# black body

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Black-body radiation curves for different temperatures

## black body

A body that absorbs all the radiation falling on it, i.e. a body that has no reflecting power. It is also a perfect emitter of radiation. The concept of a black body is a hypothetical ideal. The radiation from stars, and their effective and color temperatures, can however be described by assuming that they are black bodies.

Black-body radiation is the thermal radiation (see thermal emission) that would be emitted from a black body at a particular temperature. It has a continuous distribution of wavelengths. The graph of the energy, or intensity, of the radiation has a characteristic shape (see illustration) with a maximum value at a given wavelength, λmax. At lower temperatures the black-body radiation is mainly in the infrared region of the spectrum. As the temperature increases the maximum of the curve moves to shorter wavelengths. Curves at different temperatures follow the relationship

λmax T = 2.9 × 103

where the wavelength is measured in micrometers. This is known as the Wien displacement law. The radiation from a hot black body is greater at every wavelength than the radiation from a cooler black body. The total radiation flux thus increases rapidly with increasing temperature, as described by Stefan's law.

Stefan's law gives the total energy emitted over all wavelengths per second per unit area of a black body. The energy emitted at a particular wavelength by a black body can be predicted from Planck's radiation law. This law was derived by Max Planck from his quantum theory, propounded in 1900. If E (λ,T) is the energy emitted per unit wavelength interval at wavelength λ, per second, per unit area, into unit solid angle, by a body at thermodynamic temperature T , then

E (λ,T) = (2πhc 25)[ehc kT – 1]–1

h is the Planck constant, k the Boltzmann constant, and c the speed of light. Stefan's law and the Wien displacement law can both be derived from Planck's law.

Planck's radiation law describes only the continuous spectrum emitted by a black body. The continuous radiation from a star, as opposed to a black body, does not usually follow Planck's law exactly, although over a broad region of the spectrum the law is a good approximation.

References in periodicals archive ?
The mystery was solved yesterday when the large black object was revealed to be a filing cabinet full of dodgy offshore tax accounts arrangements involving lots of very rich people.
A dog walker found the yellow and black object on an Anglesey beach and took it to a Macclesfield auctioneers.
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They also submitted hundreds of pieces of evidence, including surveillance videos from inside Hernandez's home that showed him holding a black object that appeared to be a gun less than 10 minutes after Lloyd's killing.
The Airbus A320 was 700ft from landing when the pilot reported seeing a small black object to the left of the aircraft, the report by the UK Airprox (aircraft proximity) K Airprox (aircraft proximity) Board (UKAB) said.
"But sure enough, there was a largish black object, maybe two, very close together, in the middle of the two-mile-wide loch.
They might wander off piste and get gobbled up" - Sir Benjamin Slade on whose Somerset estate a black object, which might be a big cat, has been photographed "I have had a chauffeur for 38 years.
In March, a small black object on the ceiling of a changing room next to the Laboratory Department at Khoula Hospital was found to be a hidden camera, which was connected to the hospital's wireless network.
One diagram showed a black object resembling one of those flying saucers in 'Independence Day' hovering over the metropolis.

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