Blackbody 

• 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.

A blackbody is an ideal emitter which radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. A blackbody also absorbs all the radiant energy incident on it: i.e. no energy is reflected or transmitted.

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:

<math>I(\nu) = \frac{2h\nu^{3}}{c^2}\frac{1}{\exp\left(\frac{h\nu}{kT}\right)-1}</math>

where I(ν)δν is the amount of energy per unit surface per unit time per unit solid angle emitted in the frequency range between ν and ν+δν; h is Planck's constant, c is the speed of light and k is Boltzmann's constant.

The wavelength at which the radiation is strongest is given by Wien's law, and the overall power emitted per unit area is given by the Stefan-Boltzmann law.

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 and planets 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.

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