In order to see things in 3D each eye must see a slightly different picture. This is done in the real world by your eyes being spaced apart so each eye has its own slightly different view. The brain then puts the two pictures together to form one 3D image that has depth to it,this is called “binocular vision”- the principle behind 3D glasses.
A 3D film viewed without glasses may appear to be out of focus. The same scene is projected simultaneously from two different angles in two different colors, red and cyan (or blue or green). Here’s where those glasses come in – the colored filters separate the two different images so each image only enters one eye. Your brain puts the two pictures back together.
There are two common systems for doing this:
The red/green or red/blue system. In this system, 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.
The polarized system uses polarized lenses because they allow color viewing. Two synchronized projectors project two respective views onto the screen, each with a different polarization. The glasses allow only one of the images into each eye because they contain lenses with different polarization.