I went to see Toy Story 3 in 3D (great film, but the 3D was not worth it); so I have a pair of those cheap plastic “3D” glasses.
They’re actually polarising lenses (er, filter, not lens, as the picture is not magnified or minified, but whatever). You can use them to Do Science at home:
Above is a picture of an iPod taken through one of the lenses (of the 3D glasses). The coloured pattern is, I think, due to birefringence, but I’m not competent to explain it entirely.
So we see that the lens can be used to see patterns that are otherwise not visible. In this case, we can see stress patterns in the plastic (I think). Looking a bit more carefully at the iPod pictures, we can see that the patterns on the iPod are different for the two lenses. Thus, there are two different sorts of light that each lens passes.
I am holding the camera behind one of the lenses and shooting a picture into the mirror so I can see myself and the glasses and camera in the mirror. [edit: it's more fun to put the glasses on and look into a mirror and close one eye, but I can't show you a picture of that.]
Note that from the viewer side both of the lenses in the glasses are transparent. But as seen in the mirror, one of the lenses is opaque. Curiously the lens that is opaque in the mirror, is the one in front of the camera. How did the picture get taken!? Clearly light is able to enter the camera having passed through the right-hand lens. But the camera, behind the right-hand lens, is not visible. So light leaving the body of the camera through the right-hand lens cannot pass through the lens again when reflected in the mirror:
My explanation of this is that the light acquires a property as it passes through the lens, and the mirror changes that property so that light with the new property cannot pass through the lens. The property is the handedness of circular polarisation. The right-hand lens filters the light so it contains right-circular polarised light (say); the mirror reflects it into left-circular polarised light which cannot pass back through the lens (and into the camera sensor). Note that light that passes through the left lens and is reflected in the mirror, can pass through the right-lens.
[edit: however, I have a problem with this explanation, as it does not explain what happens when I get my laser pointer out...]
3D films work by displaying one frame through a left-circular polarising filter, and the next through a right-circular polarising filter, and alternating. A reflective screen with the right properties will maintain the separate polarisation of alternate frames. Circular polarising filters in the glasses are insensitive to viewing angle, so it’s a superior option to linear polarisation.