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For a black body (a perfect absorber) there is no reflected radiation, and so the spectral radiance is entirely due to emission. In addition, a black body is a diffuse emitter (its emission is independent of direction). Blackbody radiation becomes a visible glow of light if the temperature of the object is high enough. [19]
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is called black-body radiation. The name "black body" is given because it absorbs all colors of light.
The importance of the Lummer and Kurlbaum cavity radiation source was that it was an experimentally accessible source of black-body radiation, as distinct from radiation from a simply exposed incandescent solid body, which had been the nearest available experimental approximation to black-body radiation over a suitable range of temperatures.
For a black body, Planck's law gives: [8] [11] = where (the Intensity or Brightness) is the amount of energy emitted per unit surface area per unit time per unit solid angle and in the frequency range between and +; is the temperature of the black body; is the Planck constant; is frequency; is the speed of light; and is the Boltzmann constant.
The temperature of stars other than the Sun can be approximated using a similar means by treating the emitted energy as a black body radiation. [28] So: L = 4 π R 2 σ T 4 {\displaystyle L=4\pi R^{2}\sigma T^{4}} where L is the luminosity , σ is the Stefan–Boltzmann constant, R is the stellar radius and T is the effective temperature .
The effective temperature of the Sun (5778 kelvins) is the temperature a black body of the same size must have to yield the same total emissive power.. The effective temperature of a star is the temperature of a black body with the same luminosity per surface area (F Bol) as the star and is defined according to the Stefan–Boltzmann law F Bol = σT eff 4.
Comparison of Rayleigh–Jeans law with Wien approximation and Planck's law, for a body of 5800 K temperature.. In physics, the Rayleigh–Jeans law is an approximation to the spectral radiance of electromagnetic radiation as a function of wavelength from a black body at a given temperature through classical arguments.
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.