PHYS 445 Lecture 26  Blackbody radiation I
26  1
© 2001 by David Boal, Simon Fraser University.
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Lecture 26  Blackbody radiation I
What's Important:
•
number density
•
energy density
Text
: Reif
In our previous discussion of photons, we established that the mean number of
photons with energy
i
is
n
i
=
1
e
ß
i

1
(26.1)
Here, the questions that we want to address about photons are:
•
what is their number density
•
what is their energy density
•
what is their pressure?
Number density
n
Eq. (26.1) is written in the language of discrete states.
The first thing we need to do is
replace the sum by an integral over photon momentum states:
Σ
i
→
∫
d
3
p
Of course, it isn't quite this simple because the densityofstates issue that we
introduced before: for every spin state there is one phase space state for every
h
3
, so
the proper replacement more like
Σ
i
→
(1/
h
3
)
∫
d
3
p
∫
d
3
r
The units now work: the left hand side is a number, and so is the right hand side.
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 Spring '08
 DavidBoal
 Physics, Energy, Force, Photon, Radiation, Fundamental physics concepts, Energy density

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