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Unformatted text preview: Since wavelength is easier to measure for light than frequency, one more
frequently sees B and I in observational contexts. A graph of B(T ) shows
that hotter T 's indeed give both overall greater intensity everywhere, and
peak at shorter wavelengths. Bλ (T) 2000 K 1750 K
1500 K
1250 K 0 1 2 3
4
λ in µm 5 λ 3.5.3 Asymptotics of the Planck spectrum
For h kT , there is an exponential fallo in brightness. This is called the
\Wien limit." It is why you cannot get a tan under an ordinary incandescent
lightbulb: The temperature of its lament is around 2700 K, and so it makes
almost no UV.
For h kT the exponential can be expanded
h
h
exp kT 1 = kT +
so that for hv kT , we have the RayleighJeans law:
2
B (T ) 2c2 kT :
! 45...
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This note was uploaded on 12/29/2011 for the course AST 350 taught by Professor Dion during the Fall '09 term at SUNY Stony Brook.
 Fall '09
 dion

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