We call a collection of photons a "photon gas". The term "gas" will not only refer to the traditional understanding of air, but will refer to any mobile conglomerate of a particle. You may hear of an "electron gas" or the like in your study of thermodynamics.

We can derive the entropy of the blackbody photons to be σ = ()3.

The reason that this topic is included in thermodynamics is that it describes thermal radiation, and the results can be derived using some of the equations we have recently developed, along with some quantum mechanics.