The term “radiation” used in this area of Thermopedia is simply an alternative short name of the electromagnetic radiation (or electromagnetic wave) because we are not going to consider the acoustic radiation or the ionizing radiation emitted by radioactive substances. The visible light is the most evident example of electromagnetic radiation. It is well-known that a color of light is determined by the wavelength of electromagnetic wave: the change of color from red to violet corresponds to decreasing the wavelength from about 0.7 µm to 0.4 µm. A more short-wave radiation is called ultraviolet and we cannot see this radiation. For problems of the so-called thermal radiation, a long-wave radiation (as compared with the visible one) appears to be more important. The infrared radiation is characterized by the wavelengths up to one millimeter but we have an experience in direct sensing only the near-infrared radiation by our skin. A good example is a pleasant heat from an almost dark campfire just before its complete extinction.
In physics, it is more correct to use of the radiation frequency instead of the wavelength because the latter depends on the refractive index of the medium. At the same time, this is not so important when we are talking about the radiation propagating in a gas medium like the atmosphere.
The term “thermal radiation” means that a hot medium is a source of this electromagnetic radiation. The complete understanding of the thermal radiation physics has been reached in the beginning of the 20th century after the fundamental works by Niels Bohr on the structure of atoms and of radiation emanating from them. A quantum nature of the radiation emitted by single atoms and molecules leads to complex spectra of gases. Some more detailed discussion of this subject will be presented in the article "Physical nature of thermal radiation" following from the present introductory article.
It is very instructive that the known law for the integral equilibrium thermal radiation has been derived by Ludvig Boltzmann using the thermodynamics only. There is no any dependence of radiative flux on substance properties in this general low. The same result can be obtained by direct integration of the Plank function over the spectrum. A more detailed consideration of the fundamental laws of the equilibrium radiation can be found in the article “Basic laws of the equilibrium (black-body) radiation”.
In realistic heat transfer problems, even in the case of local thermodynamic equilibrium, there is no equilibrium of thermal radiation and the radiation field appears to be very complex. Fortunately, the polarization of electromagnetic waves is usually ignored in the heat transfer problems because the emitted thermal radiation is unpolarized (randomly polarized). Nevertheless, there are many remaining parameters which should be known for complete description of the radiation field. It is sufficient to remember that the radiation at every point of a medium depends on the direction, characterized by two angles. A traditional description of the radiation intensity field and the spectral radiative characteristics of media and surfaces are presented in the article “Physical quantities used to characterize radiation of surfaces and media”.
The above referred article deals with a phenomenological description of the radiation. This approach based of the ray (geometrical) optics is usually formulated in terms of the so-called radiation transfer theory. The resulting radiative transfer equation is similar to the Boltzmann kinetic equation for ideal gases. It is clear that this equation ignores the wave nature of electromagnetic radiation, and only the coefficients of this equation are responsible for the radiation interaction with a substance. A good example of this approach is a propagation of electromagnetic wave in a rarefied cloud of randomly positioned particles with sizes comparable with the wavelength. It is important to take into account the wave nature of the radiation in the interaction of the radiation with single particles. A problem of physical optics should be solved to obtain the characteristics of this interaction. At the same time, the geometrical optics can be used not only in a space between particles but also for the complete problem. It is sufficient to use radiative properties of an equivalent continuous medium as coefficients of the radiative transfer equation. This traditional approach is failed in the case when a characteristic size of the heat transfer region is comparable with the wavelength. The known example is a heat transfer through a very thin gap between two bodies. The thickness of this gap may be not so small to observe the wave effects, especially at cryogenic temperatures when the maximum of thermal radiation takes place at a large wavelength. The specific interference wave effects are very important in micro-scale and nano-scale heat transfer by radiation. The problems of this type are usually solved on the basis of the fluctuational electrodynamics. A more detailed discussion of various theoretical concepts employed as a basis of the radiation propagation analysis can be found in one more article following from the present introduction: “Ray optics and wave effects in radiation propagation”.