radiation pressure

Radiation pressure is the minute pressure exerted on a surface at right-angles to the direction of travel of the incident electromagnetic radiation. Its existence was first predicted by James Clerk Maxwell in 1899 and demonstrated experimentally by Peter Lebedev. In quantum mechanics, radiation pressure can be interpreted as the transfer of momentum from photons as they strike a surface. Radiation pressure on dust grains in space can dominate over gravity and explains why the tail of a comet always points away from the Sun.