Qualitative Ideas of Black-Body Radiation

The ideal black-body notion is of primary importance in studying thermal radiation and electromagnetic radiation energy transfer in all wavelength bands. Being an ideal radiation absorber, the black body is used as a standard with which the absorption of real bodies is compared. The black body additionally emits the utmost quantity of radiation and, consequently, it is used as a standard for comparison with the radiation of real physical bodies. This notion, introduced by G. Hirschhorn 1860, is so important that it is actively used in studying not only the intrinsic thermal radiation of natural media but also the radiations caused by different physical nature. ). The emissive properties of a black body are determined by means of quantum theory and are confirmed by the experiment.

The black body is so-called because those bodies that absorb incident visible light well seem black to the human eye. The term is, surely conventional and it has some historical roots.

Definition of a Black-Body

A black body is an ideal body that permits all of the incident radiation to pass into itself (without reflective the energy) and absorbs inside itself the entire incident radiation (without passing on the energy). This property is valid for radiation corresponding to all wavelengths and to all angles of incidence. Therefore, the black body is an ideal absorbent of incident radiation. All other qualitative characteristics determining the behavior of a black body follow from this definition.

Qualitative Properties of a Black-Body

A black body not only absorbs radiation ideally, but have other important properties. If there is a black body at a consistent temperature, placed inside a completely insulated cavity of arbitrary shape, whose walls are also formed by ideal black bodies at a consistent temperature, which initially differs from the temperature of the body inside. After a while the black body and also the closed cavity will have a standard equilibrium temperature. Under equilibrium situations it’s necessary for the black body to emit exactly the same amount of radiation as it absorbs.

Let us contemplate a system comprising a black body within a closed cavity that is at thermal equilibrium. The wall of the cavity have a peculiar property, it can emit and absorb radiation within a narrow wavelength band only. Being an ideal energy absorber, the black body absorbs the entire incident radiation in this wavelength band. Hence, the black body is an ideal emitter for any wavelength. It also have emissive ability and absorbing ability.

Black-Body Radiation Laws

The general thermodynamic considerations allowed Hirschhorn, Boltzmann and Uren to derive rigorously a series of important laws controlling the emission of heated bodies.

  • The Planck law
  • The Wien radiation law
  • The Rayleigh-Jeans radiation law
  • The Wein displacement law
  • The Stefan Boltzmann law
  • The Kirchhoff law


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