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Question
An em wave exerts pressure on the surface on which it is incident. Justify.
Solution
An em wave carries a linear momentum with it. The linear momentum carried by a portion of wave having energy U is given by `p=U/c`.Thus, if the wave incident on a material surface is completely absorbed, it delivers energy U and momentum `p=U/c` to the surface. If the wave is totally reflected, the momentum delivered is `p=(2U)/c` because the momentum of the wave changes from p to -p. Therefore, it follows that an em waves incident on a surface exert a force and hence a pressure on the surface.
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