A plane electromagnetic wave is incident on a material surface. The wave delivers momentum p and energy E.

Explanation: An electromagnetic wave striking a material surface

delivers both energy and momentum to the surface.

If the frequency of the wave is ν then the energy imparted will be

…. (i)

where h is the Planck’s constant.

The energy is also related to the speed of the wave as

…. (ii)

where p is the momentum and c is the speed of the wave.

From (i) and (ii)

The energy and momentum both are non zero.

We can also define a quantity known as radiation pressure which is the pressure the radiation exerts on the material surface is strikes.

The radiation pressure is given as

…. (iii)

Also the electromagnetic wave has energy so there is an intensity related to it given by

…. (iv)

From (iii) and (iv)

(because energy dissipated per unit time is the power and power = force × velocity)

So is the radiation pressure.