EXERCISE 7.6 A cubic box of volume V = L contains electromagnetic energy (photons) in equilibrium with the walls at temperature T (black-body variation). The allowed photon energies are determined by the standing waves formed by the electromagnetic field in the box. The photon energies are hw;=hck, where k; is the wavevector of the ith standing wave. Compute the pressure of this photon gas. (Note that since photons have no mass, the gas has chemical potential, ' = 0.)