A black body is one that
- A. reflects all radiation falling on it
- B. absorbs all radiation falling on it and is also the best possible emitter
- C. emits no radiation at all
- D. is black in colour at all temperatures
Explanation
A perfect absorber must also be a perfect emitter at the same temperature, otherwise it could not stay in thermal equilibrium with its surroundings. The spectrum of the radiation it emits depends only on temperature, which is why the concept is so useful for stars. A small hole in a hollow cavity is the standard practical approximation.
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