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Why the concept of an ideal blackbody had been introduced? What was its need or its significance?

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The concept of a blackbody is an idealization to describe a surface that absorbs any photon with which it comes in contact - no matter what the energy (i.e. wavelength) of that photon and no matter what its angle of incidence. It is a perfect absorber. An early empirical law (Kirchoff's law) states that this ideal body is then the most perfect of emitters at a given temperature T - no real body (at the same isothermal temperature T) emits energy at a greater rate (per unit of surface area) than does a blackbody. A blackbody is a perfect absorber and a perfect emitter (thus in principle, such an object would be invisible, unless viewed at wavelengths lying in the spectral interval over which it is itself a significant emitter).

As to the need or significance, I've read that soon after the unification of Germany the German state wished to invest in electricity as an industrial enterprise, and funded research on light and energy. It was of interest to understand the relation between the temperature of a wire filament (light bulb) and the colour (wavelength) of the light emitted. This led to the systematic experimental study of light spectra, and theoretical investigations... leading eventually to Planck's law for the emission spectrum of a blackbody (which subsumes or explains the earlier empirical laws of Wein and of Stefan-Boltzmann regarding blackbody emission spectra). The experimental realization of a blackbody, at least in the early days, was an isothermal chamber with a small orifice... and one measured the spectrum of radiation emerging from the orifice.

The concept of the perfect emitter can also be extended to gases, which however are selective emitters/absorbers (as opposed to having a continuous spectrum).

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  • $\begingroup$ A black body is a perfect absorber in thermal equilibrium at a set temperature. Objects which exhibit a range of temperatures or which are changing temperature may be perfect absorbers, but not black bodies (e.g. the Sun). $\endgroup$ – Rob Jeffries Aug 3 '19 at 6:04

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