Blackbody

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.

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:

     [I(\nu) = \frac{2h\nu^{3}}{c^2}\frac{1}{\exp\left(\frac{h\nu}{kT}\right)-1}]

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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