# When/How does thermal radiation stop (if it stops) and conduction

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## Main Question or Discussion Point

Consider two solid objects in the vacuum (of different materials, if you will) at different temperatures approaching each other until they make "perfect contact" through flat surfaces (no gaps or defects, so that thermal contact conductance effects are absent, even though interfacial thermal resistance may still be present).
Of course, before making contact, they exchange heat via electromagnetic radiation. My question is: is this heat exchange via thermal radiation still present once the bodies are making contact with each other and exchanging heat via thermal conduction? (Please explain what's going on in terms of the constituent particles of the bodies.)

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When the two surfaces make contact the now combined object will quickly reach a temperature overall that is somewhere in between that of the originally separate objects.
(unless one of the objects is composed of material with extremely low thermal conductance.)
The combined object will still emit radiative heat assuming there are no other external heat sources to complicate things.

anorlunda
Mentor
I think what the OP is trying to ask is about IR radiation, even internally in an object. Certainly, an atom can emit a photon which is absorbed by a neighboring atom. But I suspect that is included in what we call heat conduction, or ignored when conduciton is active.

https://en.wikipedia.org/wiki/Thermal_conduction said:
In conduction, the heat flow is within and through the body itself. In contrast, in heat transfer by thermal radiation, the transfer is often between bodies, which may be separated spatially. Also possible is transfer of heat by a combination of conduction and thermal radiation.

To add to that, from the same Wikipedia article: "In solids, conduction is mediated by the combination of vibrations and collisions of molecules, of propagation and collisions of phonons ([sic], not photons), and of diffusion and collisions of free electrons." So, whoever wrote that did not consider exchanged photons as part of thermal conduction. However, I guess that when measuring thermal conductivity in the laboratory it's hard to isolate those effects, so that they are lumped all together and called "thermal conduction" (I emphasize I'm guessing).
Further information: when they talk about combined conduction-radiation in that Wikipedia article, I think they refer to semi-transparent media, in the sense of this article from thermopedia: Coupled (combined) radiation and conduction.

CWatters
Homework Helper
Gold Member
If the objects are transparent to IR I see no reason why thermal radiation from one to the other shouldn't occur as well as conduction.

I almost got the full answer from anorlunda above, when he/she said "Certainly, an atom can emit a photon which is absorbed by a neighboring atom." (as a mechanical engineer, I'm ignorant about physics at this level, so this phrase alone was good guidance.)
Let met share the answer I got from physics.stackexchange.com by user Thomas Lee Abshier ND. Here I include a fragment:
...as long as the absorption/emission frequencies of two substances overlap, radiation can emit and absorb between the two masses. In summary, "Will two materials continue to exchange radiation energy once they were in intimate thermal conductive contact." The answer is yes.​

256bits
Gold Member
If the objects are transparent to IR I see no reason why thermal radiation from one to the other shouldn't occur as well as conduction.
Out of curiosity, if a substance is transparent to a frequency, what are the chances that radiation of that frequency can be produced?

256bits
Gold Member
I almost got the full answer from anorlunda above, when he/she said "Certainly, an atom can emit a photon which is absorbed by a neighboring atom." (as a mechanical engineer, I'm ignorant about physics at this level, so this phrase alone was good guidance.)
Let met share the answer I got from physics.stackexchange.com by user Thomas Lee Abshier ND. Here I include a fragment:
...as long as the absorption/emission frequencies of two substances overlap, radiation can emit and absorb between the two masses. In summary, "Will two materials continue to exchange radiation energy once they were in intimate thermal conductive contact." The answer is yes.​
You are looking at depth of penetration of radiation, and that is substance and frequency dependent. As an example, lead is commonly designated as the material to use for absorption of ionizing radiation, rather than say wood. Gamma or x-ray will penetrate into lead measured in mm, while for concrete it is cm.

hilbert2
Gold Member
Most of the energy that is transferred from the interior of the sun to its surface is in the form of thermal radiation, despite the high density of solar matter. However, the average time it takes for a photon to diffuse from the core to the surface is quite long. So, it is possible to have radiative heat transfer inside condensed phase objects.

anorlunda
Mentor
@katelr, the way you worded your original post, it is not clear if you are asking about radiation heat transfer between two objects, or internal heat transfer within one object. In the OP, you stuck two together as if to make one.

based on https://en.wikipedia.org/wiki/Stefan–Boltzmann_law
$$j*= \sigma T^4$$
the amount of energy heat radiated on a (black) body is the temperature in kelvins to the power of 4.
my thoughts are that when 2 bodies are in contact, both conduction and (black body) radiation/absorption continues
the relative amount of energy/heat exchange would thus be mainly be determined by the temperature of the bodies.
at low temperatures conduction dominates, at high temperatures (black body) radiation dominates

@katelr, the way you worded your original post, it is not clear if you are asking about radiation heat transfer between two objects, or internal heat transfer within one object. In the OP, you stuck two together as if to make one.

I'm not sure where the lack of clarity is, so I'm going to talk about a practical implication: insulation of home attics. In modern practice it is customary to place, facing the ceiling, a foil of aluminium inside the attic to reflect the thermal radiation coming from the ceiling. Products like this are commonplace. Turns out that there happens to be some talking around the internet saying that the foil of aluminium should not touch the ceiling arguing that it would otherwise not reflect the radiation. (The fact that, once in contact with the ceiling, it will transfer its heat by conduction to the aluminium is a different problem we need not talk about.) So, my post comes from wondering why the reflection would stop once the bodies are in contact.

Out of curiosity, if a substance is transparent to a frequency, what are the chances that radiation of that frequency can be produced?
Actually asking or pointing out the physics? Anyhow, your implication is correct. Emissivity and absorption are two sides of the same coin. Low absorption at a given wavelength (highly transparent or highly reflective whether specular or scattering) means low emissivity at that wavelength, i.e. low thermal glow per Kirchoff’s law.

256bits
Gold Member
Actually asking or pointing out the physics? Anyhow, your implication is correct. Emissivity and absorption are two sides of the same coin. Low absorption at a given wavelength (highly transparent or highly reflective whether specular or scattering) means low emissivity at that wavelength, i.e. low thermal glow per Kirchoff’s law.
That is the way I understood.
Really I was actually pondering, or asking, if there ever was a gothcha !