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Black body radiation

by Amith2006
Tags: black, body, radiation
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Amith2006
#1
Jul6-08, 03:43 AM
P: 423
# Hi guys! One question is troubling me a lot. It is know fact that a hollow metallic enclosure with a small 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?
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per.sundqvist
#2
Jul6-08, 04:07 AM
P: 111
Quote Quote by Amith2006 View Post
# Hi guys! One question is troubling me a lot. It is know fact that a hollow metallic enclosure with a small 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?
Its not nessesary from the black body meaning. In this simple picture of the hollow metallic enclosure I remember it was the fact that the particle is reflected "chaotic" so it never finds its way back out of the hole again, independent of what the temperature is in there.

But if you keep T constant you could use Boze-Einstein statistics for the photons and you could derive the classic properties of black-bodies, like Wien's displacement law etc. If you have a temperature gradient there you would get a perturbation of the BE-distribution and you would have some kind of drift of photons inside the body.
Andy Resnick
#3
Jul6-08, 09:22 PM
Sci Advisor
P: 5,513
Quote Quote by Amith2006 View Post
# Hi guys! One question is troubling me a lot. It is know fact that a hollow metallic enclosure with a small 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?
I don't think it's required that the black body be maintained at a constant temperature in order for it to be a well-defined thermodynamic object. I think the "requirement" for constant temperature is required for discussions about "equilibrium" (i.e. the equilibrium spectrum of a photon gas). And of course, it's useful for a metrology standard.


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