Assume that the surface is exposed to electromagnetic radiations. So, all the radiations are allowed to fall normally on the given surface.

Let the intensity of the radiation is I and the area of the surface is A.

So, the energy received by the surface in each second is given by,

E = I. A

So, number of photons received by surface in each second is given by,

For the perfectly absorbing surface,

So, force is given by,

Pressure is given by,