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Some of the wavelengths of light that are emitted from the sun will be absorbed by atoms in the outer layer of the sun and also the atmosphere of the sun, and we see this as absorption lines in the spectrum. Now, this absorbed radiation will indeed be re-emitted again, so one might think that these emission lines should "cancel out" the absorption lines. The usual explanation for why this doesn't happen is that the re-emitted light is radiated in all directions, not just towards us, meaning that to us these wavelengths will be much fainter than the other wavelengths.

But the problem I have is that this happens all around the sun (since the atmosphere is completely surrounding it), and intuitively it seems then that all of this re-emitted light should combine such that far away it would appear that the sun is radiating these wavelengths just the same as it is radiating all the other wavelengths. And if that is true, then we shouldn't see absorption lines in the spectrum.

So what is it that I am missing?

Dale
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User3141
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  • Answered at https://physics.stackexchange.com/questions/327960/why-dont-absorption-and-emission-lines-cancel-out-in-our-sun – ProfRob Dec 12 '20 at 08:52

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Although your analogy seems reasonable, if you think about it the absorption patterns will still be there. You are right that the absorbed wavelengths will be randomly emitted from the whole surface but those emitted photons are still a very small percentage compared to the photons emitted from the interior of the sun.

Bill Alsept
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When these wavelengths are absorbed the energy is “thermalized”. That means that it is distributed among many internal degrees of freedom. Thus it is not necessarily re-emitted at the same wavelength it was when it was absorbed, but rather it is re-emitted at random wavelengths according to the black body spectrum. So in the end you have specific wavelengths being absorbed but black body radiation being emitted. This leads to the observed absorption spectrum.

Dale
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  • I’m not sure about that. The elements in the suns atmosphere exhort radiation and emit specific wavelengths. It’s those specific wavelengths that are missing in the spectrum that the OP is talking about. They’re not so much missing they’re just less noticeable because they are radiated in every direction. – Bill Alsept Sep 13 '20 at 16:44
  • The point is that those elements are not isolated. They interact with the sun and the other elements of the solar atmosphere, particularly thermally. – Dale Sep 13 '20 at 17:20
  • @Dale Thanks for the answer, Dale. But if this is true, then why is everyone talking about the radiation being emitted in every direction? Perhaps the reason for the absorption spectrum is a combination of what you said and what they say? – User3141 Sep 14 '20 at 07:17
  • @FelisSuper certainly, they are closely connected. Thermalization takes time for additional interactions and so forth, and less time is needed the more interactions there are. If the radiation is emitted back towards the sun then there is much more time for thermalization. If it is emitted away from the sun then there is less time and less probability for further interactions. Anyway, it seemed that you already understood the directions thing, so I focused on the thermalization, which explains why it is not eventually re-emitted at the same wavelength and which seemed new to you – Dale Sep 14 '20 at 12:28
  • @Dale Thanks for your help again. This has got pretty complicated now, as I asked this same question on the astronomy forum, and here another explanation is given. But anyways, I don't understand how this can be a primary reason. If another wavelength is emitted, then surely that wavelength must correspond to some transition between two energy levels. So what if a hydrogen atom absorbs a photon correspondibg to a transition from the n=1 ro the n=2 state. Then there is no wavelength but the one absorbed to re-emit. – User3141 Sep 14 '20 at 17:06
  • @FelisSuper even in that case the energy can still be thermalized before it leaves the sun. The excited hydrogen atom can collide with another atom and transfer energy that way. Or a photon corresponding to the transition can be emitted and then scatter off an electron. Or it can be absorbed by a different molecule. Or ... The point is that many different things can happen before it leaves the sun, and only one of those things is a re-emission at the same wavelength followed by an escape from the sun. – Dale Sep 14 '20 at 17:45
  • Because of the existence of those other possibilities the spectrum is decreased at that line. – Dale Sep 14 '20 at 22:40