1. The problem statement, all variables and given/known data Show that the average number of photons per unit volume in the cavity for a temperature T is given by n=( I / Pi^2 . (kT/c(h-bar))^3 ) where I = integral from infinity to 0. dx x^2 ((exp (x)-1))^-1 = 2.404 2. Relevant equations You may assume the mean number of photons occupying a state at energy (strange looking e)= hbar omega when the radiation has a temperature T is <N>=1 over exp (strange looking e/kT)-1 You may also assume that the number of available states in the angular frequency rage omega to omega plus delta omega in a cavity of volume V is given by g(omega)d-omega = V/c^3 . omega^2 / pi^2 d-omega 3. The attempt at a solution I know I need to do N/V however I have no idea how available states are useful to me, perhaps I could rearrange for V, but then I dont' know what g(omega) can be used for.