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.