Question

Why is the radiation in a cavity as if it came from a "black body"?

Answer 1

A cavity can be treated as perfect absorber as whatever will come to it will completely pass it without being reflected at that point.The cavity was a reasonable way to model mathematically the electromagnetic fieldfor the radiation observed/measured from all bodies, it simplifies the mathematics, and the small hole allows for the radiation to come out and be measured. It should have the same spectrum as the one radiated from any part of the body. For thermodynamic considerations it makes no difference if the body is hollow or not. Equilibrium is equilibrium and should exist outside and inside. A small hole is the probe of what is happening inside.Suppose the cavity is held at a fixed temperature T and the radiation trapped inside the enclosure is at thermal equilibrium with the enclosure. The hole in the enclosure will allow some radiation to escape. If the hole is small, radiation passing in and out of the hole has negligible effect upon the equilibrium of the radiation inside the cavity. This escaping radiation will approximate black-body radiation that exhibits a distribution in energy characteristic of the temperature T and does not depend upon the properties of the cavity or the hole, at least for wavelengths smaller than the size of the hole