Blackbody: Difference between revisions

Latest revision as of 13:34, 26 March 2024

A hypothetical body that cannot be excited to radiate by an external source of electromagnetic radiation of any frequency, direction, or state of polarization except in a negligibly small set of directions around that of the source radiation.
The traditional definition of a blackbody—as one that absorbs all the radiation incident on it—is inadequate unless to this definition is added the requirement that the body be large compared with the wavelength of the incident radiation. The concept of radiation incident on a body is from geometrical (or ray) optics, which is never strictly valid (because all bodies are finite) and may break down completely when the body is small compared with the wavelength. This was recognized by Planck, but by almost no one who followed him. Although no strict blackbody exists, some bodies are approximately black over a limited range of frequencies, directions, and polarization states of the exciting radiation.
See blackbody radiation, Planck's radiation law, emissivity.
Planck, M. 1959. The Theory of Heat Radiation. p. 2.

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+A hypothetical body that cannot be excited to radiate by an external source of [[electromagnetic radiation|electromagnetic  radiation]] of any [[frequency]], direction, or state of [[polarization]] except in a negligibly  small set of directions around that of the source radiation.<br/> The traditional definition of a blackbody&mdash;as one that absorbs all the [[radiation]] incident on  it&mdash;is inadequate unless to this definition is added the requirement that the body be large compared  with the [[wavelength]] of the incident radiation. The concept of radiation incident on a body is  from geometrical (or [[ray]]) optics, which is never strictly valid (because all bodies are finite) and  may break down completely when the body is small compared with the wavelength. This was  recognized by Planck, but by almost no one who followed him. Although no strict blackbody  exists, some bodies are approximately black over a limited [[range]] of frequencies, directions, and  polarization states of the exciting radiation. <br/>''See'' [[blackbody radiation]], [[Planck's radiation law]],  [[emissivity]].<br/> Planck, M. 1959. The Theory of Heat  Radiation. p. 2.
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-<div class="definition"><div class="short_definition">A hypothetical body that cannot be excited to radiate by an external source of [[electromagnetic  radiation]] of any [[frequency]], direction, or state of [[polarization]] except in a negligibly  small set of directions around that of the source radiation.</div><br/> <div class="paragraph">The traditional definition of a blackbody&mdash;as one that absorbs all the [[radiation]] incident on  it&mdash;is inadequate unless to this definition is added the requirement that the body be large compared  with the [[wavelength]] of the incident radiation. The concept of radiation incident on a body is  from geometrical (or [[ray]]) optics, which is never strictly valid (because all bodies are finite) and  may break down completely when the body is small compared with the wavelength. This was  recognized by Planck, but by almost no one who followed him. Although no strict blackbody  exists, some bodies are approximately black over a limited [[range]] of frequencies, directions, and  polarization states of the exciting radiation. <br/>''See'' [[blackbody radiation]], [[Planck's radiation law]],  [[emissivity]].</div><br/> </div><div class="reference">Planck, M. 1959. The Theory of Heat  Radiation. p. 2. </div><br/>
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