The photos of John Atherton and me were taken by Robert Prior at one of Toronto's Physics and Pints Nights. John Travolta couldn`t make it that night.
The glasses that John and I are wearing (which you can order from http://www.rainbowsymphonystore.com for $0.50 each) have lenses polarized at +45 and -45. As we tilt our heads, it changes which of our partners' eyes we can see. At a 3-D movie, the projector sends out images that alternate between these polarizations. Each polarization carries the image of the scene from a slightly diferent viewpoint and so your eyes receive different viewpoints and your brain interprets this in 3-D.
A standard projections screen destroys the polarization when it reflects the lights and so a metal screen must be used. You can show this by projecting a bright beam of light (laser pointer or LED flashlight) through a polarizing filter onto a white non-metallic surface and then onto a piece of tin foil. With the non-metallic surface, both eyes can see the reflected light equally well. With the tinfoil, one eye will see the spot and the other won't.
Last spring I went to one of these new 3-D movies and brought the glasses home with me. They don't behave the same way! If you tilt your head while looking at someone else's glasses, the blacked out eye stays blacked out. This is great in the theatre, because it means you don't have to keep your head upright - you can lean on your date's shoulder. But how do they work? It turns out that they are not linearly polarized but circularly polarized. To see this, grab a long metal spring. Make a vertically polarized standing wave. Make a horizontal one. Now make one by shaking it and spinning it like a skipping rope. That one is circularly polarized. Notice how it can be polarized clockwise or anti-clockwise. Notice also how tilting your head makes a vertically polarized spring look horizontal but it doesn't change the direction of the circularly polarized spring
The 'lenses' of the glasses consist of two parts; a linear polarizer and a quarter-wave plate. In the quarter-wave plate, one component of the light travels travels slower than one perpendicular to it - this is called birefingence. The material has the right thickness to make the one component shift 180 degrees out of phase. The easiest way to get this is to do a dance like John Travolta in Saturday Night Fever. (Go on, no one is looking!) Move your hand diagonally, from the upper right to the bottom left of your body. Now, decompose that diagonal motion into its vertical and horizontal components. One hand moves up to down, while the other moves right to left. Now, what if the horizontal one was delayed so that rather than moving right to left, it moved center to right to center? Try it. Those components add up to a vector that goes from up to right to down, in other words it moves in a circle. It is circularly polarized.
Finally, if you look carefully at the cheap cardboard glasses, they are also a bit circularly polarized.