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Plank curves and emission/absorbtion spectra

by goldsax
Tags: curves, plank, spectra
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goldsax
#1
Apr5-12, 03:34 AM
P: 51
i understand that a good black body would produce a plank curve.
it is my understanding that plank curves are continuous emmision spectra..
now the sun a good approximation to a black body.... but we get and emission/absorbtion spectra..
can you please help me understand where i am going wrong..cheers
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Drakkith
#2
Apr5-12, 04:32 PM
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The emission and absorbtion lines are due to atoms and molecules in the outer atmosphere of the Sun absorbing or emitting light at specific wavelengths. The outer areas of the Sun are cool enough to allow atoms and molecules to form.
Ken G
#3
Apr5-12, 10:06 PM
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So the Sun is not a perfect blackbody. To the extent that it approximates a good blackbody, we get a continuous Planck spectrum, but when we include the details of lines that form in cooler overlying layers, we get absorption lines. This is handy-- the closeness to a blackbody means we will have a useful concept of temperature, and those deviations made by the spectral lines will give us additional information about the surface layers (especially the strength of the gravity) that we would have no other way to infer.

goldsax
#4
Apr6-12, 04:16 AM
P: 51
Plank curves and emission/absorbtion spectra

interesting point about the gravity analysis..
i think i am digging a hole for myself here..
am i to understand that the emission spectrum only arises due to the 'cooler' outer layers because the sun is not a perfect black body?
so does this mean that no body would produce a continuous spectrum?
so would there be emission/absorption lines in ALL spectral analysis?
Ken G
#5
Apr6-12, 01:42 PM
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Quote Quote by goldsax View Post
am i to understand that the emission spectrum only arises due to the 'cooler' outer layers because the sun is not a perfect black body?
Yes, though I would have said that about the absorption spectrum, just to be clear which aspects of the spectrum we are worrying about.
so does this mean that no body would produce a continuous spectrum?
To the extent that a perfect blackbody is impossible, yes, though sometimes we are remarkably close (a good example is the cosmic microwave background).
so would there be emission/absorption lines in ALL spectral analysis?
In principle, yes, though in practice we might not be able to tell above the noise (as with the cosmic microwave background).
mfb
#6
Apr7-12, 07:52 AM
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Quote Quote by goldsax View Post
so does this mean that no body would produce a continuous spectrum?
A black hole might be a perfect black body - but for usual sizes of them, their temperature is so low that it is impossible to detect their Hawking radiation.


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