A beam of light is incident normally on a metal surface, as illustrated in the figure below.
The beam of light has cross-sectional area 
and power 
. The light has wavelength 
. The light energy is absorbed by the metal and no light is reflected.
Show that a photon of this light has an energy of 
.
Calculate, for a time of 
,
the number of photons incident on the surface,
the change in momentum of the photons.
Use your answer in (a)(ii) to calculate the pressure that the light exerts on the metal surface.
.
.
.
