The force on an object exposed to electromagnetic radiation. It has been known since the days of J. C. Maxwell in the nineteenth century that electromagnetic radiation (which includes visible light) carries both energy and momentum. If radiation impinges on a material body and becomes absorbed, the energy gives rise to heat and is readily detectable. When radiation interacts with an object and is absorbed or scattered, there is also a change in the momentum of the light. By conservation of momentum, this gives rise to a force on the object. This is called radiation pressure. The magnitude of this momentum for visible light is quite small and is difficult to detect. Only near or inside stars, where the intensity is enormous, do light forces have large effects.