Concept of Black Body Radiation

Black body Radiation Concept:

Black body Radiation is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. A black body in thermal equilibrium  emits electromagnetic radiation called black-body radiation. A black body in thermal equilibrium has two notable properties:

  • It is an ideal emitter; it emits as much or more energy at every frequency than any other body at the same temperature.
  • It is a diffuse emitter; the energy is radiated isotropic-ally, independent of direction;

A perfect black body is one, which absorbs all the radiation falling on it Experimentally, a hollow body, blackened on the inside and with a small opening, is considered a typical black body.  Any radiation that enters ‘through the small .opening is reflected repeatedly from the walls until all of the energy eventually becomes absorbed.


A black body is both good absorb and radiator of energy. Of the various types of bodies heated to particular temperature, only black body radiates. the maximum amount of energy. It radiates -the same amount of energy as it absorbs. The main aspects of black body radiation, which emerge from experimental observations are:

  • At shorter wavelength region, which is ‘at higher frequency  region,intensity of radiation is low.
  • At every temperature, there is a wavelength at which energy radiated is maximum. This wavelength is called λmax value. of that temperature.
  • At higher temperatures, there is increased intensity of radiation in the shorter wavelength region:

A piece of iron, say at 375 K, is hot for the hand to touch but no visible radiation, i.e. shorter wavelength radiation, is emitted by the metal. However, with increase of temperature, it becomes progressively dull red, bright red, orange, yellow and white hot. In general,. as temperature is increased, the radiation emitted, contains more of shorter wavelength re n, whether it is from black body or an iron piece.

Tags: B.Sc

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