# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 frequency band   باند ِ بسامد   bând-e basâmadFr.: bande de fréquence   A range of frequencies that is continuous between two specified limits, selected from a more extended range of frequencies.→ frequency; → band. frequency drift   دلک ِ بسامد   delek-e basâmadFr.: dérive de fréquence   An undesired progressive change in an oscillator's frequency with time.→ frequency; → drift. frequency shift   کیب ِ بسامد   kib-e basâmadFr.: décalage de fréquence   The change in the frequency of a wave motion due to the → Doppler effect.→ frequency; → shift. frequency spectrum   بیناب ِ بسامد   binâb-e basâmadFr.: spectre de fréquence   A graphical display of the intensity of radiation or energy versus frequency.→ frequency; → spectrum. frequency switching   دگربانی ِ بسامد   degarbâni-ye basâmadFr.: 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; → switching. frequency to wavelength conversion   هاگرد ِ بسامد به موج-طول   hâgard-e basâmad bé mowj-tulFr.: conversion fréquence / longueur d'onde   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. Fresnel diffraction   پراش ِ فرنل   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. Fresnel equation   هموگش ِ فرنل   hamugeš-e FresnelFr.: 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. Fresnel integrals   درستال‌های ِ فرنل   dorostâlhâ-ye FresnelFr.: 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. Fresnel lens   عدسی ِ فرنل   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.→ Fresnel diffraction; → lens. Fresnel mirror   آینه‌ی ِ فرنل   âyene-ye Fresnel (#)Fr.: miroir de Fresnel   A pair of plane mirrors which are slightly inclined to one another. It is used for producing two coherent images in interference experiments. Fresnel rhomb   لوزی‌وار ِ فرنل   lowzivâr-ye FresnelFr.: parallélépipède de Fresnel   A piece of special glass in the form of an oblique → parallelepiped so cut that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing two internal reflections. It is a type of → quarter-wave retarder used to produce a → circularly polarized light from a → plane polarized light, or the reverse. Fresnel's biprism   دومنشور ِ فرنل   domanšur-e Fresnel (#)Fr.: biprisme de Fresnel   An optical element consisting of two small angle → prisms, joined together at their bases, used to produce two → coherent sources. The thin double prism refracts the light from a source into two overlapping beams, which produce → interference fringes. With this experiment Fresnel was able to produce interference without relying upon → diffraction to bring the interfering beams together.→ Fresnel diffraction; → bi-; → prism. Fresnel's mirrors   آینه‌های ِ فرنل   âyenehâ-ye Fresnel (#)Fr.: miroirs de Fresnel   Two plane mirrors, fitted side by side at a small angle, used to create two mutually → coherent sources in a famous → interference experiment first suggested by A. Fresnel. A point source reflected at the mirrors appears as a pair of → virtual light sources, positioned close together, which interfere with each other due to their → coherence. This arrangement removes the problem that two separate light sources do not produce observable interference on account of their incoherence. Same as Fresnel's double mirror. See also → Fresnel's biprism, → Lloyd's mirror. friction   مالش   mâleš (#)Fr.: frottement   The resisting force offered by one body to the relative motion of another body in contact with the first.From L. frictionem "a rubbing, rubbing down," from fricare "to rub."Mâleš, verbal noun of mâlidan "to rub," from, variants parmâs "contact, touching," marz "frontier, border, boundary," Mid.Pers. mâlitan, muštan "to rub, sweep;" Av. marəz- "to rub, wipe," marəza- "border, district;" PIE base *merg- "boundary, border;" cf. L. margo "edge" (Fr. marge "margin"); P.Gmc. *marko; Ger. Mark; E. mark, margin. Fried parameter   پارامون ِ فرید   pârâmun-e FriedFr.: paramètre de Fried   One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r0, becomes smaller. The Fried parameter is wavelength dependent: r0 ∝ λ6/5. On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.Named after David L. Fried, who defined the parameter 10 1966; → parameter. Friedmann equation   هموگش ِ فریدمن   hamugeš-e FriedmannFr.: équation de Friedmann   An equation that expresses energy conservation in an → expanding Universe. It is formally derived from → Einstein's field equations of → general relativity by requiring the Universe to be everywhere → homogeneous and → isotropic. It is expressed by H2(t) = (8πG)/(3c2)ε(t) - (kc2)/R2(t), where H(t) is the → Hubble parameter, G is the → gravitational constant, c is the → speed of light, ε(t) is the → energy density, k is the → curvature of space-time, and R(t) is the → cosmic scale factor. See also → Big Bang, → accelerating Universe. See also → Friedmann-Lemaitre Universe.Named after the Russian mathematician and physical scientist Aleksandr Aleksandrovich Friedmann (1888-1925), who was the first to formulate an → expanding Universe based on Einstein's theory of → general relativity ; → equation. Friedmann-Lemaitre Universe   گیتی ِ فریدمن-لو‌متر   giti-ye Friedmann-LemaîtreFr.: univers Friedmann-Lemaître   One of the first → cosmological models to incorporate Einstein's → general relativity, predicting that → galaxies should be → receding from each other due to → cosmic expansion.→ Friedmann equation; Georges Edouard Lemaître (1894-1966), a Belgian cosmologist and priest who proposed a first sketch of the → Big Bang theory; → universe. frigid   سجن   sajan (#)Fr.: glacial, froid, glacé, frigide   Very cold in temperature.From Latin frigidus "cold, chill, cool," from stem of frigere "be cold;" related to noun frigus "cold, coldness, frost," from PIE root *srig- "cold;" cf. Gk. rhigos "cold, frost."Sajan "very cold," variants šaja,, sajâm, šajad, Oss. I. syjyn/syd, D. sujun/sud "to freeze," Yaghnobi ši-, Yazghulami šed/šiy- "to freeze," Shughni šitô , Sariqoli š(i)tu "cold, ice." Proto-Ir. *saiH-/siH- "to freeze." Skt. syā- "to freeze, coagulate, become rigid." fringe   فریز   fariz (#)Fr.: frange   1) One of the alternating bright or dark bands produced by → interference or → diffraction. 2) Wavy patterns due to the layered structure of → CCDs. These interference effects are prominent when emission lines such as the night-sky emissions are present.From M.E. frenge, from O.Fr. frange, from V.L. *frimbia, metathesis of L. fimbriæ "fibers, threads, fringe," of uncertain origin.Fariz, contraction of farâviz "fringe, lace, edging," from far-, par-, variant pirâ- "around, about" (Mid.Pers. pêrâ; O.Pers. pariy "around, about," Av. pairi "around, over;" Skt. pari; Indo-Iranian *pari- "around;" PIE base *per- "through, across, beyond;" cf. Gk. peri "around, about, beyond;" L. per "through") + âviz "anything suspended; a place where things are hung up; a border, margin," from âvixtan, âvizidan "to hang, suspend;" Mid.Pers. âwextan "to hang;" Av. vij- "to shake, swing," frauuaēγa- "swinging forward;" cf. Skt. vej- "to dart up or back, move up;" Proto-Iranian *uij- "to shake, swing."