Radiation, as far as I understand, is the transfer of energy through electromagnetic waves. The energy emitted from a hot body is known as thermal radiation. However, the accepted answer in this stack makes a distinction between thermal and coherent radiation.

What precisely is the difference characterizing the classification of the two kinds of radiation?


1 Answer 1


Coherent light usually implies it is also temporally coherent, meaning there is only one frequency in the electromagnetic radiation. Coherent radiation is usually emitted by a laser, so that the light is monochromatic, i.e. only one frequency.

Thermal radiation, by definition, contains multiple frequencies, distributed according to the Planck distribution.

  • $\begingroup$ In this article they say that thermal radiation describes heat transferred. So is thermal radiation about the electromagnetic waves or the heat contained in electromagnetic waves? $\endgroup$ Commented Oct 5, 2020 at 8:15
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    $\begingroup$ Thermal radiation is the EM waves that a body emits just because of its temperature. In perfect thermal equilibrium, the spectrum is perfectly Planck distributed. The body may be emitting waves because of things other than temperature. In that article they talk about remote controls, which use infrared radiation to do other stuff. So that radiation does not come from the remote's temperature, but from the small laser (or whatever) inside of it that is generating that extra infrared. $\endgroup$ Commented Oct 5, 2020 at 8:22
  • $\begingroup$ "The body may be emitting waves because of things other than temperature. ".. what are these other ways? $\endgroup$ Commented Oct 5, 2020 at 8:37
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    $\begingroup$ Well if it's a laser, there are processes inside that generate photons, whose frequency is determined by the cavity length. If it's a WiFi router, there's a circuit designed to emit that type of radiation... etc. $\endgroup$ Commented Oct 5, 2020 at 8:41

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