Everytime, I read about blackbody, I always get confused at the point where it is written
Under thermal equilibrium conditions , the blackbody radiation depends only on temperature. ..... . At equilibrium , electromagnetic waves bounce around with the walls of the cavity.
What is this equilibrium all about? Why is it important here?? If there is thermal equilibrium, why does the body radiate?? Please help.
Answer
The equilibrium mentioned in that quotation is between the radiation field and the walls of the container. The walls of the container are imagined to be held at some fixed temperature by some (unspecified) means. Under those conditions, the spectrum of the radiation in the cavity is that of a blackbody at the temperature of the walls. The radiation field and the walls exchange energy, but since they are in equilibrium the temperature of both remains the same. The walls radiate and absorb equally.
So far I haven't mentioned anything about radiation that leaves the cavity. In the ideal case of a closed cavity, the discussion is about the walls of the cavity and the field within. One could poke a small hole in the cavity, allowing some of the radiation to escape. In the limit that the hole is infinitesimally small, the disturbance caused by the hole is negligible, and the radiation that leaks out has very nearly a blackbody spectrum. This is a blackbody radiator. Usually the temperature of the blackbody is much higher than the surroundings into which the radiation radiates. Thus the blackbody radiator is not in thermal equilibrium with it's surroundings.
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