Re: [Phys-l] Blackbody radiation

["Bob LaMontagne" <rlamont@postoffice.providence.edu>]

It's kind of like finding the efficiency of a Carnot engine (relatively
easy) and then using that value for other reversible engines that may be too
complicated to easily calculate their efficiency. One can demonstrate that
both engines must have the same efficiency or else the 2nd Law of
Thermodynamics is violated.

A similar argument can be made for blackbody radiators - put any two in an
isolated space and let them come to equilibrium. They must both emit and
absorb the same at every wavelength or else their temperatures will change -
which would imply they are not really in equilibrium. The construction of
the device becomes irrelevant.

Bob at PC

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Michael Edmiston
> Sent: Wednesday, May 17, 2006 9:09 AM
> To: PHYS-L Maillist
> Subject: [Phys-l] Blackbody radiation
>
> 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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