In physics a blackbody is an object that absorbs all light that falls onto it (and thus reflects none). Despite the name, blackbodies do radiate light, but they just don't reflect any. The spectrum (amount of light emitted at each wavelength) of a blackbody is very characteristic, and depends entirely on its temperature.
- The light emitted by a blackbody is called blackbody radiation.
- The differences between an object's spectrum and that of an idealized blackbody are sufficient to allow one to determine the chemical composition of the object.
The term "black body" was introduced by Gustav Kirchhoff in 1862. The spectrum of a blackbody was first derived by Max Planck, who had to assume that electromagnetic radiation could propagate only on discrete packets, or quanta.
The intensity of radiation from a blackbody at temperature T is given by the Planck's Law of Radiation:
In the laboratory, the closest thing to a blackbody is a small hole to a cavity with a non-smooth, black surface. In astronomy, such objects as stars are frequently regarded as blackbodies, though this may be a bad approximation. An almost perfect blackbody spectrum is exhibited by the cosmic microwave background radiation.