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My initial guess is yes there is a blackbody spectrum of photons inside a solid.

The process that emits thermal photons won't "know" that it is deep inside a solid and not near the surface, so the emission of thermal photons would be the same. They don't get very far.

So I'd like to ask: Is there a blackbody spectrum of photons inside a solid?

Let's assume that "inside" means in the center of a body with a temperature $T$ and that the body is at least several absorption lengths in size for photons near $k_BT$. Let's also assume the self-absorption is not so overwhelmingly strong that it significantly alters the emission process itself.

I believe that the answer will be the same for dielectrics, semiconductors and metals, but if the answer differs for these please mention it.

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    $\begingroup$ Also would be worth thinking about the reflectivity, some photons are evanescent not because of absorption but reflection. $\endgroup$ – KF Gauss May 13 '19 at 0:00
  • $\begingroup$ You understand that most solids are not black bodies, yes? $\endgroup$ – DanielSank May 13 '19 at 5:52
  • $\begingroup$ @DanielSank yes indeed, which is why I've constrained the problem the way I have in terms of the medium being strongly self-absorbing. If you'd like to add an answer that addresses separately the shape of the spectrum from it's integrated power and mention emissivity, please do so. $\endgroup$ – uhoh May 13 '19 at 6:14
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In steady state condition (same temperature across the volume) situation is indeed slightly different. There is a continuous black-body emission spectra in both cases, but I cannot tell if they are absolutely identical.

Photons that are emitted deep inside - indeed don't know they are deep, and follow the same rules.

Main difference is that atom in the middle of the solid has vibrating lattice all around it, and hence see more field disturbances all around. Atom at the edge of the material - does not feel vibrating lattice from once side. Still spectra is continuous in both cases.

If material has some transparency windows - emission from deep levels will partly reach the surface and will change overall emission spectra.

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    $\begingroup$ I'm pretty sure this is the beginning of the right answer, but I wish it linked to some published discussion or relied on some fundamental physics. If you ever take a break from overclocking wrist watches and photographing ducks, it would be great if you could go a little deeper with this answer. Thanks! ;-) $\endgroup$ – uhoh May 19 '19 at 13:25

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