An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The wavelength in a given diffraction order for which the efficiency curve reaches its maximum.

→ blaze; → wavelength.

Mowjtul→ wavelength; beliz→ blaze.

Center of a filter passband measured at 50% of peak transmittance.

→ center; → wavelength.

1) In an interference filter, the wavelength of peak transmission.
2) In a spectrograph, the wavelength corresponding to the middle of the range covered by the grating or grism.

Central, adj. from → center; → wavelength.

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.

→ coherence; → length.

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.

Wavelength at which the transmittance of a filter, or the detectivity of a detector, has fallen to one-half its peak value.

→ cutoff; → wavelength.

The wavelength of the wave associated with a → particle as given by the → de Broglie equation.

→ de Broglie equation; → wavelength.

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.

→ debye (D); → length.

The focal length of an imaging system, which consists of several lenses or mirrors.

→ effective; → focal length.

The distance between the optical center of a lens, or the surface of a mirror, and its focus.

→ focal; → distance.

Deriving the → wavelength of an undulatory phenomenon from its → frequency, and vice versa.
1) For → electromagnetic waves: λ = c / f, where λ is the wavelength, c is the → speed of light in → meters per second and f the frequency in → hertz. It can be written as: λ (m) = 2.998 × 10⁸ / f (Hz).
2) For → sound waves: λ = C / f, where C is the → sound speed. For air at temperature 0°C, λ (m) = 332 / f (Hz).

→ frequency; → wavelength; → conversion.

The distance traveled by light along a straight → geodesic in one → Hubble time. Also called the → Hubble radius, → Hubble distance, and → cosmic horizon.

→ Hubble; → length.

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_s²/Gρ)^1/2 = 0.2 pc (T/10 K)^1/2(n_H2/10⁴ 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.

→ Jeans; → length.

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.

Same as → Lorentz contraction.

→ length; → contraction.

1) Having or being of great length; very long.
2) Tediously verbose; very long; too long (Dictionary.com).

From → length + -y.

Kešnâk "lengthy" (Bardsiri, Kermâni), from kešidan, kašidan "to draw, protract, trail, drag, carry," → tide. Bardesir, Kermân

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.

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; → parameter.

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).

→ mixing; → length; → theory.

An approximate value of the → focal length, used for the classification of lenses, mirrors, or cameras, as opposed to the measured one.

→ nominal; → focal; → length.