Photons have both wave and particle characteristics and observe wave particle duality. Though they have no mass, they carry momentum via the De Broglie equation
Then for example whenever they are reflected from a mirror or interact with matter, an exchange of momentum takes place. At all stages total momentum is conserved as in classical mechanics, even if the actual definition of momentum changes slightly and is not now equal to
Suppose then that a photon is bounced between two mirrors a distance
apart.
The photon travels with speed
the speed of light, so travels a distancein
seconds. Before returning to the mirror it must travel to the second mirror, undergo reflection and return to the first one. The time taken to return is then
and the photon strikes the mirror
times per second. Each time it strikes the mirror it transfers momentum equal to
to the mirror so the force exerted on the mirror is equal to the rate of change of momentum
The pressure exerted is tiny. One photon with a wavelength of 
bouncing between mirrors 1 m apart exerts a pressure of
The pressure exerted by photons is the principle behind the solar sail. Photons from the Sun bounce off a large flat sail in space, pushing the sail away from the Sun. A very large surface area must be used to generate a force sufficient to accelerate the sail to an appreciable speed.
