$($a$)$ You direct a $19\mathrm{.}0$ mW laser beam toward a surface at normal incidence. The surface is made of a material that completely absorbs the radiation. What is the radiation pressure on the surface if the cross section of the beam has a diameter of $2\mathrm{.}75$ mm when it arrives at the surface?

N$/$m$2$

$($b$)$ You now direct the same laser beam, with the same power and beam cross section, at normal incidence on a mirror where $100\%$ of the beam is reflected from the surface. What is the total pressure on it due to the incident and reflected energies?