zamân-e oft-e âzâd
Fr.: temps de chute libre
The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: tff = (3π/32 ρ0 G)1/2, where ρ0 denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.
Fr.: objets flottants
A population of → substellar objects which are not bound to stars; they are detected in young star clusters. Their masses, estimated from their fluxes, is several Jupiter masses, lower than those of → brown dwarfs. Their formation is not yet explained. Among the envisaged possibilities: 1) These objects form like stars, from protostellar core collapse and subsequent accretion; 2) they form as low-mass members of small groups, and are ejected from the group; 3) they form like planets within circumstellar disks of higher-mass objects, but are ejected either due to internal dynamics or external interactions.
Barâxt, → object; šenâvar "that swims, floats," from šenâ "swimming;" Mid.Pers. šnâz "swim," šnâzidan "to swim;" Av. snā- "to wash, swim;" cf. Skt. snā- "to bathe, to wash;" L. nare, natare "to swim" (Fr. nage, nager, natation; Sp.nadar, natacion).
gosil-e âzâd-âzâd (#)
Fr.: emission libre-libre
The state of being free or at liberty.
From M.E. fredom, from O.E. freodom, from freo "free; noble, joyful," → free.
Âzâdi "freedom," noun from âzâd, → free.
Fr.: loi de Freeman
A statistical finding about "normal" → spiral galaxies, whereby there is an upper limit on the mean central → surface brightness of disks. This value is constant for different spiral types, amounting to 21.65 ± 0.30 mag arcsec2 in the B band.
Named after K. C. Freeman (1970, Ap.J. 160, 811); → law.
1) yax bastan; 2) rocidan
1) To change from the liquid to the solid state by loss of heat; become hardened into ice.
Freeze, from O.E. freosan "turn to ice," from P.Gmc. *freusanan (cf. O.H.G. friosan, Ger. frieren "to freeze"), from *freus-, from PIE base *preus- "to freeze" also "to burn" (cf. Skt. pruśva- "hoar-frost, ice;" L. pruina "hoar-frost," Skt. pruśta- "burnt;" Albanian prus "burning coals;" L. pruna "a live coal").
1) Yax bastan, from yax "ice" + bastan "to bind, shut; to congeal, coagulate." The first component yax, from Av. aexa- "ice, frost," isav-, isu- "icy, chilly," cf. Sarikoli (Pamir dialect) īš "cold," P.Gmc. *isa- (O.N. iss, O.Fris. is, Du. ijs, Ger. Eis). The second component bastan, from Mid.Pers. bastan/vastan "to bind, shut;" Av./O.Pers. band- "to bind, fetter," banda- "band, tie" (cf. Skt. bandh- "to bind, tie, fasten;" PIE *bhendh- "to bind;" Ger. binden; E. bind). 2) Rocidan, → coagulate.
1) yaxbast; 2) roceš
Fr.: gel, congélation
1) The phase transition of a substance passing from the liquid to the solid state;
the opposite of → fusion. In meteorology, the freezing of water.
Verbal noun from → freeze.
Fr.: point de congélation
1) The temperature at which a liquid of specified composition changes into a solid under
a specified pressure.
French Republican Calendar
gâhšomâr-e jomhuri-ye Farâncé
Fr.: Calendrier républicain, Calendrier révolutionnaire français
A calendar composed by Fabre d'Eglantine and others during the French Revolution which divided the year into 12 months of 30 days each, with five odd days called → Sansculottides. The year started at → autumnal equinox and the months were: Vendémiaire (Vintage), Brumaire (Fog), Frimaire (Frost), Nivôse (Snow), Pluviôse (Rain), Ventôse (Wind), Germinal (Buds), Floréal (Flowers), Prairial (Meadows), Messidor (Harvest), Termidor (Heat), Fructidor (Fruits). The week consisted of 10 days, and was called a Décade; each 10th day of Décade (called Décadi) was a day of rest. The calendar was used by the French government for about 12 years, from late 1793 to 1805, when it was suppressed by Napoleon.
M.E. Frensh, French, O.E. Frencisc "of the Franks," from Frank; republican, from republic, from Fr. république, from L. respublica, from res publica "public interest, the state," from res "affair, matter, thing" + publica, feminine of publicus "public;" → calendar.
basâmad (#), feregi (#)
The number of complete oscillations per unit time of a vibrating system. The reciprocal of the → period, T.
From L. frequentia "assembly, multitude, crowd."
Basâmad, from bas "many, much" (Mid.Pers. vas "many, much;"
O.Pers. vasiy "at will, greatly, utterly;" Av. varəmi "I wish,"
vasô, vasə "at one's pleasure or will," from vas- "to will, desire, wish")
+ âmad past stem of âmadan "to occur, to come, to become"
O.Pers. gam- "to come; to go,"
Av. gam- "to come; to go," jamaiti "goes;"
Proto-Iranian *āgmatani; Skt. gamati "goes;"
Gk. bainein "to go, walk, step;" L. venire "to come;"
Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come;
PIE root *gwem- "to go, come").
Fr.: bande de fréquence
A range of frequencies that is continuous between two specified limits, selected from a more extended range of frequencies.
Fr.: dérive de fréquence
An undesired progressive change in an oscillator's frequency with time.
Fr.: décalage de fréquence
The change in the frequency of a wave motion due to the → Doppler effect.
Fr.: spectre de fréquence
A graphical display of the intensity of radiation or energy versus frequency.
Fr.: permutation de fréquence
A mode of observation in radio astronomy in which the telescope remains at a fixed position and data is accumulated while the center of the receiver passband is switched between signal and offset frequencies. Data accumulated at the offset frequency is subtracted from the signal frequency data before storage as a frequency switched scan. → beam switching.
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.
parâš-e Fresnel (#)
Fr.: diffraction de Fresnel
The diffraction effects obtained when either the source of light or observing screen, or both, are at a finite distance from diffracting aperture or obstacle. → Fraunhofer diffraction.
Named after Jean Augustin Fresnel (1788-1827), French physicist, a key figure in establishing the wave theory of light. His earlier work on interference was carried out in ignorance of that of Thomas Young (1773-1829), English physician and physicist, but later they corresponded and were allies; → diffraction.
Fr.: équation de Fresnel
For an electromagnetic wave incident upon the interface between two media with different indices of refraction, one of a set of equations that give the → reflection coefficient and → transmission coefficient at the optical interface. These coefficients depend on the polarization degree of the incident wave.
Fr.: intégrales de Fresnel
Two integrals that involve quadratic equations in the sine and cosine functions and are defined as: C(x) = ∫ cos (πt2/2) dt and C(y) = ∫ sin (πt2/2) dt, integrated from 0 to x. They are quite frequently used in optics studying → Fresnel diffraction and similar topics. The Fresnel integrals are also used in railway and freeway constructions. These integrals may be evaluated to arbitrary precision using → power series. Alternatively the amplitudes may be found graphically by use of → Cornu's spiral.
adasi-ye ferenel (#)
Fr.: lentille de Fresnel
An optical lens composed of a series of rings of glass so curved that they all have the same focus. It is flat on one side and ridged on the other making it possible that nearly every ray of light from the source be re-directed out on a horizontal path. This design enables the construction of lenses of large aperture and short focal length avoiding thus the large weights and volumes of material which would be required in conventional lenses. The first Fresnel lens, designed for use in a lighthouse on the river Gironde, was installed in France in 1823, and by the 1850s many examples were in use everywhere. Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. They can also be used as magnifiers and projection lenses. Nowadays, Fresnel lenses made of optical plastics are widely used for various applications.