Day 1.6 Polarization:
1) Put on a pair of 3-D polarizing glasses. Close one eye and look at another filter. Rotate the filter.
a) Write down all your observations. Be quantitative if possible.
Instructions: Each student should have a pair of the cardboard linear polarizing 3D glasses to wear and another
half a pair to hold in their hands. These can be purchased for 50 cents each from
http://www.rainbowsymphonystore.com
The first filter makes things darker. The second filter has an effect that ranges from nothing to completely
blocking the light. These are 90o apart.
b) How could you model polarized light and filters with a large spring?
Instructions: Ask a student to make a standing wave that is polarized vertically, horizontally, diagonally and
circularly. They usually have no trouble figuring out that polarization refers to which way it is vibrating. You
could have the rest of the class form two rows on either side of the spring to act as a filter. Only vertically
polarized standing waves will be transmitted.
c) Draw vector diagrams to explain what happens when the filters are in the same direction, at 90o to
each other and at 45o to each other.
d) How much light that gets through one filter will also get through filters at 0o, 90o and 45o to the first?
The first will be 100% for an ideal polarizing filter. There is also some regular absorption in a real filter, which
is noticeable if you have many of them all at the same angle of polarization. The first will be 0% for an ideal
filter. However, if you look through crossed polarizers directly at a light bulb, you will be able to see the light.
It seems reasonable, that 50% of the light should be transmitted for 45o. However, the component that passes is
71% of the initial vector. The intensity of light, like many waves, is proportional to the amplitude squared. If
our diagrams are showing amplitude, then the intensity of the light that passed will be 71% x 71% = 50%.
e) How much of the original unpolarized light gets through the first filter?
None of the horizontal components get through and all of the vertical components get through. The light is
randomly polarized in all directions, so half gets through.
2) Put on a pair of glasses. Close one eye and look at another filter. Rotate the filter until it looks black.
Insert a third filter in between the first two.
a) What happens? Draw vector diagrams to explain what happened.
b) If the three filters are at 45o to each other, how much of the original light will pass through all three?
Half gets through the first, half of that through the second and half of that through the third = 12.5%.
c) How much light will four filters at 30o to each other let through?
Half gets through the first, then cos2 (30) = ¾ of this gets through the next. Altogether there will be ½ x ¾ x ¾ x
¾ = 21.1 %. Notice how four filters passes more light than the three in the previous question.
3) The cardboard 3-D glasses are either polarized vertical/horizontal or plus/minus 45o. How can you tell
which type these are? Hint: What would happen to each if you wore one pair and looked at another pair
and then turned that second pair upside down or back to front?
If the glasses are H/V and you close one eye, the blacked out filter will change when you switched then front to
back but not if you turn them upside-down.
If the glasses are diagonal and you close one eye, the blacked out one will reverse when you turn them upside
down and not if you switch them front to back.
I believe they are designed this way because they are so easy to fold them the wrong way.
4) Put on the glasses, close one eye and look at the glare from light reflecting off windows or shiny desks.
a) Tilt your head. What happens? How can you make the glare disappear?
The glare disappears when your head is tilted at 45 to horizontal – clockwise for one eye and counter clockwise
for the other.
b) What does this tell you about reflected light?
Reflected light is polarized either parallel or perpendicular to the surface. (It is parallel.)
c) Why are polarized sunglasses better than non-polarized glasses?
They specifically reduce glare. This allows you to see everything else.
5) Place a piece of Icelandic Spar on a tiny dot.
a) Rotate the rock and look through it. How does the dot change?
Another dot appears which rotates around the original one.
b) What happens when you look at it through the glasses with one eye?
One of the dots disappears. Which dot switches, changes as you switch eyes.
c) What does this tell you about light passing through Icelandic Spar?
The light is split into two beams, with light polarized in opposite directions.
d) This material is birefringent. Explain how this is different from a polarizing filter? p. 494
Birefringent means double refracting. Light refracts by different amounts depending on its polarization, so they
take different paths. This means that a crystal can separate light into the two polarizations unlike a filter which
transmits one and absorbs the other.
6) Put on the glasses and look at another polarizer so that no light gets through.
a) Place a plastic fork between crossed polarizers. What do you see? Explain this.
The fork lets lots of light through. It must have rotated the angle of polarization.
b) Plastic shows elastic birefringence. What happens if you squeeze the tines? Explain.
You see lots of colours and they change position as you squeeze and as you rotate your head. Refraction
depends on frequency – and so does double-refraction.
7) Put on the glasses and look at an LCD screen (calculator, watch, laptop screen, LCD projector).
a) What do you notice?
Many of these use polarized light and the image will change as you tilt your head.
b) Go to Physics 2000. http://www.colorado.edu/physics/2000/index.pl How do LCD screens work?
Instructions: This site does a great jab explaining polarized light with lots of simulations etc. Encourage your
students to go through the whole section.
The key ingredients in these devices are liquid crystals – the L and the C of Liquid Crystal Displays. These are
able to rotate the polarization, as was seen in question #2 with many filters in slightly changing orientation. The
amount of rotation is changed by an electrical signal changing how the crystals are arranged.
8) Put on the plastic 3-D glasses, close one eye and look at another pair of these. Tilt your head.
a) What do you notice that is different from the cardboard glasses?
Which lens is blacked out doesn’t switch after 90o.
b) How is this an advantage?
You can tilt your head at the theatre and the 3-D glasses will still work properly.
9) The plastic 3-D glasses have regular linear polarizers like the cardboard glasses. They also have a
quarter-wave plate which turns them into circular polarizers.
a) Explain why tilting your head doesn’t matter.
The filters and light are either clockwise or counter-clockwise. Tilting does not change these.
b) Explain how the quarter-wave plate forms circularly polarized waves.
Instructions: This needs a moving 3-D explanation. Have the students stand and use their two arms to show the
horizontal and vertical components of the diagonal polarization that you demonstrate with one arm. Then use
two arms to show them the components after one has been slowed by a quarter-cycle and have them use one
arm to show the resultant polarization. Afterwards, they can draw diagrams similar to the ones below.
Consider light polarized at 45. It has components that move up/down and left/right and they do this in phase.
Half a cycle is shown below on the left.
The quarter-wave plate slows one component by a quarter of a cycle. The result is on the right.
10) Look at http://www.youtube.com/watch?v=KM2TkM0hzW8&feature=related – especially 2:00 – 6:00
a) How are microwaves different from and similar to visible light?
Microwaves have much larger wavelengths – on the order of 10-2 m instead of 10-7 m. They travel at the same
speed and exhibit similar behaviours refraction, interference, polarization etc. The microwaves are produced
already polarized.
b) How are polarizing filters for light and microwaves different and how are they similar?
The microwave filters are larger, with spacings similar to the wavelengths.
c) How do polarizing filters absorb light?
Instruction: This correct explanation is quite different from the easier to understand but ‘wrong’ model of a
picket fence and vibrating rope. If the light’s polarization (direction of the electric field vibrations) direction is
parallel to the filter – it gets absorbed. What about the magnetic field? It is perpendicular to the electric field
and could play the role of the rope. This changing field will induce an electric field which will produce a current
in the wire. Why don’t they define polarization with respect to the magnetic field instead? I don’t know and I
don’t think it is really important to go past the picket fence model in high school.
If the electric field is parallel to the conducting wires - or molecules in the visible light polarizers - they will
produce a current in the wire. This takes energy away from the electric field.
Textbook:
10.1
p. 498 # 1 -5