The pressure (or force) on a body illuminated by electromagnetic radiation. By ordinary standards radiation pressure is extremely small. For example, the radiation pressure on an object exposed to intense sunlight is about 10¹¹ times smaller than sea level atmospheric pressure. But for small (comparable to or smaller than the wavelengths of visible and near-visible radiation) particles subjected to only the gravitational attraction of the sun and the repulsive radiation pressure of its radiation (e.g., cometary particles), radiation pressure is not negligible, which accounts for the curvature of the tails of comets.