What is Black-Body Radiation?
Any object has the property of continuously radiating, absorbing, and reflecting electromagnetic waves. The radiated electromagnetic waves are different in each band, that is, they have a certain spectral distribution. This spectral distribution is related to the characteristics of the object itself and its temperature, so it is called thermal radiation. In order to study the law of thermal radiation that does not depend on the specific physical properties of matter, physicists have defined an ideal object, the black body, as a standard object for the study of thermal radiation.
Blackbody radiation
- what is
- ideal
- Planck's law of radiation (Planck) gives the specific spectral distribution of blackbody radiation. At a certain temperature, the unit surface
- Earth Blackbody Radiation
- B (, T) = 2hc2 / 5 / exp (hc / KT) -1
- B (, T) Spectral radiance of black body (W · m ^ -2 · Sr ^ -1 · m ^ -1)
- The formula of the relationship between the black body spectral radiation output M (, T) and the wavelength and thermodynamic temperature:
- M = c1 / [ ^ 5 (exp (c2 / T) -1)], where c1 = 2hc ^ 2, c2 = hc / k.
- Blackbody energy density formula:
- E * d = c1 * v ^ 3 * dv / [exp (c2 * v / T) -1)]
- E * dv represents the blackbody radiation energy density in the frequency range (v, v + dv).
- radiation wavelength (m)
- Tblack body absolute temperature (K, T = t + 273k)
- Cspeed of light (2.998 × 10 ^ 8m / s)
- hPlanck's constant, 6.626 × 10 ^ -34 J · S
- KBoltzmann constant, 1.3806505 * 10 ^ -23J / K basic physical constant
- It can be seen from Figure 2.2 (missing):
- At a certain temperature, there is an extreme value of the black body's spectral radiance. The location of this extreme value is related to temperature. This is Wien's law of displacement
- m T = 2898 (m · K)
- m wavelength at the maximum blackbody spectrum radiance (m)
- Tabsolute temperature of black body (K)
- According to Wien's displacement law, we can estimate that when T ~ 6000K, m ~ 0.48m (green). This is the approximate maximum spectrum in solar radiation
- Spectral density distribution of solar radiation
- When T 300K, m 9.6m, this is where the roughly maximum spectral radiance of the earth's objects is radiated.
- At any wavelength, the spectral radiance of the high-temperature blackbody is definitely greater than the spectral radiance of the low-temperature blackbody, regardless of whether this wavelength is at the spectral maximum radiance.
- If B (, T) is integrated for all wavelengths and also for each radiation direction, then Stefan-Boltzmann law can be obtained. The unit area of a black body with an absolute temperature of T in unit time The total energy radiated in all directions of inward space is B (T)
- B (T) = T4 (W / m2)
- is the Stefan-Boltzmann constant, which is equal to 5.67 × 10-8 W / m2 · K4
- But there is no such ideal black body in the real world, so what is the way to describe this difference? For any wavelength, the emissivity is defined within a small wavelength interval of that wavelength. Ratio of radiant energy. Obviously, the emissivity is a positive number between 0 and 1. Generally, the emissivity depends on material characteristics, environmental factors and observation conditions. If the emissivity is not related to the wavelength, the object can be called a gray body, otherwise it is called a selective radiator.