Classical thermodynamics, heat transfer Definition
Black body
Definition
A black body is an idealised body that absorbs all incident electromagnetic radiation, for every wavelength and every direction of incidence. So it reflects and transmits none. Black-body radiation is the thermal radiation emitted by a black body in thermal equilibrium at a given temperature.
Remarks
Example: a small hole in the wall of an opaque cavity whose walls are at uniform temperature. Radiation that enters the hole is reflected many times inside and is absorbed before it leaves, so the hole absorbs almost all incident radiation, as required. It is the standard laboratory approximation of a black body.
Non-example: a polished silver plate. It reflects most of the visible light falling on it, so it fails the absorption condition.
A black body in equilibrium is a perfect emitter: at each temperature, wavelength and direction, no body emits more. Its emission is the same in all directions when measured as radiance, that is, it is a diffuse emitter.
Reading path
To understand this page from the ground up, read these 3 pages in order. The foundations come first.
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- no evidence level
- Electromagnetic radiation concept
- Thermal radiation concept
- Black body definition
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About this page
| Tag | 33MF92TMW9 |
|---|---|
| URI | m2://33MF92TMW9 |
| Kind | definition |
| Writing | W0 stub |
| MSC | 80A21, 78A40 |
| Defines | Black body |
| Uses | Thermal radiation, Electromagnetic radiation |
| Changed | 3 November 2026 |
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