blackbody 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 → 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. |
blackbody curve 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 photosphere 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 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 spectrum 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 temperature 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. |
body jesm (#) Fr.: corps Any material object characterized by its physical properties. Body, from O.E. bodig "trunk, chest," related to O.H.G. botah, of unknown origin. Jesm, from Ar. jism "body, corps." |
celestial body axtar (#), jesm-e âsmâni (#) Fr.: corps céleste |
constrained body jesm-e pâvandidé Fr.: corps lié A → rigid body whose displacement is prevented because it is under → constraint or a body that can move only in certain directions. Contrasted with → free body. |
embody tanigândan Fr.: exprimer, concrétiser, incarner 1) To give a concrete form or body to an idea or quality. From en- "in" + → body Tanigândan, from Mid.Pers. tanig "bodily, corporal," from tan "body," → if and only if, + -ig, → -ik, + -ândan suffix of transitive verbs. |
free body jesm-e âzâd (#) Fr.: corps libre A → rigid body not constrained with other bodies and which from any given position can be displaced in any direction in space. Opposite of → constrained body. |
gray body jesm-e xâkestari (#) Fr.: corps gris A hypothetical body which emits radiation at each wavelength in a constant ratio, less than unity, to that emitted by a black body at the same temperature. |
heavenly body jesm-e âsmâni Fr.: corps céleste |
many-body problem parâse-ye N jesm Fr.: problème à N corps The mathematical problem of solving the equations of motions of any number of bodies which interact gravitationally. More specifically, to find their positions and velocities at any point in the future or the past, given their present positions, masses, and velocities. Many, from M.E. mani, meni, O.E. monig, manig; → body; → problem. |
n-body problem parâse-ye n-jesm Fr.: problème de n-corps The mathematical problem of studying the behavior (e.g., velocities, positions) of any number of objects moving under their mutual gravitational attraction for any time in the past or future. Same as the → many-body problem. |
perturber body jesm-e parturandé Fr.: corps perturbateur A celestial body that causes a perturbation in the orbit of another body. |
Planck's blackbody formula 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. |
primary body jesm-e naxostân Fr.: corps principal The body that is being orbited; such as the → Sun in the → solar system. As regards → multiple star systems, the most massive, or → primary star. See also: → secondary body. |
restricted three-body problem parâse-ye seh jesm-e forudâridé Fr.: problème restreint à trois corps A special case of the → three-body problem in which the → mass of one of the bodies is negligible compared to that of the two others. If the relative motion of the two massive components is a circle, the situation is referred to as the → circular restricted three-body problem. An example would be a space probe moving in the → gravitational fields of the → Earth and the → Moon, which revolve very nearly in circles about their common → center of mass. |
rigid body jesm-e saxtpây Fr.: corps rigide Mechanics: A system of many particles whose positions relative to one another remain fixed. |