It is know fact that a cavity with a hole is a perfect black body. When electromagnetic waves of any frequency are incident on the hole, it is completely absorbed after a number of reflections. So, the condition for absorptivity=1 is satisfied as required for it to be a perfect black body. But it is said that the inner walls of the cavity should be maintained at constant temperature. What is the purpose of having this? Does it act as a source of radiation for obtaining a continuous spectrum?
QUOTE (plasma+Jul 6 2008, 03:45 PM)
So, the condition for absorptivity=1 is satisfied as required for it to be a perfect black body. But it is said that the inner walls of the cavity should be maintained at constant temperature. What is the purpose of having this? Does it act as a source of radiation for obtaining a continuous spectrum?
Plasma;
Yes, in order to be a perfect blackbody, it must not only be a perfect 'absorber' but also be a 'perfect' emitter of radiation.
Stefan-Boltzmann's law states that the rate that a body emits radiation (per unit area), E, is directly proportional to the 4th power of the body's absolute temperature, T.
E = (sigma)T^4
Change in wall temperature is therefore a reflecton of the fact that emission and absorption rates are different.
JW
Plasma;
Yes, in order to be a perfect blackbody, it must not only be a perfect 'absorber' but also be a 'perfect' emitter of radiation.
Stefan-Boltzmann's law states that the rate that a body emits radiation (per unit area), E, is directly proportional to the 4th power of the body's absolute temperature, T.
E = (sigma)T^4
Change in wall temperature is therefore a reflecton of the fact that emission and absorption rates are different.
JW
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