black body

(redirected from Dark object)
Also found in: Dictionary, Thesaurus.
Black-body radiation curves for different temperaturesclick for a larger image
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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
In video taken of the van driver's arrest on Monday, after the white vehicle ploughed into lunch-hour crowds, the suspect can be seen pointing a dark object at officers.
Osborn (University of Warwick) and colleagues interpret these events as eclipses by a very large, dark object that's in an 808-day orbit around the star.
Before entering this exhibition of Berlin-based artists Martin Ebner and Kitty Kraus, curated by Audrius Pocius and Nicholas Matranga, one might have spotted a dark object piercing the concrete facade of the Contemporary Art Centre's monolithic modernist building.
"The evidence of the taxi driver is that he saw a dark object in the carriageway which he thought was a bin bag and initially only had time to steer rather than brake in an attempt to avoid the tyres of the taxi running over the object.
The halogen lamp in this case was to symbolise light whereas the case where some part of the PV is covered with a dark object is to symbolise the cloudy condition.
I could see large dark object in my field of view, but there was no way I could distinguish it as a tank.
Rescuers believe they have found the plane on the sea floor off Borneo, after sonar detected a large, dark object beneath waters near where debris and bodies were found on the surface.Ships and planes had been scouring the Java Sea for Flight QZ8501 since Sunday, when it lost contact during bad weather about 40 minutes into its flight from Surabaya to Singapore.
NNA - A sonar image showing a large, dark object under the sea was presumed to be the missing AirAsia plane, an official with IndonesiaA's search and rescue agency said on Wednesday.
Indonesian rescuers believe they have found the wreck of a crashed AirAsia plane on the ocean floor off Borneo, after sonar detected a large, dark object beneath waters where debris and bodies were found floating.
He picked up a dark object and put it down his trousers and he moved towards me."
He has a raptor skin on his head, probably a personal protector, and a long dark object across his back which could be a war or pipe bundle.