How LCDs use liquid crystals and polarized light

An LCD screen uses the sunglasses trick to switch its colored pixels on or off. At the back of the screen, there's a large bright light that shines out toward the viewer.

 In front of this, there are the millions of pixels, each one made up of smaller areas called sub-pixels that are colored red, blue, or green. Each pixel has a polarizing glass filter behind it and another one in front of it at 90 degrees. 

That means the pixel normally looks dark. In between the two polarizing filters there's a tiny twisted, 

nematic liquid crystal that can be switched on or off (twisted or untwisted) electronically. When it's switched off, it rotates the light passing through it through 90 degrees, 

effectively allowing light to flow through the two polarizing filters and making the pixel look bright. When it's switched on, it doesn't rotate the light, which is blocked by one of the polarizers, and the pixel looks dark. Each pixel is controlled by a separate transistor (a tiny electronic component) that can switch it on or off many times each second.