Black body
A body which completely absorb radiations of all wavelengths incident on it .
A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperaA black body in thermal equilibrium has two notable properties
- It is an ideal emitter: at every frequency, it emits as much energy as or more energy than any other body at the same temperature.
- It is a diffuse emitter: the energy is radiated isotropically, independent of direction
An ideal body is now defined, called a black body. A black body allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the black body is a perfect absorber for all incident radiation .
Black body radiation
The radiations emitted by black bodies are called black body radiations .
Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body
(an opaque and non-reflective body) held at constant, uniform
temperature. The radiation has a specific spectrum and intensity that
depends only on the temperature of the body.The thermal radiation spontaneously emitted by many ordinary objects can be approximated as blackbody radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium.
A black-body at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. At higher temperatures, black bodies glow with increasing intensity and colors that range from dull red to blindingly brilliant blue-white as the temperature increases.
Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, and it is believed that they emit black-body radiation with a temperature that depends on the mass of the black hole.
The term black body was introduced by Gustav Kirchhoff in 1860. Black-body radiation is also called complete radiation or temperature radiation or thermal radiation.
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