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All mass with a temperature creates infrared light, but what is the cause of this? How is it created?

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marked as duplicate by John Rennie, GiorgioP, JMac, Kyle Kanos, M. Enns Apr 29 at 18:01

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Matter consists of multiple atoms. Temperature is a measure for how fast these atoms are moving on average:

$$v \propto \sqrt{\frac{k_B T}{m}}$$

With the Boltzmann constant $k_b$, temperature $T$ and mass $m$.

Every atom is surrounded by electron orbitals that have a discrete angular momentum. If energy is transmitted to one electron e.g. via collision it can move to a higher orbital (excitation) or even be ionized. This leaves a hole in the former spot, which will be reoccupied by an electron from a higher level. Because it takes energy to move an electron to higher levels, energy gets released once the lower level is reoccupied. The energy gets released as a photon with frequency $f$ which is correlated to the energy via $E=h f$, with Planck constant $h$.

For the right energy differences between orbitals this leads to infrared radiation. The energy needed to excite an electron comes from the collisions from the atoms inside matter.

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    $\begingroup$ What happens if you just have one atom? How does it it emit thermal radiation? $\endgroup$ – boyfarrell Apr 29 at 9:26
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    $\begingroup$ It doesn't. One atom is not a thermal medium $\endgroup$ – infinitezero Apr 29 at 9:30
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    $\begingroup$ It’s in detailed balance with it environment at thermal equilibrium; it emits at the same the rate it absorbs. So a single atom must still emit thermal radiation. To be clear I am thinking of a single atom interacting with photon gas of a finite temperature. $\endgroup$ – boyfarrell Apr 29 at 9:32
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    $\begingroup$ If it has an environment there is more than a single atom. The energy required can not only come from collisions but also from radiation. So an atom can absorb an earlier emitted photon which is exactly the energy needed to excite one of its electrons. $\endgroup$ – infinitezero Apr 29 at 9:38
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    $\begingroup$ So , I assume that matter with enough heat will produce visible light instead, but does that mean that cold enough matter will create microwaves or radio waves? $\endgroup$ – Melvin Apr 29 at 9:43

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