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any object having the continuous radiation, absorption, emission of electromagnetic waves art. Radiated electromagnetic waves in each band is different, i.e. having a certain spectral distribution. This spectral distribution characteristic of the object itself and its temperature, and thus is called heat radiation. To study was not dependent on the specific material of the heat radiation law, physicists defines an over the object - blackbody (black body), as a study of the standard heat radiation body.
meaning blackbody
means that the so-called blackbody all incident electromagnetic wave is absorbed, neither reflection nor transmission (of course, still blackbody radiation outward). Obviously the real nature of the black body does not exist, but many feature is a better approximation of a blackbody (on some bands).
Planck's radiation law
SUMMARY
Planck's law of radiation (Planck) is given a specific spectral distribution of black body radiation, at a certain temperature , black body per unit area per unit time, per unit solid angle of radiation and the unit wavelength interval energy
B (λ, T) = 2πhc2 / λ5 · 1 / exp (hc / λRT) - 1
B (λ, T) - blackbody spectral radiance (W, m-2, Sr-1, μm-1)
λ- radiation wavelength (μm) < / p>
T- blackbody absolute temperature (K, T = t + 273k)
C- speed of light (2.998 × 108 m · s-1)
h- P Planck constant, 6.626 × 10-34 J · S
K- Boltzmann constant (Bolfzmann), 1.380 × 10-23 J · K-1 basic physical constants
significance
can be seen from Figure 2.2: ① at a certain temperature, the presence of a blackbody spectral radiance extremum, the position of the extreme value related to temperature, which is Wien's displacement law (Wien) < absolute temperature of the wavelength spectrum at the maximum radiance of a blackbody (μm)
T- blackbody - / p>
λm T = 2.898 × 103 (μm · K)
λm (K)
according to Wien's law, we can estimate the time when T ~ 6000K, λm ~ 0.48μm (green). This is approximately the maximum solar radiation at the spectral radiance.
When T ~ 300K, λm ~ 9.6μm, which is substantially at a maximum spectral radiance of the earth object radiation.
② strengths at any wavelength, spectral radiance greater than the absolute temperature of blackbody spectral radiance of a blackbody low temperature, regardless of whether the wavelength of the maximum spectral radiance at.
If the B (λ, T) for all wavelengths integration, while also integrating the respective radiation directions, then the fan can be obtained Manchester - Boltzmann's law (Stefan-Boltzmann), T is the absolute temperature blackbody per unit area within the unit time, the total energy spatial directions radiated to B (T)
B (T) = δT4 (W · m-2)
δ is Stefan-Boltzmann constant, is equal to 5.67 × 10-8 W · m-2 · K-4
gray body radiator with a selective
but in reality this ideal world does not exist black body, then what is there to portray this difference? ratio of blackbody radiation energy and at the same temperature radiant energy within a small wavelength interval of any wavelength, defined emissivity of the wavelength, the real object. Emission was clearly positive number between 0 and 1, the emission rate depends on the general physical characteristics, viewing conditions and environmental factors. If the emissivity independent of wavelength, then the object can be called a gray body (grey body), otherwise called selective radiator.