rationalskepticism.org

If you go to see a 3D movie and they're using the RealD 3D projection system, they give you (sell you) a pair of specs in which each lens is polarised to accept light at 90 degrees to that accepted by the other lens. Naturally you retain these and play with them. I found they behave very oddly when you look in a mirror.

At first all seems fine: in your reflection you can see the specs, and behind the specs you can see your eyes, only a little dimmed. But if you close one eye - say, the right - the opposite lens suddenly appears black: the only eye you can now see is the one that's closed! Swap eyes and the effect reverses: you can only ever see the closed eye, not the one that's actually doing the seeing!



I tried to get my head around this, thinking about the light passing through the lenses twice and such, and wondered if the polarity of light flips 90 degrees on being reflected. But a clear explanation from somebody with a solid understanding of the matter would be much appreciated. I've attempted to capture the flavour of the problem with the photo below: you can see that the lens that the camera isn't pointing through is clear, while the one that it is pointing through is almost black.