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I found a pair of polarizing "3D glasses" lying around, and tried to look at myself in the mirror while wearing them.

To my utter confusion, when closing the left eye and only looking through the right eye, I could not see the right eye in the mirror. The light could not pass through the same polarized lens twice. (I could, however, see the closed left eye clearly.)

I would expect the opposite to be true, as light going out the right lens with polarization X and coming back in with the same polarization X should pass through unaffected.

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4 Answers 4

up vote 19 down vote accepted

See the Wiki article on Polarized 3D glasses. Most likely, you have a pair of circularly polarized glasses. The mirror reverses the circular polarization.

The article on Circular polarization does it better than I would be likely to achieve in less than an hour or two. Or Hyperphysics, or Google.

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3  
This is also how some "glare reducing" filters for computer monitors work. That said, this answer could stand being expanded a bit to explain the bare essentials of the difference between linear and circular polarization. –  dmckee May 13 '11 at 16:52
    
Peter Morgan is exactly right; you have circular polarizing lenses there, which is to say linear polarizers with half-plates between the polarizers and your eyes. Both regular polarizing sunglasses and IMAX 3D are linear, and will behave more like you would have expected. –  Terry Bollinger Feb 10 '13 at 5:07
    
@TerryBollinger I believe you mean a quarter wave plate, and it would need to be in front of the linear polarizer not between it and your eye. –  Chris Mueller Mar 26 at 14:59
    
Good catch and exactly right on both points. The quarter-wave plates in front translate the two circular polarizations into the two linear polarizations, and the linear polarizers closer to your eyes then select between them. –  Terry Bollinger Mar 27 at 1:47

Cinema 3D glasses (at least those made by Read-D) are circular polarized. This has the advantage that the polarized light reflected from the screen doesn't depend on the angle between your eye and the screen and so you can move your head around while watching. But when you look in a mirror the rotation direction is reversed on reflection.

The shutter glasses used with most home 3D LCD-TV have a linear polarizer and when turned off you can see through them in a mirror.

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Assuming 3D RealD glasses - uses circular polarized light to cancel images.

To understand better, ask yourself why do I see my eyes when both eyes are opened?

The answer is that your brain mixes two reflected images:

  • one image contains your left lens blacked
  • the other image contains the right lens blacked

Brain mixes to images to coming from two eyes

When you close an eye, you cancel one image.

Why in one image you see a black lens?

The mirror reverses the circular polarization from right-handed to left-handed or vice versa. See the image below.

One of the lens is blacked

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See How do 3D glasses work - Sixty Symbols (Youtube) for a video explanation. The key is that the light has to pass through the lens twice, if it goes through once you just see the polarized light, if it goes through twice it doesn't see light at all. See the image on the right. enter image description here

Reference: Image from http://www.exo.net/~pauld/summer_institute/summer_day8polarization/polarizerfencemodel600.jpeg

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3  
No, and in fact I'm sure the original poster understood all that. The question is basically why it's not the case on the left. After all it's passing through the same lens twice. And the real answer is given above: circular polarisation. –  Retarded Potential Feb 11 '13 at 2:52
1  
@RetardedPotential is correct. This answer does not correspond to the question. –  Deestan Feb 11 '13 at 8:27

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