Radiation pressure

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. 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) (see also Climate model).
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Noun
1. the minute pressure exerted on a surface normal to the direction of propagation of a wave
(synonym) corpuscular-radiation pressure
(hypernym) pressure, pressure level, force per unit area
(hyponym) acoustic radiation pressure

The force (per unit area) exerted by a photon of electromagnetic radiation when it collides with a particle of matter. The greater the momentum of the photon, the greater the radiation pressure. This effeict is clearly seen with a comet where its tail is always pointing away from the sun.