by blocking all the waves except those in one
plane with polarizing filters. Polarizing filters can also be used to detect polarized light.
Polarization by reflection occurs when light reflects off of a surface of water, waxed hoods of
cars and windows. Students have experienced the glare from their local pool and know that the
glasses reduce that glare. Sporting goods stores also sell “fishing glasses” to block the glare
associated with reflection from the surface of a river or stream. Polaroid sunglasses block one
half of the un-polarized sunlight. Reflected light (glare) from horizontal surfaces is horizontally
polarized, so vertically oriented polarized sunglasses block almost most of the glare.
The English physicist Sir David Brewster discovered that the degree of reflective polarization
depends on the material from which the reflection takes place and the angle of incidence.
Brewster found maximum polarization occurs when the angle of incidence plus the angle of
refraction equals 900. The Brewster angle is related to the index of refraction of the reflecting
surface, tan =n (where n is the index of refraction).
Polarization via refraction occurs with Iceland Spar - a rare form of calcite. Iceland Spar has two
different indices of refraction and doubly refracts. Placing a piece of Iceland Spar over large
print produces two images. The light splits into two beams as it refracts through the crystal. The
ray of light is split into two rays that form a 900 angle. If a polarizing filter is placed over the
crystal and rotated, one of the images, the image that is oriented 900 to the polarity of the filter,
will disappear.
Finally, polarization also occurs when light is scattered while traveling through a medium, such
as the atmosphere. When light strikes the atoms of a medium, the electric field component
causes the electrons of the atoms to vibration, creating a small, induced dipole. The oscillating
dipoles then produce electromagnetic waves with the electric field parallel to the electric field of
the incoming light and perpendicular to the direction of propagation. These waves, in turn, strike
neighboring atoms, creating induced dipoles and forcing them into oscillations at the same
original frequency. These oscillating dipoles produce additional electromagnetic waves. None
of the dipoles, however, produce light along the axis of the dipole. This absorption and
reemission of light waves causes the light to be scattered about the medium. The scattered light
in the forward direction is completely unpolarized. In a direction away from that direction, light
is partially polarized, and at an angle of 900 to the direction of the incident light, the light is
completely polarized.
You can easily observe polarization by scattering using a polarizer. Locate the sun and then
examine the sky at roughly 900 to the sun’s rays. You’ll find that as you rotate the polarizer the
Jan Mader and Mary Winn
sky will appear very dark as much of the scattered light is polarized. Photographers know that
this partial polarization of scattered light produces a washed-out sky and so they correct the
problem by using a Polaroid filter. So the secret of capturing vivid blue skies in photographs lies
in the physics of polarization!
1. Set up a light bulb and view it through one polarizing filter. Describe what you see.
Ans: The intensity does not change as you rotate the filter.
2. Using two filters view the light bulb, hold one filter stationary and rotate the other.
Describe what you see.
Ans: The intensity decreases to a minimum and then increases to a maximum twice as
the filter is rotated 360°. The effect of a polarizing filter on un-polarized light is to
reduce the intensity, but the orientation of the filter does not affect the intensity.
However, when the light passes through one filter, it is polarized. When it encounters
a second filter, the light will not be diminished at all the axes are in the same
direction. If they are crossed at a non-right angle, some light will be filtered. If the
axes are crossed at right angles, a minimum of light will get through the second filter.
3. Place a glass plate on the table and view the reflection of the light bulb in the glass
plate using only one filter. Rotate the filter and describe what you see.
Ans: The intensity decreases to a minimum and then increases to a maximum.
4. Place a mirror on the table and view the reflection of the light bulb in the mirror using
only one filter. Rotate the filter and describe what you see.
Ans: The intensity does not change
5. Place a container of water on the table and view the reflection of the light bulb on the
water’s surface using only one filter. Rotate the filter and describe what you see.
Ans: The intensity decreases to a minimum and then increases to a maximum.
6. Print your name on a piece of paper and place a calcite crystal over your name.
Describe what you see. Now view your name using only one polarizing filter. Rotate
the filter and describe the changes.
Ans: the double image disappears and then reappears as the filter is rotated.
7. Place 2 pieces of cellophane between two polarizing filters and view the “sandwich”
in light. What do you see?
Ans: A rainbow of changing colors appears as the filters are rotated.
8. Place two filters one on either side of a piece of formed plastic and look at different
places on the plastic and describe the differences that are apparent when there is more
stress on the shape.
Ans: A rainbow of colors is more pronounced where there is more stress on the object
i.e. where there is a greater curve or bend.
Jan Mader and Mary Winn
9. Take your polarizing filter, ONE ONLY, outside and look at the following. Decide
which surfaces or objects polarize light and which surfaces or objects do not polarize
light.
a. Look at the sky. DO NOT LOOK DIRECTLY AT THE SUN. Polarization is
angle dependent. The light from the sky is partially polarized. Maximum
polarization is at 90° to the sun
b. Look at clouds. Polarization is angle dependent.
c. Look at a mud puddle Polarization is angle dependent.
d. Look at the hood or windshield of a car Polarization is angle dependent.
e. Look at your reflection in a window from various angles Polarization is angle
dependent.
Summing Up:
1. Based on your observations, which types of surfaces polarize light?
Ans: Horizontal surfaces polarize light if they are not highly reflective or
metallic.
2. If you were to go to a store to buy a pair of “polarizing” sunglasses, what procedure
could you use to determine if the glasses were actually polarizing? Hint: you may not
break them to get 2 separate polarizers.
Ans: Take a pair of glasses and rotate them while observing a reflection from the
surface of a glass display case. If the lenses are polarized the reflection’s
intensity will change as the filters are rotated.
3. How might you position a polarizing filter on your windshield and headlights to
minimize glare at night?
Ans: If all polarizing filters were oriented at a 450 to the upper right on both
headlights and windshields the approaching cars headlights would have a
polarized light that would be perpendicular to the orientation of the polarized
filter of the windshields. This would minimize the glare of approaching headlight.
4. What advantage might an engineer have in viewing a model of a structure in
polarized light?
Ans: Stressed areas would appear multicolored under polarized light when
viewed with a filter. This would show the location of the weakness due to stress in
the structure.
Jan Mader and Mary Winn