A theoretical object that is simultaneously a perfect → absorber
(it does not reflect any radiation) and a perfect → emitter
of → radiation in all → wavelengths
and whose radiation is governed solely by its → temperature.
Blackbody radiation cannot be explained by → classical physics.
The study of its
characteristics has, therefore, played an important role in the development of
→ quantum mechanics.
A blackbody can be realized in the form of a cavity with highly
absorbing internal walls and a small aperture. Any ray entering
through the aperture can leave the cavity only after
repeated reflection from the walls. When the aperture is
sufficiently small, therefore, the cavity will absorb practically all
the radiation incident on the aperture, and so the surface of the
aperture will be a black body.
The light within the cavity will always interact and exchange energy with the material
particles of the walls and any other material particles present. This interaction will
eventually → thermalize
the radiation within the cavity, producing a → blackbody spectrum,
represented by a → blackbody curve.
See also
→ blackbody photosphere;
→ blackbody radiation;
→ Planck's blackbody formula;
→ Planck's radiation law;
→ Rayleigh-Jeans law;
→ Stefan-Boltzmann law;
→ thermalization;
→ Wien's displacement law.

The characteristic way in which the → intensity
of → radiation emitted by a
→ blackbody varies with its → frequency
(or → wavelength), as described by
→ Planck's radiation law. Also referred to as the
→ Planck curve.
The exact form of the curve depends only on the object's
→ temperature. The wavelength at which the
emitted intensity is highest is an indication of the temperature of
the radiating object. As the temperature of the blackbody increases, the peak wavelength
decreases (→ Wien's displacement law) and the total energy being
radiated (the area under the curve) increases rapidly
(→ Stefan-Boltzmann law).

A curve displaying → blackbody radiation intensity versus the
wavelength for a given temperature, according to
→ Planck's blackbody formula. It is an asymmetrical curve
with a sharp rise on the short wavelength side and a much more gradually sloping
long-wavelength tale. Same as → Planck spectrum.

A formula that determines the distribution of intensity of radiation
that prevails under conditions of thermal equilibrium at a temperature
T:
B_{v} = (2hν^{3} / c^{2})[exp(hν / kT) - 1]^{-1}
where h is Planck's constant
and ν is the frequency.