Fr.: matière intergalactique
Very low density material found indirectly in the space between galaxies. → intergalactic absorption.
Fr.: milieu intergalactique
The matter or environment between the galaxies of a → cluster.
ionization correction factor (ICF)
karvand-e aršâyeš-e yoneš
Fr.: facteur de correction d'ionisation
A quantity used in studies of → emission nebulae to convert the → ionic abundance of a given chemical element to its total → elemental abundance. The elemental abundance of an element relative to hydrogen is given by the sum of abundances of all its ions. In practice, not all the ionization stages are observed. One must therefore correct for unobserved stages using ICFs. A common way to do this was to rely on → ionization potential considerations. However, → photoionization models show that such simple relations do not necessarily hold. Hence, ICFs based on grids of photoionization models are more reliable. Nevertheless here also care should be taken for several reasons: the atomic physics is not well known yet, the ionization structure of a nebula depends on the spectral energy distribution of the stellar radiation field, which differs from one model to another, and the density structure of real nebulae is more complicated than that of idealized models (see, e.g., Stasińska, 2002, astro-ph/0207500, and references therein).
Fr.: fractionnement isotopique
A slight difference between the → abundances of → isotopes of the same → chemical element owing to → physical or → chemical → processes. It results in the → enrichment or → depletion of an isotope. Same as → isotopic fractionation.
Fr.: fractionnement isotopique
Same as → isotope fractionation.
Fr.: contraction de Kelvin-Helmholtz
After the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907) and the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → contraction.
Fr.: facteur de Landé
The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1.
After Alfred Landé (1888-1976), a German-American physicist, known for his contributions to quantum theory; → facteur.
Fr.: dernier contact
Same as → fourth contact at an eclipse.
law of refraction
qânun-e šekast (#)
Fr.: loi de réfraction
One of the two laws governing → refraction of light when it enters another transparent medium: a) The → incident ray, normal to the surface, and refracted ray, all lie in the same plane. b) → Snell's law is satisfied.
Fr.: contraction de longueur
Same as → Lorentz contraction.
line of action
xatt-e žireš, ~ koneš
Fr.: ligne d'action
Of a force, the straight line along which the force → vector is directed. The action of a force on a → rigid body does not change when its point of application is displaced along the line of action. Hence, forces applied to a rigid body can be regarded as non-localized, or sliding, vectors.
1) The act or process of liquefying or making liquid.
Fr.: contraction de Lorentz
The decrease in the length of a body moving in the direction of its length as measured by an observer situated in that direction. The shortening factor is [1 - (v/c)2]1/2, where v is the relative velocity and c light speed.
Fr.: facteur de Lorentz
In → special relativity, an important parameter which appears in several equations, including → time dilation, → length contraction, and → relativistic mass. It is defined as γ = 1 / [1 - (v/c)2]1/2 = dt/dτ, where v is the velocity as observed in the reference frame where time t is measured, τ is the proper time, and c the → velocity of light. Same as Lorentz γ factor.
Fr.: fraction de masse
The fractional amount (by mass) of a given element or nuclide in a given composition.
Fr.: impact météoritique
A striking of a meteorite against another body, especially the solar system planets or satellites.
Fr.: réaction nucléaire
A process in which the energy, composition, or structure of an atomic nucleus changes.
Fr.: réacteur nucléaire
A device in which a nuclear fission chain reaction is maintained and controlled for the production of nuclear energy or radioactive isotopes.
nulling fraction (NF)
Fr.: fraction de phase d'arrêt
Fr.: activité optique
The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).