How is infrared light created?

Question

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

Answer 1

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.