3D glasses work on the same motivation as that of a motion picture. Exploiting the weaknesses of the human sight. While motion makes use of the property of persistence of vision of the human eye, 3D glasses make use of the shortcomings of our binocular vision system.

The Binocular Vision System is the ability to judge how far an object is away from us. However, if we close one of our eyes, we find a marked decrease in the accuracy with which we perceive how far the objects are from us.

You may experiment this for yourself by asking your friend to throw some object at you from some distance while you attempt to catch it with one eye closed. You will find that you are faultering sometimes with your eye closed as compared to when both your eyes are open.

The main reason why this Binocular Vision System exists is because the human eyes are spaced two inches apart and thus will experience two different visions, though the differences will be very slight. This forces the brain to correlate the images by calculating the differences between the images and giving us a final information through which we perceive our world.

Now you can imagine that it is the differences in the visions of the two eyes that makes us perceive whether things are close or far away. 3D glasses do exactly this! They manipulate the images going into each eye and produce a feeling of objects being much closer than they really are.

How do they do this? The 3D movie that you watch is actually being projected by two different projectors. Thus, you are watching two different views of the same movie. What 3D glasses do is convey only one of these views to each eye. So if the movie is being projected by two projectors named A and B, your 3D glasses may allow your left eye to see only view A and your right eye to see view B.

This is possible through a phenomenon called polarisation and thus 3D glasses are made out of polarised glass which only allow certain frequencies of light to pass through while obstructing the others.

This creates an illusion of some objects in the images being closer than the others and thus the 3D effect is obtained.