Radiation pressure

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Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the energy flux density divided by the speed of light. If the radiation is totally reflected, the radiation pressure is doubled. This happens because the waves that make up electromagnetic radiation carry momentum with them and pass that momentum on when they reach a surface.

For example, the radiation of the Sun at the Earth has an energy flux density of 1370 W/m2, so the radiation pressure is 4.6 µPa (absorbed).