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I’ve recently been scouring my Finnish high school level textbooks and online physics forums for an answer to the following question: ”How is thermal radiation created on a molecular level?” or ”What thermal radiation actually is?” In some places I hear it being described as the motion of charged particles which leads to radiation, and in other places I’ve heard it being explained as the atoms getting excited by the heat. I’m not sure whether either of these are correct, whether they are synonymous nor whether they contradict one another.

Help would be much appreciated, I am high school level in physics but would still like a ’deeper’ understanding on this subject. Thank you in advance.

Max123456789
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Thermal radiation is radiation (photons) that is emitted by matter, where the energy or frequency spectrum is characterized by the temperature of the matter.

The temperature of the matter will determine the relative number of atoms and molecules in their various energy levels, the amount of ionisation, the distribution of speeds of atoms and molecules in a gas or the frequency spectrum of vibrational modes in a solid.

Thermal radiation can take many forms. It can be in the form of sharp, discrete spectral lines arising from the emission of photons corresponding to transitions between energy states (e.g. vibrational and rotational) in a molecule or due to transitions between discrete energy levels in an atom. That is, molecules are able to rotate or vibrate, such that they have rotational and vibrational energies that take on discrete values. How many molecules are in which energy states is determined by the temperature. Molecules more frequently occupy higher energy states at higher temperatures. They may transition to a lower energy state by emitting photons with energies corresponding to the energy difference. Similarly, electrons in atoms occupy discrete energy levels and can move between them by absorbing or emitting photons of the right energies. Higher temperatures mean more atoms in higher ("excited") energy states.

But thermal radiation can also be a continuous spectrum of frequencies, for example light emitted as a result of free electrons combining with ions; the acceleration of free electrons by ions in a hot gas; or the overlapping emission from many, closely spaced vibrational states in a solid.

ProfRob
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  • What do you mean when you say ’vibrational or rotational’ in terms of transitions between the energy states of, say, an atom? – Max123456789 Dec 03 '20 at 00:36
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    @Max123456789 I didn't say that. Molecules have vibrational and rotational states. Atoms don't. They have different energy states defined by their electrons. – ProfRob Dec 03 '20 at 00:39
  • Oh, sorry for misrepresenting your answer, wasn’t intentional. Then I jusr don’t understand what you mean by rotational or vibrational states of a molecule. Could you help me by explaining briefly what they are and how they relate to thermal radiation exactly? – Max123456789 Dec 03 '20 at 00:41
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How is thermal radiation created on a molecular level?” or ”What thermal radiation actually is?

Thermal radiation is nothing more than photons. Sometimes it is photons with an energy corresponding to visible light (we can see that - like when a hot piece of metal glows red) and sometimes it is at non-visible wavelengths (like when we stand next to something warm, but apparently normal looking but can feel heat radiated from it).

So where do these photons come from ?

They are released as atoms and molecules release energy and the only way they can do that is to radiate a photon. Typically this will be from an electron which is at some energy level in a substance moving to a lower energy level. The photon is that "packet" of energy.

StephenG - Help Ukraine
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  • Ok, thank you. But what does wikipedia mean when it says the following: ”Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter.” – Max123456789 Dec 02 '20 at 23:45
  • EM radiation is (surprise) photons. Thermal radiation is a term used when the origin of the EM radiation is thermal in nature and this is often described as particle motion (because this is what classical physics thinks in terms of, whereas quantum theory is more complex and "motion" is not quite the right mental picture in quantum theory). When we heat something up it often does move faster (e.g. air molecules in a pressure cooker). Hence "motion". – StephenG - Help Ukraine Dec 03 '20 at 00:04
  • This could be improved by discussing the difference between discrete spectra and continuous. A hot tube of neon gas produces photons only at very specific frequencies corresponding to electron transitions, so why does a hot bar of iron produce a continuous spectrum? – Paul T. Dec 03 '20 at 00:16
  • I’ve also just been wondering about that same question. Doesn’t it have to do with the energy levels becoming ’staggered’ in solids due to the atoms being binded so close together while in a gas we can treat these atoms as individual systems with their own energy levels? More exact information on this would be appreciated. – Max123456789 Dec 03 '20 at 00:19
  • @PaulT. This would be more appropriate as a separate question - we like questions (and answers) as tightly focused as possible on Physics SE. I suspect you'll find an answer to this already if you search the site, but I could be wrong. – StephenG - Help Ukraine Dec 03 '20 at 00:23
  • @Max123456789 Please see my comment above in response to PaulT – StephenG - Help Ukraine Dec 03 '20 at 00:24
  • I understand your comment but I don’t see that there is a problem in discussing it here as it is closely related to the original question if not even critical to actually understanding thermal radiation. – Max123456789 Dec 03 '20 at 00:29