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I've been looking around about LCD monitors, and how they polarise light. When a pixel needs to be black, the light is "twisted" so that it can't go through the polarising sheet in front. What happens to this light? Does it relfect back into the screen? Surely that would mean the screen would get quite hot if you leave it on a black image for a while? (but I've never noticed that happen).

What happens to the energy (from the electricity used to create the light) when it's blocked by polarisation.

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up vote 11 down vote accepted

It is possible to "block" light based on its polarization in a number of ways. In the situation you are describing, where the light hits a polarizing filter, it is simply absorbed by the filter. The filter does indeed heat up, and in fact if you put your hand near the screen you can usually feel that it is quite warm.

It is also possible to have a polarized mirror, which either reflects or transmits light based on its polarization. In this case the mirror doesn't absorb any* of the energy, so it doesn't heat up.

*In the ideal case. In reality, of course, there is some small amount of absorption, but it can usually be ignored.

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