Radiation pressure

Electromagnetic waves, sound waves, the waves of a vibrating string etc carry momentum and exert, on an obstacle in their path, a force equal to the rate of momentum change produced by the obstacle. A very general energy theorem is applied to the calculation of the momentum, and shows that, in an unbounded medium, waves of intensity J have momentum J/v2 per unit volume, where v is the velocity of propagation. In a dispersive medium the phase velocity must be used to calculate the radiation pressure, i.e. the radiation force per unit area, in terms of intensity.