Fr.: longueur d'onde de blaze
The wavelength in a given diffraction order for which the efficiency curve reaches its maximum.
Fr.: longueur d'onde centrale
Center of a filter passband measured at 50% of peak transmittance.
Fr.: longueur d'onde centrale
1) In an interference filter, the wavelength of peak
Fr.: longueur de cohérence
The distance over which an electromagnetic wave train maintains a specified degree of coherence.
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.
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.
de Broglie wavelength
mowjtul-e de Broglie
Fr.: longueur d'onde de Broglie
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: λD = 743(Te/ne)1/2, where Te is the electron temperature in → electron-volts and ne 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.
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.
Fr.: longueur de Hubble
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: λJ = (π cs2/Gρ)1/2 = 0.2 pc (T/10 K)1/2(nH2/104 cm-3)-1/2, where cs is the → sound speed, G is the → gravitational constant, ρ is the gas density, T is the gas temperature, and nH2 is the molecular hydrogen density.
derâzâ (#), tul (#)
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.
Fr.: contraction de longueur
Same as → Lorentz contraction.
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
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 (HP), l = αHP, 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/HP. 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.