Re: [Phys-l] Blackbody radiation

["Bob Sciamanda" <trebor@winbeam.com>]

The text "Modern Physics" by Serway et al derives as follows:
"We shall now derive Planck's blackbody formula by quantizing the standing 
electromagnetic waves in the cavity.  Instead of Rayleigh's assumption that 
a standing wave can have any energy from 0 to infinity, we postulate that a 
standing wave of frequency f can have have only discrete energies E = nhf . 
. . " pg 52 of the 1989 first edition.

The cavity is still a valid BB, but the energy of each mode is quantized to 
nhf, n= an integer.

( In the above quote from Serway, I would replace "standing wave" by 
"standing wave mode".)

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message ----- 
From: "Edmiston, Mike" <edmiston@bluffton.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, May 17, 2006 12:00 PM
Subject: Re: [Phys-l] Blackbody radiation


> Bob Sciamanda said, "I think the argument is from thermodynamic
> principles and asserts that the energy spectrum of a black body in
> equilibrium with a definite temperature environment should be
> independent of the details of its structure.  Thus in calculating the
> universal BB spectrum function dE(lamda) one is free to choose any valid
> BB construction for the calculational model."
>
> Okay, that agrees with my thinking.  The problem is that the textbooks
> have just finished following the Rayliegh derivation (standing waves) in
> order to show it leads to the "ultraviolet catastrophe."  Then when they
> switch to the Planck derivation, they continue to use the "cavity with
> standing waves model."  I think this model is not necessary, but
> students get confused and think that standing waves are a necessary
> component of the quantum explanation of BBR.  That, in fact, was how my
> son posed the question to me.  He said, "I don't see what standing waves
> have to do with it.  Can you explain that to me?"  And I couldn't.
>
>
> Michael D. Edmiston, Ph.D.
> Professor of Chemistry and Physics
> Bluffton University
> Bluffton, OH 45817
> (419)-358-3270
> edmiston@bluffton.edu
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
>
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.392 / Virus Database: 268.5.6/338 - Release Date: 5/12/2006