Fun with 3-D glasses

We saw How to Train Your Dragon in 3-D at a theater today, and kept the 3-D glasses to experiment with. (Luke will be happy to use them as sunglasses when we return to Jos.) Next time you see a 3-D movie, keep those glasses and try some things to see how they work.

If you’ve played with polarized sunglasses or polarizing photo filters before, you might have some expectations, as I did, about how these glasses would work. For example, you might expect the left lens to darken a blue sky at one angle, and the right at an angle 90-degrees from the first. Try it and see. Try these other experiments, too, which work best when looking into a fairly bright, featureless view such as the sky or bright wall.

• Put on one pair of 3-D glasses, then look through another pair, just as if you were wearing two pairs one in front of the other. Do you see any difference when looking through the right or left eye?
• Rotate the glasses you’re holding, keeping the bows pointed toward you (i.e., keeping the lenses in the same plane as the ones you’re wearing). Any differences?
• Now flip the second pair of glasses back-to-front so that you’re looking through them as if they were on a person looking at you. Again, close one eye and then the other, and look through the both pairs of lenses. Do both right and left lenses still appear tinted?
• With the second pair of glasses still back-to-front, rotate them 90-degrees as in the second step. What happens now?
• Bring the back-to-front glasses very near one of the lenses you’re wearing, and view a bright scene while rotating the lens.
• Wearing just the one pair of glasses, look into a mirror, again closing one eye and then the other.

Have you figured out what’s going on? I’m catching on to the way circular polarization is used, but I still don’t understand why there it makes such a difference whether you look through the lenses in one direction (front-to-back) or the other (back-to-front).