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## Kirchhoff's law of thermal radiation*See also: Kirchhoff's laws for other laws named after Kirchhoff*.
In thermodynamics, ## Additional recommended knowledgeAn object at some non-zero temperature radiates electromagnetic energy. If it is a perfect black body, absorbing all light that strikes it, it radiates energy according to the black-body radiation formula. More generally, it is a "grey body" that radiates with some emissivity multiplied by the black-body formula. Kirchhoff's law states that: **At thermal equilibrium, the emissivity of a body (or surface) equals its absorptivity.**
Here, the absorptivity (or Kirchhoff's Law has a corollary: the emissivity cannot exceed one (because the absorptivity cannot, by conservation of energy), so it is not possible to thermally radiate more energy than a black body, at equilibrium. In negative luminescence the angle and wavelength integrated absorption exceeds the material's emission, however, such systems are powered by an external source and are therefore not in thermal equilibrium. This theorem is sometimes informally stated as ## References- Evgeny Lifshitz and L. P. Pitaevskii,
*Statistical Physics: Part 2*, 3rd edition (Elsevier, 1980). - F. Reif,
*Fundamentals of Statistical and Thermal Physics*(McGraw-Hill: Boston, 1965).
Categories: Thermodynamics | Electromagnetic radiation |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Kirchhoff's_law_of_thermal_radiation". A list of authors is available in Wikipedia. |