Physics 23 Laboratory
Polarization
I.
Polarization of Light
A.
Look at the room lights through one of the polarizing filters provided. Describe how the filter affects
what you see. Does rotating the filter have an effect?
B.
Hold a second polarizing filter in front of the first, and look at the room lights again.
Describe how the filter affects the light that you see. How does rotating one of the filters affect what
you see in this case?
On the basis of your observations so far, why is it appropriate to use the term filter to describe these
pieces of apparatus?
How is the behavior of the polarization filters different from the effect of colored acetate filters?
You have learned that light may be thought of oscillating
electric and magnetic fields. If the electric field in all
Direction of propagation
parts of the light beam oscillate along a single direction,
then the light is said to be linearly polarized, plane polarized,
or simply, polarized. For example, the diagram to the
right represents a polarized light wave moving in the +xdirection in which the electric field oscillates only in the
y-direction. By convention, the direction along which the
electric field oscillates (in this case the y-direction) is
called the direction of polarization of the light beam. If the
electric field oscillates in different, random directions, within the same light beam, that beam is said to be
unpolarized.
II.
Polarizing filters
The light transmitted by a polarizing filter (or polarizer) depends upon the relative orientation of the
polarizer and the electric field in the light wave. Every polarizer has a direction of polarization, which is
often marked by a line drawn on it. The electric field of the transmitted wave is equal to the component of
the electric field of the incident wave that is parallel to the direction of polarization of the polarizer.
A.
Do the room lights produce polarized light? Explain how you can tell from your observations so far.
B.
Suppose that you had two marked polarizers (i.e., their directions of polarization are marked).
Predict how you should orient the polarizers with respect to one another so that the light
transmitted through the polarizers would have (1) maximum intensity or (2) minimum intensity.
Discuss your reasoning with your partners and then check your predictions.
When two polarizers are oriented with respect to each other such that the light transmitted through
them has minimum intensity, the polarizers are said to be crossed.
C.
Suppose that you had a polarizer with its direction of polarization marked. How could you use this
polarizer to determine the direction of polarization of another (unmarked) polarizer? Explain your
reasoning.
D.
A beam of light is incident on a polarizer, as shown in the side view diagram below. The light is
polarized in a direction that is tilted by an angle  with respect to the polarizer's direction of
polarization. (See front view diagram below.) The vector, Eo, shown in the diagram represents the
amplitude of the electric field of the incident light at one instant at a particular point in space. The
corresponding magnetic field vector, Bo, which is not shown, represents the amplitude of the
magnetic field of the incident light.
Polarizer
Incident
light
Side view
Direction of
polarization
of the filter

Eo

Front view

Resolve the vector Eo , into two components: (1) the component of the electric field that is
transmitted by the polarizer and (2) the component of the electric field that is absorbed by the
polarizer.
What is the direction of the electric field that is transmitted by the polarizer? How, if at all, is it
different from the direction of the electric field of the incident light? Explain.
Write an expression for the magnitude of the electric field of the transmitted light, in terms of Eo and
.
Write an expression for the intensity of the transmitted light in terms of Io, the intensity of the
incident light, and . Show all work. (Hint: If the amplitude of the electric field were halved, how
would the intensity change?)
E.
An observer is looking at a light source
through two polarizers as shown in the side
view diagram at right. The polarizers are
crossed, that is, they are oriented so that the
light transmitted through them has
minimum intensity.
1.
Light
source
Crossed polarizers
Observer
Y
X
Suppose that a third polarizer were inserted at the position marked X, shown above.
Predict how, if at all, this change would affect the intensity of the light reaching the observer.
Does your answer depend on the orientation of the third polarizer? Discuss your reasoning
with your partners.
Check your prediction experimentally. (Borrow a third polarizer from another group.) If your
prediction was incorrect, identify those parts of your prediction that were wrong.
How can you apply your results from part D to help you account for your observations?
Support your answer with one or more diagrams.
2.
Suppose that instead a third polarizer were inserted at the position marked Y, shown above.
Predict how, if at all, this change would affect the intensity of the light reaching the observer.
Does your answer depend on the orientation of the third polarizer? Discuss your reasoning
with your partners.
F.
Consider a beam of unpolarized light that is incident on a polarizer. What is the intensity of the
transmitted light in terms of Io, the intensity of the incident light? (Hint: We can think of
unpolarized light as equal amounts of light that are polarized parallel and perpendicular to the
direction of polarization of the polarizer.)
III. Polarization by reflection
Light reflected from smooth, nonconducting surfaces is, in general, partially polarized. However, at one
particular angle, called the polarizing angle or Brewster’s angle, the reflected light is plane polarized.
Polarizing
filter
A.
Place a glass plate in the polarization apparatus
Incoming light
and position the apparatus in front of a window or
under a bank of ceiling lights such as the glare
coming from the lights is reflected from the glass
at a range of angles. Observe the glare through a
Glass
polarizer and rotate the polarizer. What did you
plate
observe as the polarizer was rotated? What does it
mean when we say that the light is partially
polarized?
B.
Rotate the polarizer until the light passing through the polarizer is of minimum intensity. Now vary
the angle of observation until the intensity of the light in further minimized. Ensure that the
reflected light is minimized by slightly rotating the polarizer and varying the angle of observation.
When a minimum is reached, measure the polarizing angle, and from this measurement, find the
index of refraction of the glass plate.
C.
Explain how you can use this method of observing polarized light in order to determine the plane of
polarization of the polarizing filter?
D.
Discuss with your partners the mechanism involved in order that this effect is produced. (Think
about how electromagnetic waves are emitted by an antenna.)
IV. Polarization by scattering
Light that is scattered from molecules and small particles is polarized. For example, when unpolarized
sunlight interacts with oxygen and nitrogen molecules in the atmosphere, the scattered light is polarized.
The effect is most pronounced when the incoming unpolarized light and the scattered light make an
angle of 90o with each other. In other words, the effect is most easily observed by looking straight
upward through a polarizing filter at sunrise or sunset.
A.
Go outside with a polarizing filter. Face away from the sun and look through the polarizer at the
sky. (DO NOT LOOK AT THE SUN.) Rotate the polarizer. What did you observe?
B.
Rotate the polarizer until the intensity of the light coming from the sky is minimized. How is the
plane of polarization of the polarizer related to the direction of the sun?
Discuss with your partners how and why this effect occurs? (Hint: It’s similar to reflection.)
IV. Polarization by double refraction (birefringence)
An unpolarized beam of light passing through a double refracting material, such as calcite, emerges as
two beams that are mutually polarized. This phenomenon is a result of the material being anisotropic –
light travels at different speeds in different directions through the material.
A.
Draw a dot (~1mm) on a piece of paper. Place the calcite crystal on top of the dot. What do you see?
B.
Rotate the calcite crystal. What happens?
C.
Use a polarizer to observe the dots. Slowly rotate the polarizer. What happens?
D.
What can you conclude about the two images?
E.
Take two polarizing filters and rotate them until the light passing through them is a minimum.
Place a double refracting material, such as cellophane, hard plastic, etc., between the filters. What do
you observe? What happens when one of the filters is rotated? Can you explain why these effects
occur?