Why can't my eye see itself in the mirror through polarizing 3D-glasses? 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.
 A: 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



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.

A: 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.
A: 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.
A: 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.

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