Note that visible radiation occupies a very narrow spectrum band from 400 to 760 nm. A blackbody is a perfect absorber (and a perfect emitter ). Since the absorptivity and the emissivity are interconnected by Kirchhoff’s Law of thermal radiation, a blackbody is also a perfect absorber of electromagnetic radiation.įor an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.Ī blackbody absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence, and its absorptivity is equal to unity, which is also the highest possible value. These collectors waste very little solar energy through the emission of thermal radiation. For example, solar heat collectors incorporate selective surfaces with very low emissivities. Emissivity plays an important role in heat transfer problems. Real objects with emissivities less than 1.0 (e.g., copper wire) emit radiation at correspondingly lower rates (e.g., 448 x 0.03 = 13.4 W/m 2). The surface of a blackbody emits thermal radiation at the rate of approximately 448 watts per square meter at room temperature (25 ☌, 298.15 K). Real objects do not radiate as much heat as a perfect black body, and they radiate less heat than a black body and therefore are called gray bodies. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.0. A body that emits the maximum amount of heat for its absolute temperature is called a blackbody.Ī blackbody is an idealized physical body that has specific properties. Therefore, various materials emit different amounts of radiant energy even when they are at the same temperature. It is known that the amount of radiation energy emitted from a surface at a given wavelength depends on the material of the body and the condition of its surface, as well as the surface temperature. This law determines the most likely frequency of the emitted radiation. This law describes the spectrum of blackbody radiation, which depends only on the object’s temperature. This law gives the relationship between the emissivity and absorptivity of an object. The following laws are associated with blackbody radiation:
Blackbody radiation is also called thermal radiation, cavity radiation, complete radiation, or temperature radiation. The term blackbody was introduced by German physicist Gustav Kirchhoff in 1860.
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