eli5 movie theater Glasses

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How do those old red and blue movie theater Glasses make movie 3d? And are the new movie glasses similar do they do something different

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If you ever looked at the screen without the glasses on, you’d notice that there was weird coloring around the edges of different things; specifically, red and blue “shadows”. Those correspond to the color of the lenses on the 3D glasses, which makes each eye see a different image.

Your brain interprets that as 3D, because the other common situation where each eye sees a different image is when there is an object that is three-dimensional and somewhat near your face (if you hold your finger up to your nose and close one eye at a time, you’ll see that each eye sees the finger in a different place; a different image). So by filming the scene with two cameras (one with a red filter, one with a blue filter) or by manipulating the film and making you wear the 3D glasses, the filmmaker can trick your brain into thinking that the objects in the movie are coming out of the screen.

Your depth perception comes from the fact that your left eye doesn’t see the exact same thing as your right eye. So to create a 3D effect, you need to somehow show your two eyes a slightly different image.

The old way to do this was to have a red layer and blue layer on top of each other. The blue lens filtered out the blue layer for one eye, the red lens filtered out the red layer for the other.

Modern 3D glasses are a bit more high tech. The red and blue filters obviously mess with the colors of the movie. So new glasses flicker in sync with the screen. The right lens will go dark so only your left eye sees the screen. Then the glasses switches to the right eye and the screen switches to a slightly different perspective. Each eye still get a slightly different image, but colors are less messed up.