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Gustav Kirchhoff (1824–1887) . In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium.
In 1761, Benjamin Franklin wrote a letter describing his experiments on the relationship between color and heat absorption. [7] He found that darker color clothes got hotter when exposed to sunlight than lighter color clothes. One experiment he performed consisted of placing square pieces of cloth of various colors out in the snow on a sunny day.
The Earth has an albedo of 0.3, meaning that 30% of the solar radiation that hits the planet gets scattered back into space without absorption. The effect of albedo on temperature can be approximated by assuming that the energy absorbed is multiplied by 0.7, but that the planet still radiates as a black body (the latter by definition of ...
The planets are solar thermal collectors on a large scale. The temperature of a planet's surface is determined by the balance between the heat absorbed by the planet from sunlight, heat emitted from its core, and thermal radiation emitted back into space. Emissivity of a planet is determined by the nature of its surface and atmosphere. [11]
The law was formulated by Josef Stefan in 1879 and later derived by Ludwig Boltzmann. The formula E = σT 4 is given, where E is the radiant heat emitted from a unit of area per unit time, T is the absolute temperature, and σ = 5.670 367 × 10 −8 W·m −2 ⋅K −4 is the Stefan–Boltzmann constant. [28]
Color temperature is a parameter describing the color of a visible light source by comparing it to the color of light emitted by an idealized opaque, non-reflective body. The temperature of the ideal emitter that matches the color most closely is defined as the color temperature of the original visible light source.
The total absorption ratio a(T, i) of that body is dimensionless, the ratio of absorbed to incident radiation in the cavity at temperature T. (In contrast with Balfour Stewart's, Kirchhoff's definition of his absorption ratio did not refer in particular to a lamp-black surface as the source of the incident radiation.)
For example, comparisons in the B (blue) or V (visible) range lead to the so-called B-V color index, which increases the redder the star, [39] with the Sun having an index of +0.648 ± 0.006. [40] Combining the U (ultraviolet) and the B indices leads to the U-B index, which becomes more negative the hotter the star and the more the UV radiation.