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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.

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:

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 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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