I have a very rudimentary understanding of electromagnetic radiation and how it corresponds to temperature.

It is my understanding that any object above absolute zero first starts emitting radiation in infrared, and then as its temperature increases, it starts emitting in visible light from red to orange to white to blue and so on.

But why does it start at infrared? Why not start at radio waves or microwaves first?

  • 2
    $\begingroup$ They do emit in radio and microwaves too. Planck's law tells you the amount of energy that is radiated by a black body at temperature $T$ and frequency $\nu$. $\endgroup$ – Stratiev Jun 10 '20 at 12:32
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    $\begingroup$ ...what @Stratiev said, but note: In order for the peak frequency of the emission spectrum to be down in the "radio" waves, the temperature of the body must be extremely cold—like, single-digit cold on the Kelvin scale. And, at those temperatures, the total amount of power radiate will be extremely small. $\endgroup$ – Solomon Slow Jun 10 '20 at 14:37
  • $\begingroup$ @Stratiev That appears to be an answer, not a comment. Please consider making it one. $\endgroup$ – ACuriousMind Jun 10 '20 at 17:23
  • $\begingroup$ @SolomonSlow Please also consider answering questions in answers, not comments. $\endgroup$ – ACuriousMind Jun 10 '20 at 17:26
  • $\begingroup$ @SolomonSlow Reading your commentary was the starting point for me to answer the question. I hope it does not hurt you as others think it does. Anyway I’m curious to see your answer $\endgroup$ – HolgerFiedler Jun 10 '20 at 17:59

The simple answer is that your understanding of this phenomenon is incorrect.

enter image description here (From Wikipedia)

Planck's law tells us that objects in thermal equilibrium at non-zero temperature emit electromagnetic radiation at all wavelengths, but their peak emission occurs at $$\lambda_{peak} = \frac{2.898 \times 10^{-3}\text{m}\cdot\text{K}}{T}$$ where $T$ is the temperature. enter image description here (From Britannica)

A peak wavelength of $10^{-5}$ m (which is in the infrared range) corresponds to a temperature of $290$ K. That's why objects at or around the average surface temperature of the Earth have peak wavelengths in the infrared. However, an object at $0.1$ K corresponds to a peak wavelength of about 3 cm, which is comfortably in the microwave regime.


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