1. The problem statement, all variables and given/known data I know how to derive the density of states for an ideal gas by using the energy equation: E_n = A*n^2, where A = (h_bar^2*pi^2)/(2mL^2) but what about for a 'photon gas'? Do I use the same energy equation as above, or the following: E_n = (h_bar*pi*c/L)*n 2. Relevant equations 3. The attempt at a solution I think I'd use the second equation, but solutions to the assignment said use the first equation. I'm confused!