What is thermal radiation on a molecular level?

Question

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.

Answer 1

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.

Answer 2

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.