Demonstrating how a backlit LCD actually works.
An LCD has a backlight, and a liquid crystal layer sandwiched between two linear polarizers oriented at 90 degrees relative to each other.
Typically, unpolarized backlight passes through the first linear polarizing filter. It then passes through the liquid crystal layers (for red, blue, and green), which selectively rotate the polarization based on desired intensity at each pixel. Then the front linear polarizer functions to block the appropriate color channels by the appropriate amounts based on the angle between its orientation, and the linear polarization angle of the light at the local pixel.
I've removed the front polarizer from the LCD monitor. So all you are seeing is the backlight. The on-screen information is encoded in the polarization rotation angles at each pixel, but it can only be revealed with a linear polarizer. Notice how the native filter orientations are 45 degrees relative to the screen boundaries. Try and guess why!
Also notice how intensities get flipped when I hold the polarizer at the wrong angle, giving us a negative of the intended image.
