Polarization of Light

D. James Chichester            Lincoln-Way High School
                               1801 East Lincoln Highway
                               New Lenox IL 60451
                               (815) 485-7655


Objectives:

This lesson is most appropriate for high school, but could be revised with a 
little thought and effort for lower grades.

Students will discover that polarization filters will be able to block incoming 
light from passing through when filters are positioned perpendicularly to 
each other.

Students will be able to explain why the filters block some of the incoming 
light and also redirects incoming light.

Students will be able to explain some of the uses of polarization filters within 
our society. 

Materials Needed:

3 polarization filters, about 1.5 inches by 1.5 inches, per student
3 wood gates that will simulate polarization filters
Several small layered pieces of cheap transparent tape, per student
One transparent protractor
One pair of polarized sunglasses
Small sandwich baggie for students filters and tape swatch
Length of surgical tubing
note: polarization filter sheets, 1 square foot, can be purchased at the 
American Science and Surplus for $7.50 and then cut with sturdy scissors


Strategy:

Pass out one baggie with the three filters and swatch of transparent tape 
layerings to each student.  Tell the students to take out two filters and play 
with them for a while.  After some time several students should discover that 
when you hold them on top of each other and rotated just right that you can't 
see through the overlapped area.  Have those students explain how to do this 
with the filters and allow everyone to do the same.  Why does this happen? 

Most light travels in waves that comes towards us vertically, horizontally, and 
at all other angles of tilt.  With this illustration on the board, hold one 
wooden fence vertically in front of your picture and ask what direction will 
pass through the screen.  Hold the same fence horizontal and ask what direction 
of light would pass through this fence.  Hold the fence at an angle that matches 
one of your illustrated angled light rays.  Which direction light would pass 
through this filter?  We're driving home the idea that the fence only allows the 
same direction light through it that the fences slats are held.  This is a 
concrete example of the polarization filter and light rays.  

Send a vertical wave with the tubing through one fence that is held vertically, 
the wave can get through the fence.  Send the vertical wave through two vertical 
fences, the wave can get through.  Send a vertical wave through the first fence 
which is vertical, and the try to turn the second fence horizontal.  Did the 
wave get through any of the fences, both of the fences, why?  We're trying to 
explain why the filters blocked the light when they were turned just right, 
perpendicular to each other.  

Students should now experiment with the three filters and see if they can 
discover that if the middle is at a 45 degree angle to the bottom, and the top 
is perpendicular to the bottom filter that will either give you a hole in the 
middle of your sheet, or an "unhole" in your blacked out background.  The 
angeled filter redirects the vertical or horizontal light a small amount, thus 
allowing some of the light from the second filter to pass through the third 
filter.  This can be illustrated by holding the three fences up in front of 
themselves and taking it step by step with the students. 

Place the transparent tape swatch between the two filters.  Students should 
notice different colors of light in the tape depending upon the thickness of the 
tape.  The tape can also be layered on a small piece of cut glass.  They could 
even make their own art projects from this.  Placing the clear protractor in 
place of the tape gives a similar appearance.  The dark bands of light represent 
where the plastic met when the protractor was poured in the mold.  These are 
the relief spots, or points of weakest strength.  Engineers can do the same by 
surrounding parts with transparent plastic and viewing the test pieces under 
stress through polarization filters.  Note: the tape is birefringent, meaning 
certain colors of light pass through it at different speeds separating out the 
colors of light. 

Have the students find surfaces in the room that reflect light with a glare.  
Now view the glare through one of their filters while rotating it at different 
angles of rotation.  They should discover that reflected glares are eliminated 
with a polarization filter.  The reflected "glare" light is polarized the same 
direction as the reflecting surface.  Ex: horizontal desktop...horizontally 
polarized light,  vertical watch face...vertically polarized light.  Ask 
students when they normally encounter light that reflects in a glared fashion, 
and if they normally like looking at this glared light.  

Place one filter in front of sunglasses lens, test to see if sunglasses are 
polarized.  Now test a pair of polarizing sunglasses.  Hey, just like when they 
had two polarization filters.

Optional: Take one filter outside on a sunny day and view the sky, not the sun, 
through the filter.  Is the sunlight slightly polarized?  It should be somewhat 
polarized depending upon the weather for the day.

Big Hint: Illustrate your ideas with the filters on an overhead projector so 
everyone can see what you are doing.  Be careful not to melt the filters on the 
screen and to not tell them what to do with the filters, let them do the 
discovering and experience the phenomena for themselves.

Performance Assessment:

What direction would the polarizers be in a pair of polarizing sunglasses if you 
used them while driving your car or fishing on a lake? 

References:

Conceptual Physics by Paul Hewitt