Show that the momentum 
of a photon of electromagnetic radiation with wavelength 
is given by
where 
is the Planck constant.
Use the expression in (a)(i) to show that a photon in free space that has a momentum of 
is a photon of red light.
A beam of red light of intensity 
is incident normally on a plane mirror, as shown in the figure below. The momentum of each photon in the beam is .
All of the light is reflected by the mirror in the opposite direction to its original path. The cross-sectional area of the beam is 
.
Show that the number of photons incident on the mirror per unit time is 
.
Use the information in (b)(i) to determine the pressure exerted by the light beam on the mirror.
.
