Thermal radiation: what is it?

We qualify as thermal rays the amount of energy that a body whose temperature differs from absolute zero yields in the form of radiation of electromagnetic nature and wavelength between 0.1 and 100 micrometers.

Thermal radiation and heat

Thermal radiation is, along with convection and conduction, one of the physical phenomena which authorize the transfer of heat. Its special feature: it does not require the presence of an intermediate material medium and can therefore transport heat through a vacuum. Thus, it is thanks to thermal radiation that the Sun transmits its heat to us, almost instantly.

Radiation as a function of temperature

Thermal radiation results from the transition of electrons between two states ofenergy. According to the law of Planck, the passage from an energy level to a lower energy level is accompanied by theepisode of radiation.

Thus, a body placed at a given temperature emits electromagnetic waves of frequencies different. The distribution of these frequencies – or the spectrum of the radiation – and the amount of energy emitted depend on the temperature of the body. The higher this is, the more intense the thermal radiation is at short wavelengths. A body which, for a given temperature, emits the maximum amount of energy is called a “black body”.