blaze wavelength mowjtul-e beliz Fr.: longueur d'onde de blaze The wavelength in a given diffraction order for which the efficiency curve reaches its maximum. → blaze; → wavelength. Mowjtul→ wavelength; beliz→ blaze. |
center wavelength mowjtul-e markaz Fr.: longueur d'onde centrale Center of a filter passband measured at 50% of peak transmittance. → center; → wavelength. |
central wavelength mowjtul-e markazi Fr.: longueur d'onde centrale 1) In an interference filter, the wavelength of peak
transmission. Central, adj. from → center; → wavelength. |
coherence length derâzâ-ye hamdusi Fr.: longueur de cohérence The distance over which an → electromagnetic wave train maintains a specified → degree of coherence. The coherence length is related to the → coherence time multiplied by vacuum → velocity of light. |
Compton wavelength mowjtul-e Compton, tul-e mowj-e ~ Fr.: longueur d'onde de Compton, longueur d'onde Compton The quantum wavelength of a particle with a highly relativistic velocity. The Compton wavelength is given by h/mc, where h is Planck's constant, m is the mass of the particle, and c the light speed. For an electron, the Compton wavelength is about 2.4 × 10^{-10} cm, intermediate between the size of an atomic nucleus and an atom. → Compton; → wavelength. |
cutoff wavelength mowj-tul-e boré Fr.: longueur d'onde de coupure Wavelength at which the transmittance of a filter, or the detectivity of a detector, has fallen to one-half its peak value. → cutoff; → wavelength. |
de Broglie wavelength mowjtul-e de Broglie Fr.: longueur d'onde de Broglie The wavelength of the wave associated with a → particle as given by the → de Broglie equation. |
Debye length derâzâ-ye Debye (#) Fr.: longueur de Debye A characteristic length scale in a → plasma, determined by the temperature and number density of the charged particles. The Debye length (in cm) is given by the expression: λ_{D} = 743(T_{e}/n_{e})^{1/2}, where T_{e} is the electron temperature in → electron-volts and n_{e} is the electron density in cm^{-3}. Particles which pass each other at distances smaller than the Debye length interact directly. Outside the Debye length particle interactions are dominated by collective effects. In typical laboratory plasmas the Debye length is small compared with the plasma dimension, so that collective effects are important. |
effective focal length derâzâ-ye kânuni-ye oskarmand Fr.: longueur focale effective The focal length of an imaging system, which consists of several lenses or mirrors. → effective; → focal length. |
focal length derâzâ-ye kânuni Fr.: longueur focale The distance between the optical center of a lens, or the surface of a mirror, and its focus. |
frequency to wavelength conversion hâgard-e basâmad bé mowj-tul Fr.: conversion fréquence / longueur d'onde Deriving the → wavelength of an undulatory phenomenon from
its → frequency, and vice versa. → frequency; → wavelength; → conversion. |
Hubble length derâzâ-ye Hubble Fr.: longueur de Hubble The distance traveled by light along a straight → geodesic in one → Hubble time. Also called the → Hubble radius, → Hubble distance, and → cosmic horizon. |
Jeans length derâzâ-ye Jeans (#) Fr.: longueur de Jeans The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud's internal pressure is sufficient to resist collapse. The Jeans length is defined by: λ_{J} = (π c_{s}^{2}/Gρ)^{1/2} = 0.2 pc (T/10 K)^{1/2}(n_{H2}/10^{4} cm^{-3})^{-1/2}, where c_{s} is the → sound speed, G is the → gravitational constant, ρ is the gas density, T is the gas temperature, and n_{H2} is the molecular hydrogen density. |
length derâzâ (#), tul (#) Fr.: longueur A distance determined by the extent of something specified. → Jeans length M.E. length(e), O.E. lengthu "length," from P.Gmc. *langitho, noun of quality from *langgaz (root of O.E. lang "long," cognate with Pers. derâz, as below) + -itho, abstract noun suffix. Cognate with O.N. lengd, O.Fris. lengethe, Du. lengte. Derâzâ quality noun of derâz "long," variants Laki, Kurdi derež; Mid.Pers. drâz "long;" O.Pers. dargam "long;" Av. darəga-, darəγa- "long," drājištəm "longest;" cf. Skt. dirghá- "lon (in space and time);" L. longus "long;" Gk. dolikhos "elongated;" O.H.G., Ger. lang; Goth. laggs "long;" PIE base *dlonghos- "long;" tul loan from Ar. ţaul, used in → wavelength. |
length contraction terengeš-e derâzâ Fr.: contraction de longueur Same as → Lorentz contraction. → length; → contraction. |
lengthy kešnâk (#) Fr.: long, interminable 1) Having or being of great length; very long. From → length + -y. Kešnâk "lengthy" (Bardsiri, Kermâni), from kešidan, kašidan "to draw, protract, trail, drag, carry," → tide. Bardesir, Kermân |
mixing length derâzâ-ye âmizeš Fr.: longueur de mélange In a → turbulent flow, the average distance traveled by a → convective cell before it dissolves into its surroundings and deposits its energy. The mixing length is of the order of the → pressure scale height (H_{P}), l = αH_{P}, where α is the → mixing length parameter. See also → mixing length theory. |
mixing length parameter pârâmun-e derâzâ-ye âmizeš Fr.: paramètre de la longueur de mémange In the → mixing length theory, a parameter, α, that relates the → mixing length, l, to the → pressure scale height: α = l/H_{P}. It is usually supposed that α is of order unity. Changes in α correspond to variations in the efficiency of the → convection, hence the transfer of heat. |
mixing length theory (MLT) negare-ye derâzâ-ye âmizeš Fr.: théorie de la longueur de mélange A theory dealing with heat transport by → turbulence which includes an elementary treatment of → convection. The central idea is that an unbalanced → buoyancy force drives a → convective cell to move through a distance, called the → mixing length, before the cell dissolves and joins the ambient medium. In this theory an adjustable → mixing length parameter α is used. The theory, originally due to L. Prandtl (1925), was first applied to the Sun by L. Biermann (1932, Z. Astrophys. 5, 117). |
nominal focal length derâzâ-ye kânuni-ye nâmenâl Fr.: distance focale nominale An approximate value of the → focal length, used for the classification of lenses, mirrors, or cameras, as opposed to the measured one. |