which is exactly equal to the energy density u.
If on the other hand, the surface reflects the wave, the pressure would be twice the above value.
The above is true for waves at normal incidence. If the radiation is diffuse, i.e., if it strikes the wall from all directions, it essentially consists of plane waves travelling in all directions. If the radiation is isotropic, the intensity of the wave is the same in all directions. The contribution to the pressure comes from those waves which are travelling in a direction which has a component along the normal to the surface. Thus on an average a third of the radiation is responsible for pressure. The pressure for an absorbing surface is u/3 while that for a reflecting surface is 2u/3.
The existence of radiaton pressure can be verified experimentally. The curvature of a comet’s tail is attributed to the radiation pressure exerted on the comet by solar radiation.