Re: [Phys-l] Blackbody radiation

[Roger Haar <haar@physics.arizona.edu>]

Hi,
    
	If I remember correctly, one can argue that all
blackbodies produce the same radiation distibution
based on thermodynamics and detail balance. A
cavity is just a simple and useful case to study
theoretically.  Once the distribution is solved
for a cavity, it must be the same for all
blackbodies.  The quantized atomic structure gets
blurred into conduction bands in a conductive
solid. ( In klystrons, one could think in terms of
phonons being emitted and absorbed.)  I believe it
was known in Planck's time that a heat solid
insulator did not produce a perfect black body
radiation distribution, but cavities were the best
way to produce this distribution.  I would assume
that someone tested different cavity sizes and
shapes, finding them all just as good.

Sincerely,
 Roger Haar
 
*********************************************
Michael Edmiston wrote:
> After my son asked me some questions about blackbody radiation I realize
> I have questions of my own about the usual textbook treatment of BBR.
>
> Basically I don't understand the standing wave approach to BBR.  I know
> it is historic, but the historic approach doesn't yield the correct
> result.  Although I understand standing waves in excited cavities such
> as klystron tubes and magnetrons, using standing waves for a heated
> cavity doesn't seem right once we know about quantized atomic structure
> and emission of photons from atomic transitions.
>
> First of all, hot objects that are not cavities will emit radiation that
> is nearly BBR.  That is, the cavity is not really necessary for getting
> the basic BBR curves.  The sun is not a cavity radiator.  So what do
> standing waves have to do with BBR from hot objects that don't have
> cavities?
>
> Second, the idea that the cavity walls must be electric field nodes
> doesn't seem quite right if the source of the photons is electronic
> transistions in the atoms on the surface of the walls.
>
> Modern physics textbooks on my shelf explain the historic standing wave
> approach leading to the Rayleigh-Jeans result, but then continue using
> the standing wave approach when switvhing over to the Planck quantum
> picture.  That doesn't make sense to me.  Am I missing something?
>
> Michael D. Edmiston, Ph.D.
> Professor of Physics and Chemistry
> Bluffton University
> Bluffton, OH 45817
> (419)-358-3270
> edmiston@bluffton.edu
>
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