A beam of visible light has a wavelength of 513 nm and shines perpendicularly on a surface. As a result, there are 4.2 x 1018 photons per second striking the surface. By using the impulse-momentum theorem (Section 7.1), get the average force that this beam applies to the surface when (a) the surface is a mirror, so the momentum of every photon is reversed after reflection, and (b) the surface is black, so each photon is absorbed and there are no reflected photons.