The pressure exerted by an electromagnetic wave of intensity I(Wm2) on a non-reflecting surface is [c is the velocity of light]

When a surface intercepts electromagnetic radiation, a force and a pressure are exerted on the surface. As the surface is non-reflecting, so, it is completely absorbed and in such case the force is F=IAcwhere 1 is intensity of light' A is area of surface and c is velocity of electromagnetic wave.The radiation pressure is the force per unit area i.e., P=FA=Ic.