Infrared radiation is electromagnetic in nature and to have a wavelength rather longer than that of visible light, extending from about 750 nm to some 400 000 nm (400 mm, 0.4 mm). All bodies emit radiation, the intensity and wavelength distribution depending on the nature of the body itself and its temperature. Infrared radiation is invisible to the human eye but is used in Earth resource satellites, by the military in night glasses, for spotting areas of high heat loss from buildings and for detecting hot spots in power cables.

Prévost suggested that all bodies radiate energy, but that those with a higher temperature radiate more energy than those at lower temperatures.

An ideal absorber is one that absorbs all the radiation that falls on it and also emits the maximum amount of radiation possible for that area at that temperature. Such a body is known as a black body and the radiation emitted by it as black body radiation.