Question: How do 3D glasses work?

Simone Sturniolo answered on 17 Jun 2013:

They don’t. 3D glasses are a scam!
…of course I’m just kidding. Sort of. Let me elaborate.
3D glasses work by “cheating your brain” into believing you’re looking at something that is 3D, while you really are not. See, there’s a reason why we have two eyes: if you try and close one eye at a time, you’ll realize that, due to their different positions, the two eyes give us different perspectives on the world we’re looking at. You’ll also see that, if you try to catch a ball while keeping one eye closed, it’ll be harder than with both of them. This is because our brain uses the combined information from our two eyes to calculate the distance of an object from us. In other words, if the ball is far, both your eyes will see it in the same place, but if it’s close, it will be in very different points of your ‘field of vision’ (the extreme example is your nose, which due to being stuck in the middle of your face is to the extreme left of the field of vision of your right eye, and to the extreme right of your left one).
When you go and watch a 3D movie, you’re actually seeing two movies: one shot for your left eye, and one for your right eye, from different cameras and with different points of view. They are projected both at the same time on the screen. So why don’t you see a confused mess instead of beautiful 3D images? Well, if you take out your 3D glasses, you DO see a confused mess (try it, if you haven’t. Possibly during a boring scene). The 3D glasses do the trick. See, the two projectors that project the two versions of the movie use something called ‘polarized light’. That’s a bit complex to explain now, but the point is, the lenses of your 3D glasses let only ‘one kind’ of polarized light pass. So your right eye lens will let only the image of the ‘right eye movie’ pass, and your left eye lens… you get the trick. In this way, your eyes see two different movies, and you get the impression that the images are in 3D. The same could be achieved if you put two tiny screens in front of your eyes, or simply by using red and green lenses to look at red and green images. That old system though made for an horrible color scheme – forget the beautiful blue aliens from Avatar. The polarized light trick instead allows for perfect photography.
If you want to experiment more with your eyes and the tricks that they can play on you, though, search for something called “stereograms”. They are drawings that you can make 3D by simply crossing a bit your eyes. The system is the same, but in this case the trick comes from the fact that you are holding your eyes at a different angle than your brain thinks they should be. Give ’em a try!