blackbody سیهجسم siyah-jesm (#)
*Fr.: corps noir*
A theoretical object that is simultaneously a perfect → *absorber*
(it does not reflect any radiation) and a perfect → *emitter*
of → *radiation* in all → *wavelength*s
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*. → *black*; → *body*. |

blackbody curve خم ِ سیهجسم xam-e siyah-jesm
*Fr.: courbe de corps noir*
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*). → *blackbody*; → *curve*. |

blackbody photosphere شیدسپهر ِ سیهجسم šidsepehr-e siyah-jesm
*Fr.: photosphère de corps noir*
The → *blackbody* surface of the → *Universe*
defined at a → *redshift* of about
*z* ≥ 2 × 10^{6}.
This is distinct from the → *last scattering* surface,
in other words the → *cosmic microwave background radiation (CMBR)*,
which refers to *z* = 1100. Prior to the epoch of the blackbody photosphere
the distortions from the → *Big Bang* are exponentially
suppressed. → *blackbody*; → *atmosphere*. |

blackbody radiation تابش ِ سیهجسم tâbeš-e siyah-jesm (#)
*Fr.: rayonnement de corps noir*
The radiation emitted by a blackbody at a given → *temperature*.
The → *distribution* of radiation with
→ *wavelength* is given by
→ *Planck's blackbody formula* or
→ *Planck's radiation law*. → *blackbody*; → *radiation*. |

blackbody spectrum بیناب ِ سیهجسم binâb-e siyah-jesm (#)
*Fr.: spectre de corps noir*
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*. → *blackbody*; → *spectrum*. |

blackbody temperature دمای ِ سیهجسم damâ-ye siyah-jesm (#)
*Fr.: température de corps noir*
The temperature at which a blackbody would emit the same radiation
per unit area as that emitted by a given body at a given temperature. → *blackbody*; → *temperature*. |

Planck's blackbody formula دیسول ِ سیهجسم ِ پلانک disul-e siyah jesm-e Planck
*Fr.: formule du corps noir de Planck*
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. → *Planck*; → *blackbody*;
→ *formula*. |