I've been trying to wrap my head around how 3d glasses work. I think I'm finally there, but it took making this diagram to get to the bottom of it (although it really confounded things for a bit since my monitor's screen - one of the subjects in my experiment - has a polarized screen).

If you can get your hands on a pair ($16 and they usually come with a free screening of a Pixar or James Cameron movie) they're really fun to play with. Especially if you take the lenses out of the frames.

The ones I have are from RealD 3D who uses circularly polarized lenses instead of linear (which was the source of much of my confusion). Try standing in front of a mirror with the glasses on and alternate closing your left and right eye while looking at you your eyes. Spooky!

Here is a photo of two lenses (1 horizontal over 2 vertical taped to the top of my computer) with diagram:

Explanation:
#1 Horizontal lens on top of #2 Vertical lens

• R1: 2VS - Little or no effect
• R2: 2VM - Little or no effect
• R3: 1HS - Little or no effect
• R4: 1HM - No light passes through
• R5: 1HS over 2VS - No light passes through
• R6: 1HM over 2VM - No light passes through

Key: 2VM = Lens side 2, vertically aligned, viewing monitor

Or you can look at it like this:

Click on the thumbnail below for the full results.