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Blakut t1_jarffae wrote

Reply to comment by GypsyV3nom in Why does a Thorium gas mantle, when incandescent, emit more light in the visible spectrum than in infrared, when compared with a black body with the same temperature? by [deleted]

No it's not combustion without oxygen. Spectral line emission usually originates from diffuse gases and plasmas, which exist also in flames, i.e. combustion, but not from solids. It is in the gas form that an atom can be excited, and then deexcite by emission of a photon, with little collisions happening while in the excited state. A solid block of metal (or other material) has a continuous spectrum, resulting from the velocity distribution of atoms in its composition, which usually resembles the Planck profile. Gases (plasmas!) also have a continuous spectrum, but that's a different story. If it was like the other poster said, a hot rod made of copper would glow blue when heated, while in reality the glow color depends on temperature of the rod.

Why does a solid block of thorium, when heated, emit more in the visible spectrum than in the infrared when compared to a black body of the same temperature? Is this claim even accurate?

Nevermind, found the correct answer: glow is due to oxides and chemical reactions AROUND the solid block of metal. However, the wiki article lists this effect as distinct from the black body spectrum of the object, and leaves the impression that even without it, the solid metal would glow more than a black body at those temperatures.
"Candoluminescence is the light given off by certain materials at elevated temperatures (usually when exposed to a flame) that has an intensity at some wavelengths which can, through chemical action in flames, be higher than the blackbody emission expected from incandescence at the same temperature.[1] The phenomenon is notable in certain transition-metal and rare-earth oxide materials (ceramics) such as zinc oxide, cerium(IV) oxide and thorium dioxide."

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4a61756d65 t1_jarlw2n wrote

It's true! Googling spectral emissivity may give you extra info.

Thermal radiation (both in gases and solids!) can be extremely simplified to

  1. Electron gets excited, likely through atom/atom collision, either through literal collision in a gas or vibrations in a solid.
  2. Electron comes back down and emits a photon

If your material has a nice continuous energy band (something that doesn't happen in gases, but happens in metal), the electron can get excited and unexcited without many restrictions, so you get a plank type distribution. If the electron is restricted to belong to certain bands (like in gases and some solids) emission will have to be restricted to those bands.

In bulk materials with large optical density (such as solids) you have to account for reabsorption, which happens preferentially at the emission frequencies, and the fact that your system is out of equilibrium starts to matter a lot. You get a mess instead of clean bands, but you don't get black body either!

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Blakut t1_jarmjpp wrote

>Electron gets excited, likely through atom/atom collision, either through literal collision in a gas or vibrations in a solid.
>
>Electron comes back down and emits a photon

You can have thermal emission only from point charged particle, you don't need electron levels, no? I don't have experience with solids, but this was my understanding, that it is proportional to the distirbution of velocities of atoms in the object, much like the free-free emission is a continuous spectrum too. I understand that different objects have different emissivities, and that emission at different wavelengths is different, but why would that be the case outside electron transitions? Could a possible crystalline lattice play an effect in this, restricting movements of individual atoms?

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4a61756d65 t1_jarn8c0 wrote

Yeah, you can derive some forms of thermal emission from pretending electrons are classical point charges. That doesn't mean doing it will always explain the real world correctly! That's why we need quantum physics.

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Blakut t1_jarouro wrote

But the planck spectrum uses quantum physics... It cannot be explained classically.

edit: idk why i considered gases (which i thought i know) and solids (which i know i don't know) as so different. Kirchoffs law applies to solids too, so if a solid is a poor absorber at a wavelength, it must be a good emitter.

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4a61756d65 t1_jarq3pl wrote

Ah, you're right, but you're not talking too much about the emitter when you show plank, just about the EM field itself (and if you do it somewhat rigorously you at least mumble the words ergodicity/equidistribution, which fail in dilute gases with quantum electrons for example, so you get emission lines) I'm saying classical electrons are not enough to explain emission fully even in solids (that being said I don't know much/anything about the thorium case specifically)

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Blakut t1_jars76x wrote

no they are not, i was thinking of something completely different. The stuff i work with, astrohysical sources, usually have two kinds of spectra: continuum which can come from a few things, such as free free emission, synchrotron emission, thermal emission from dust, and spectral line emission. The thermal part of the spectrum is usually very close to the ideal black body and deviations happen moslty because of geometry of particles (such as dust) and other stuff along the way, so i can separate the continuum from the spectral part. This of course gave me the wrong impression that most objects (also on earth) must have a spectrum close to a black body, and deviate from that only slightly because of things "around" the emitting body, such as gases absorbing/emitting, without considering that of course, Kirchoffs law applies to solids and everyday objects too (i only studied this in relation to gasses and some line emission scenarios).

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