With a B. Sc and a Ph.D in Physics I know about polarized light. Watching Avatar

Question

With a B. Sc and a Ph.D in Physics I know about polarized light. Watching Avatar in 3D, and removing the 3D glasses I could see the two separate images of "distant" objects. Peering though one "lens" and rotating the glasses around the light rays axis, I see the image is unchanged, i.e. did not blank out one image and reveal the other as expected Huh? How does Avatar's 3D work? Looking back at the projection booth I could see three bright "spots" one above the other. The top or the bottom one could be extinguished by rotating the glasses.

Explanation / Answer

Just based on your careful description, here's my guess.

You've noticed that two of the 3 beams are indeed linearly polarized, oppositely. (If circular polarizations had been used, the blocked beam wouldn't have been sensitive to rotating the lens.) Now when those beams bounce off the screen, quite a bit of the polarization is lost. Thus when you look through the rotated lenses, nothing completely goes away when you look at the screen.
There should be enough polarization left for the intensity of the images to be significantly different through the different polarizers,

So far that wasn't guess. Here's the guess part. Maybe the brain then uses that intensity difference to construct a 3-D image.

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