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It has been said that RF consists of IR photons (here (now deleted; screenshot) however opinions seem to differ):

The smallest energy for photons is in the range of the infrared radiation. Longer wavelengths are related to modulated EM radiation called radio waves and micro waves.

Is the above statement correct? What the spectrum of these photons looks like and depends on? Does it change if the emitter is cooled (to near 0K perhaps) or heated? Can we measure those individual photons to say something about the emitter (and/or the reflector) that we cannot say based on the radio wave as a whole?

A popular answer, suitable for anyone with engineering background, is preferable.

Emilio Pisanty
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Eugene Ryabtsev
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1 Answers1

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We should start with the obligatory warning that a beam of EM radiation is not a swarm of photons. See What is the relation between electromagnetic wave and photon? for more on this.

With that out out of the way, there is no lower limit on the energy of a photon. Radio waves of arbitrarily low frequencies still gain and lose energy in units of one photon i.e. $h\nu$. For example the famous 21cm line is produced when a hydrogen atom transitions between two electronic states and emits a photon.

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John Rennie
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