As the temperature of a black body decreases, its intensity also decreases and its peak moves to longer wavelengths. Shown for comparison is the classical Rayleigh–Jeans law and its ultraviolet catastrophe .
A black body is an idealized physical body that absorbs all incident
electromagnetic radiation , regardless of frequency or angle of incidence. A white body is one with a "rough surface [that] reflects all incident rays completely and uniformly in all directions." [1]
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 temperature alone (see figure at right), not by the body's shape or composition.
A black body in thermal equilibrium has two notable properties: [2]
1. 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.
2. It is a diffuse emitter: the energy is radiated isotropically , independent of direction.
An approximate realization of a black surface is a hole in the wall of a large enclosure (see below). Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber. The radiation confined in such an enclosure may or may not be in thermal equilibrium, depending upon the nature of the walls and the other contents of the enclosure. [3][4]
Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.0. A source with lower emissivity independent of frequency often is referred to as a gray body .[5][6] Construction of black bodies with emissivity as close to one as possible remains a topic of current interest. [7]
In astronomy , the radiation from stars and planets is sometimes characterized in terms of an effective temperature , the temperature of a black body that would emit the same total flux of electromagnetic energy.
Definition
The idea of a black body originally was introduced by Gustav Kirchhoff in 1860 as follows:
...the supposition that bodies can be imagined which, for infinitely small thicknesses, completely absorb all incident rays, and neither reflect nor transmit any. I shall call such bodies perfectly black, or, more briefly, black bodies .[8]
A more modern definition drops the reference to "infinitely small thicknesses": [9]
An ideal body is now defined, called a blackbody. A blackbody 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 blackbody is a perfect absorber for all incident radiation.