You may have read science fiction stories in which the hero, fed up with an increasingly polluted and disgusting earth, hooks his life support pod to a gigantic silvery plastic sail out beyond the orbit of Mars, and sails off to the far reaches of the solar system, powered by the pressure of sunlight. This is probably nuts, but sunlight would exert some force on such a sail. Light does carry momentum as well as energy; it's just that the momentum is very small. There are two ways to write the formulas for this effect, and both will be given here.
If you know the intensity, I = <S>, of the radiation, then it is most
convenient to use a formula for the pressure, P, (force per unit area)
exerted by the radiation.
The radiation pressure depends on whether the radiation is absorbed or reflected.
If it is reflected, the object which reflects it must recoil with enough momentum
to stop the incoming wave, and then send it back out again, while if it is absorbed,
the object must only stop it.
Hence, the reflection formulas give answers that are twice the size of those given
by the absorption formulas.
The radiation pressure formulas are
If you know the total amount of energy U in a pulse of radiation,
as you might for a laser pulse, then it is most convenient to
use formulas for the amount of momentum p imparted to an object
that either absorbs or reflects the pulse:
Because of the factor of c in the denominators of these pressure and momentum formulas, these effects are usually quite small.