Return to Physics Index
Scott R. Welty Maine East High School
Park Ridge IL 60068
This mini-teach was created with high school students in mind but can
work with grades as low as maybe 4th or 5th grade. The idea is to show what
polarized light is and show some examples and applications from the "real
*Each student receives 2 square pieces of polarizing sheet, 2-3 inches on a side.
They will also receive 1 smaller piece perhaps only 1 inch on a side. You'll
also want enough material left over to make two larger filters (maybe 6 inches
on a side) for your demo use on the overhead. This material is available from
American Science Center, $7.50 for about a square foot. Two or three of these
sheets is probably enough for a class set.
*A standard PSSC coil spring.
*You need to build 3 mini picket fences. 1x1 inch stock is fine. I built mine
with 4 slats each about 3 feet high. The spacing of the slats should allow the
passage of the coil spring with enough clearance to allow the spring to vibrate
*Several rolls of cheap celophane tape. Try some different brands but usually
if it has a yellowish tint to it it's the stuff you want.
*Plexiglass squares or glass squares approximately the same size as the student
Pass out two polarizers to each student. Ask them to take a few minutes to
observe light passing through the filters individually and in series. Suggest
various orientations and let them "play for a few minutes". Ask them what they
have observed and duplicate what they tell you on the overhead with your
filters. Eventually you should get to the point where rotating one polarizer 90
degrees with respect to the other alternately turns the light on and off.
Now to the spring. Suspend one end of the spring in a clamp while you hold the
other end suspended about 4 feet off the ground in front of the class. Send
signals down the spring and review or discuss basic ideas of wave motion. Now
pass the spring through one of your picket fences. Have a student hold the
fence with the slats vertical. Now ask which kind of vibrations sent by you
will be passed on by the fence, up and down or side to side. Do it! Now add a
second student and a second fence. Arrange them so they are both vertical (the
fences that is!). Note that the vertical vibrations that pass through the first
fence also pass through the second fence. Now have the second student slowly
turn his polarizer 90 degrees and note that the vertical vibrations from the
first fence are now blocked by the second fence.
At this point, the students will hopefully see the connections between the demo
with their polarizers and the spring and you can say, if you like, that a
polarized wave is one that is only vibrating in one direction.
Question: What will happen if we put a third polarizer between two already
crossed polarizers? Pass out another polarizer to each student but perhaps
smaller than the original ones. Most will think that nothing will happen but if
the middle one is at 45 degrees to the plane of polarization of the other two
light will again get through. The students will eventually discover this and
you can duplicate it on the overhead. This is curious. Now try to simulate
this with your spring and fences. Have the two outer fences at right angles to
each other and the middle one vertical. As you have a student rotate the middle
fence 45 degrees you should once again get some waves through the third
polarizer. The middle one re-steers the polarized wave to have vibrations
somewhat in the direction that the last one will pass.
Give each student a piece of glass or plastic with one piece of celophane tape
on it. Again ask the students to place the tape between crossed and uncrossed
polarizers and try various orientations of the tape and polarizers. Give them a
few minutes to play with this. You might ask along the way if the tape is just
another polarizer and how can you tell if it is or not? You will find that the
tape will redirect the polarized light similar to what the third polarizer did
and yet the tape is NOT itself a polarizer. You can tell because you cannot
turn the light on and off by using one known polarizer and one piece of tape.
(Note for the interested: the tape is birefringent. It has two indices of
refraction depending on the direction of the vibrations and when you present
polarized light at 45 degrees to these directions some of the wave goes down the
"fast" axis while some goes down the "slow" axis. This can cause a twist in the
direction of the polarization thus allowing the passage of light through the
Now, either on the overhead or individually with the students or both, try
making layers of tape and viewing light through various layers layed down
parallel to each other. You should begin to see a color effect depending on how
many layers. This tends to be "brand dependent" as to what color you get with
how many layers. Try viewing the colors with the polarizers crossed and
uncrossed. They change! Is there a rule for what colors change to what other
Optional - Kyro syrup will also have this twisting effect and not unlike the
tape the color you see when the syrup is between crossed polarizers depends on
the depth of the syrup. You can compare containers of different depths of
syrup. You'll need some sort of container with a flat clear bottom.
Many cheap plastic items such as protractors will show colors which indicate
locations of stress when viewed between crossed polarizers. You can also try a
piece of plastic such as from a zip lock bag. Hold it between crossed
polarizers and slowly stretch it. You should see colors appear indicating
locations of stress.
Note: Many of the things above require you to manipulate things between crossed
polarizers. If you are one of those with only two hands you may want to put one
polarizer on the overhead stage while you hang the other in front of the
overhead lens. Watch out for heat on this one on some overheads.
Ask the students to find a source of polarized light in the room and to explain
how they know that this light is polarized. They may not say the light from one
of the polarizers.
Many reflections off of non-conductors are partially polarized such as from a
table top. You can prove that this light is polarized by looking at it through
ONE polarizer and rotating it. If this light source gets dim and bright then
you are in fact looking at polarized light.