Polarized Light

Scott R. Welty                 Maine East High School
                               2601 Dempster
                               Park Ridge IL 60068
                               (847) 825-4484

Objective:

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 
world".

Materials Needed:

*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 
 freely. 
*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 
 polarizers. 

Strategy:

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 
last polarizer.) 

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 
colors?

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. 


Performance Assessment:

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.