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Old Apr 2nd 2013, 01:38 AM   #1
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Dissipation versus scattering

Hey, was wondering if someone here might confirm or correct me on this.

Absorption spectra. If I point a spectrograph at a clear sky, the way I understand it, the missing wavelengths are due to those photon being absorbed by the molecules in the atmosphere after which the energy is dissipated by collisions with other molecules? And that this process is something separate from the scattering phenomena which turns the daytime sky blue? So, in other words, if our atmosphere had been stock full of molecules absorbing across the blue wavelengths, the sky wouldn't be blue since those portions of the spectra would be dissipated?
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Old Apr 2nd 2013, 08:36 AM   #2
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Originally Posted by Devlan View Post
Hey, was wondering if someone here might confirm or correct me on this.

Absorption spectra. If I point a spectrograph at a clear sky, the way I understand it, the missing wavelengths are due to those photon being absorbed by the molecules in the atmosphere after which the energy is dissipated by collisions with other molecules? And that this process is something separate from the scattering phenomena which turns the daytime sky blue? So, in other words, if our atmosphere had been stock full of molecules absorbing across the blue wavelengths, the sky wouldn't be blue since those portions of the spectra would be dissipated?
Unless you point at the Sun, which is the source of the light, the only photons you will detect are those which are scattered in the atmosphere - predominantly blue.

Perhaps you are mixing this up with the statement that absorption lines in the solar spectrum are the result of absorption in the atmosphere of the Sun.
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Old Apr 2nd 2013, 09:18 AM   #3
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Oh, sorry, yea - it was sloppy writing more than misunderstanding, I believe. Of course I meant aiming it directly at the sun, so that what you'd measure is the spectrum minus whatever gets absorbed on the way, be it helium in the sun's atmosphere or nitrogen in Earth's.
To rephrase a little:
What I'm curious about is whether the particular photons missing where we are, measuring the direct beam, turn up somewhere else or whether their energy is dissipated. So if we have a batch of 955.5 photons coming in with the direct solar beam, and they get absorbed by some nitrogen in our atmosphere so we only measure a gap at their position in the spectrum where we are - are they scattered and turn up somewhere else or are they dissipated before that?
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Old Apr 2nd 2013, 10:33 AM   #4
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Originally Posted by Devlan View Post
To rephrase a little:
What I'm curious about is whether the particular photons missing where we are, measuring the direct beam, turn up somewhere else or whether their energy is dissipated. So if we have a batch of 955.5 photons coming in with the direct solar beam, and they get absorbed by some nitrogen in our atmosphere so we only measure a gap at their position in the spectrum where we are - are they scattered and turn up somewhere else or are they dissipated before that?
It is possible that after the atom is excited or ionized by absorption of the photon, it can be deexcited by emitting a photon of the same energy. It is much more likely that it will deexcite in a number of smaller steps - that is, by capture of a slower electron and emission of one or more photons of longer wavelength. The electron ejected in ionization will thermalize, contributing heat to the atmosphere.
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