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Kirchhoff's original contribution to the physics of thermal radiation was his postulate of a perfect black body radiating and absorbing thermal radiation in an enclosure opaque to thermal radiation and with walls that absorb at all wavelengths. Kirchhoff's perfect black body absorbs all the radiation that falls upon it.
Of particular importance, although planets and stars (including the Earth and Sun) are neither in thermal equilibrium with their surroundings nor perfect black bodies, blackbody radiation is still a good first approximation for the energy they emit. [5] The term black body was introduced by Gustav Kirchhoff in 1860. [6]
The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature". [52] It has a nearly ideal Planck spectrum at a temperature of about 2.7 K. It departs from the perfect isotropy of true black-body radiation by an observed anisotropy that varies with angle on the sky only to about one part in ...
A black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation. The ratio of any body's emission relative to that of a black body is the body's emissivity, so a black body has an emissivity of one. Absorptivity, reflectivity, and emissivity of all bodies are dependent on the wavelength of the ...
Thus Kirchhoff's law of thermal radiation can be stated: For any material at all, radiating and absorbing in thermodynamic equilibrium at any given temperature T, for every wavelength λ, the ratio of emissive power to absorptive ratio has one universal value, which is characteristic of a perfect black body, and is an emissive power which we ...
Emissivity of a body at a given temperature is the ratio of the total emissive power of a body to the total emissive power of a perfectly black body at that temperature. Following Planck's law , the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths.
A video taken while trekking toward the large, man-made body of water through the snow-covered Sierra Nevadas shows her laughing with her friend. “Wooo!” she shouts, breath visible in the ...
Gustav Robert Kirchhoff (German: [ˈgʊs.taːf ˈkɪʁç.hɔf]; 12 March 1824 – 17 October 1887) was a German chemist, mathematician and physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy and the emission of black-body radiation by heated objects. [1] [2] He also coined the term black body in 1860. [3]