Here's one you can try at home

If you have been to see one of the new 3D films which have been released recently, you might still have a pair of the 3D glasses you receive in order to watch it. If you do, then here's an interesting little experiment you can try at home, in order to figure out how they work.

You need a pair of the glasses, and a mirror. Pop the glasses on, and look at the mirror. Now, close one eye, and note which lens appears dark. Then do the other one. Now which lens turns dark?

For those without the glasses, the answer is that the lens in front of the open eye appears to go dark.

You may be familiar with the concept of polarisation from science classes. You'll no doubt therefore be aware that mirrors do not alter the plane of light's polarisation. A moment's thought will convince you that 3D glasses based on plane-polarisation would not behave that way when viewed through a mirror. Instead, they would block light which had passed through the other lens, not the same lens.

What's going on is that the lenses are circularly polarised (wiki), one clockwise, the other anti-clockwise. Mirrors do change the handedness of circularly-polarised light, so when you look through a lens at your reflection in a mirror, the lens in front of the open eye appears to go dark.

If you want a large-scale picture of this mirror effect, find yourself a soft football (e.g.), and drop it to the ground while spinning it about its vertical axis. As it returns to you, you'll see that it is spinning in the opposite direction [1].

You might wonder how you can produce circularly-polarised light. Linear polarisation is comparatively easy: you find a substance which strips out everything except the modes in one orientation, and you're done. We've known about this for ages. But circular polarisation? That needs a bit of subtlety (wiki). You linearly polarise your light at 45° to start with, but then you need something called a wave plate (wiki). This doesn't block light of different polarisations: instead, it slows down one orientation relative to its perpendicular. So you arrange everything carefully with just enough of this wave-plate to change the phase of one of the two modes by a quarter of a wave, and when they come out the other side, presto! Circularly polarised light. It's very clever.

There's one last fun thing you can do with the glasses, and I haven't a clue how to explain this yet. Looking at my flat-screen monitor through the glasses, I find that if I turn my head by 45° clockwise or anti-clockwise, the white backgrounds are tinted either yellow or blue respectively. The effect is the same through either or both lenses. Why that happens, I do not know.

[1] It's a nice demonstration, but I still haven't convinced myself of a full mechanical explanation for that effect.