I have read the Wikipedia article on how a cavity can represent a black body radiator but it has left me with 3 main questions.
It says that the radiation inside the cavity is in thermal equilibrium with the walls of the cavity implying that the photons have a temperature but I don't understand how this can be given that they have no mass.
It states that it is the actual hole in the box and not the cavity that is the black body and that a black body is a perfect absorber and emitter. Surely then if the only radiation entering the cavity comes from a single beam of light passing through the hole then all of this radiation must come out of the hole an not from the walls of the box which is very unlikely.
It states that at a constant temperature this spectra will be produced but what decides the temperature, if it is an external source would this not alter the equilibrium inside the cavity. If the fixed temperature is a result of the source of radiation entering through the hole then isn't the equilibrium being formed between the source and the cavity walls rather than the centre if the cavity and the walls.
This probably doesn't make a lot of sense but I would appreciate any answers :)