Black body, absorption

Both emission and absorption processes rely on the background radiation, which is present throughout the universe and which has a wavelength distribution characteristic of a black body and a temperature of about 2.7 K. This radiation is a consequence of the big bang with which the universe supposedly started its life. 

Black body A hypothetical body that has an absorptance and an emissivity of... 

The emissive power of a fireball, however, will depend on the actual distribution of flame temperatures, partial pressure of combustion products, geometry of the combustion zone, and absorption of radiation in the fireball itself. The emissive power ( ) is therefore lower than the maximum emissive power (E ) of the black body radiation ... 

Surfaces will absorb radiant heat and this factor is expressed also as the ratio to the absorptivity of a perfectly black body. Within the range of temperatures in refrigeration systems, i.e. - 70°C to + 50°C (203-323 K), the effect of radiation is small compared with the conductive and convective heat transfer, and the overall heat transfer factors in use include the radiation component. Within this temperature range, the emissivity and absorptivity factors are about equal. 

For a grey body, the emissivity and the absorptivity are, by definition, independent of temperature and hence equation 9.115 may be applied more generally showing that, where one radiation property (a, r or e) is specified for an opaque body, the other two may be obtained from equations 9.115 and 9.124. KirchofPs Law explains why a cavity with a small aperture approximates to a black body in that radiation entering is subjected to repeated internal absorption and reflection so that only a negligible amount of the incident radiation escapes through the aperture. In this way, a - e = 1 and, at T K, the emissive power of the aperture is aT4. 

Figure 8.30. Theoretical energy flux for radiation from the sun as a black body at 5,800 K. The spectrum reaching the surface of the earth is modified through absorption in the infrared region by H2O and CO2 and in the...

Thermal radiation emitted by an object can be continuous, discontinuous or, in most cases, a mixture. A continuous radiation profile corresponds to an ideal black body, where only the temperature of the emitting object determines the emission profile. Discontinuous thermal emission spectra are caused by photons emitted during the relaxation of excited vibrational states. Since vibrational states are quantised, this results in emission bands at the wavelengths of the corresponding IR absorption bands. 

The radiation coming from a black body is continuous in wavelength but dispersing the radiation from a star reveals absorption lines, which can be identified clearly... 

The preceding calculation of the thermal energy balance of a planet neglected any absorption of radiation by molecules within the atmosphere. Radiation trapping in the infrared by molecules such as CO2 and H20 provides an additional mechanism for raising the surface temperature - the greenhouse effect. The local temperature of a planet can then be enhanced over its black body temperature by the atmosphere. 

In complete equilibrium, the ratio of the population of an atomic or molecular species in an excited electronic state to the population in the groun d state is given by Boltzmann factor e — and the statistical weight term. Under these equilibrium conditions the process of electronic excitation by absorption of radiation will be in balance with electronic deactivation by emission of radiation, and collision activation will be balanced by collision deactivation excitation by chemical reaction will be balanced by the reverse reaction in which the electronically excited species supplies the excitation energy. However, this perfect equilibrium is attained only in a constant-temperature inclosure such as the ideal black-body furnace, and the radiation must then give -a continuous spectrum with unit emissivity. In practice we are more familiar with hot gases emitting dis-... 

Atoms and molecules absorb only specific frequencies of radiation dictated by their electronic configurations. Under suitable conditions they also emit some of these frequencies. A perfect absorber is defined as one which absorbs all the radiation falling on it and, under steady state conditions, emits all frequencies with unit efficiency. Such an absorber is called a black body. When a system is in thermal equilibrium with its environment rates of absorption and emission are equal (Kirchhoff s law). This equilibrium is disturbed if energy from another source flows in. Molecules electronically excited by light are not in thermal equilibrium with their neighbours. 

BLACK BODY. This term denotes an ideal body which would, if it existed, absorb all and reflect none of the radiation felling upon it its reflectivity would be zero and its absorptivity would be 100%. Such a body would, when illuminated, appear perfectly black, and would be invisible except its outline might be revealed by the obscuring of objects beyond. The chief interest attached to such a body lies in the character of the radiation emitted by it when heated and the laws that govern the relations of the flux density and the spectral energy distribution of that radiation with varying temperature. 

We now consider the transition rates for absorption of and stimulated emission induced by the black body radiation and compare these rates to the spontaneous... 

The black body photons can also be absorbed as the atoms in the n state make the transition to a higher lying n t state. Both the stimulated emission and absorption rates are given by... 

Fig. 5.8 Black body radiative transfer signals in Na located between parallel conducting plates for 29d —> 30p (left-hand side) and 28d —> 29p (right-hand side) as a function of the absorption frequency. The cutoff frequency is vc = 1/2d = 1.48 cm-1, where d is the plate separation. The increase in the transfer rate at v = vc (left-hand side) is due to the "switching on of the radiation polarized parallel to the plates (from ref. 24).

Absorption,1 field ionization Variable temperature black body 3.3 10 17... 

In 1900 Max Planck proposed a solution to the problem of black-body radiation described above. He suggested that when electromagnetic radiation interacts with matter, energy can only be absorbed or emitted in certain discrete amounts, called quanta. Planck s theory will not be described here, as it is highly technical. In any case, Planck s proposal was timid compared with the theory that followed. He supposed that quanta were only important in absorption and emission of radiation, but that otherwise the wave theory did not need to be modified. It was Einstein who took a more radical step in 1905 (the year in which he published his first paper on the theory of relativity and on several other unrelated topics). Einstein s analysis of the photoelectric effect is crucial, and has led to a complete change in the way we think of light and other radiation. 

In connection with observations of emission from hot equilibrium gases, it should be pointed out that the observed emission spectrum must be simply the product of the black body intensity distribution, times the emissivity of the gas. Therefore the emission spectrum contains no more information about the excitation mechanism than does the absorption spectrum of the same gas at the same temperature and path length. 

Finally, we should mention Kirchhoff s law. The emissivity e expresses which fraction a body of temperature T emits to bodies of lower temperature. If e = 1, we speak of black-body radiation, otherwise of gray-body radiation. Kirchhoff s law compares the emissivity a with the absorptivity a of a body when exposed to incident radiation from a body with a higher temperature and states that... 

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