Human beings come equipped with two eyes and an absolutely amazing binocular vision system. For objects up to about 20 feet (6 to 7 meters) away, the binocular vision system lets us easily tell with good accuracy how far away an object is. For example, if there are multiple objects in our field of view, we can automatically tell which ones are farther and which are nearer, and how far away they are. If you look at the world with one eye closed, you can still perceive distance, but your accuracy goes down and you have to rely on visual cues or focusing distances, both of which are slower.
To see how much of a difference the binocular vision system makes, have a friend throw you a ball and try to catch it while keeping one eye closed. Also try it in a fairly dark room or at night, where the difference is even more noticeable. It is much harder to catch a ball with one eye than with two eyes.
The binocular vision system relies on the fact that our two eyes are spaced about two inches (5 cm) apart. Each eye therefore sees the world from a slightly different perspective, and the binocular vision system in your brain uses the difference to calculate distance. Your brain has the ability to correlate the images it sees in its two eyes even though they are slightly different. The correlator can pick out objects in the two scenes it sees and calculate how far apart an object is between the two images. Objects that are further apart in the two images are closer than objects that are not so far apart. (See this page for a demo).
If you have ever used a Viewmaster or a stereoscopic viewer, you have seen your binocular vision system in action. In a Viewmaster, each eye is presented with an image, and the images are created by two cameras that photograph the same image from slightly different positions. Your eyes can correlate these images automatically because each eye sees only one of the images.
In a movie theater, the reason why you wear 3D glasses is to feed different images into your eyes just like the Viewmaster does. The screen actually displays two images, and the glasses cause one of the images to enter one eye and the other to enter the other. There are two common systems for doing this:
There are some more complicated systems as well, but because they are expensive they are not used widely. For example, in one system a TV screen displays the two images alternating one right after the other. Special LCD glasses block the view of one eye and then the other in rapid succession. This system allows color viewing on a normal TV, but requires you to buy the special equipment.
- Polarization - At Disney World, Universal studios and other 3D venues, the preferred method uses polarized lenses because they allow color viewing. Two synchronized projectors project two views onto the screen, each with a different polarization. The glasses are able to allow only one of the images into each eye because they contain lenses with different polarization as well.
- Red/Green or Red/Blue - Polarization cannot work on an ordinary TV screen, so the red-green system is used. Two images are displayed on the screen, one in red and the other in blue (or green). The filters on the glasses allow only one image to enter each eye, and your brain does the rest. You cannot really have a color movie when you are using color to provide the separation, so the image quality is not nearly as good as with the polarized system.
Here are several useful links: