Radiation Pressure

Electromagnetic waves transport energy. To remind ourselves of that fact, all we have to do is to go to the beach on a sunny day and stay there for a while without sunscreen. The warmth that we feel on our skin (and the sunburn ...) is, as everybody knows, due to the sunlight. And the sunlight is nothing but electromagnetic waves. Thus electromagnetic waves transport (= radiate) energy from the surface of the Sun to the surface of Earth.

But with the energy that electromagnetic waves carry, there also is momentum. Maxwell showed that if electromagnetic radiation falls on an object during a time interval $\Delta$t, and if the energy absorbed by the object from the electromagnetic radiation is U, then the magnitude of the momentum delivered to the object is

\[ \rm \mathbf{p = \frac{U}{c}} \] (for total absorption of U)

If the electromagnetic wave is totally reflected by the object, then the momentum delivered to the object is

\[ \rm \mathbf{p = \frac{2U}{c}} \] (for total reflection of U)

The reason that the momentum is a factor of two bigger in the second case is that the object needs to reverse the momentum of the incoming radiation and thus receives a bigger recoil.

The momentum change of the object means that a force is exerted on it. The force divided by the surface area exposed to the radiation then gives us the pressure.

In carefully controlled experiments, this radiation pressure can be measured, in particular if one uses laser light as the electromagnetic radiation.

Radiation pressure can also be seen in the tails of comets. Due to the radiation pressure the comets' tails always point away from the sun.