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frequency shift kib-e basâmad Fr.: décalage de fréquence The change in the frequency of a wave motion due to the → Doppler effect. |
frequency spectrum binâb-e basâmad Fr.: spectre de fréquence A graphical display of the intensity of radiation or energy versus frequency. |
frequency switching degarbâni-ye basâmad 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. → frequency; → wavelength; → conversion. |
Fresnel diffraction 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 hamugeš-e Fresnel 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. → Fresnel diffraction; → equation. |
Fresnel integrals dorostâlhâ-ye Fresnel Fr.: intégrales de Fresnel Two integrals that involve quadratic equations in the sine and cosine functions and are defined as: C(x) = ∫ cos (πt^{2}/2) dt and C(y) = ∫ sin (πt^{2}/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 diffraction; → integral. |
Fresnel lens 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 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 diffraction; → mirror. |
Fresnel rhomb lowzivâr-ye Fresnel Fr.: 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 diffraction; → rhombus. |
Fresnel's biprism 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 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. → Fresnel diffraction; → mirror. |
friction 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 pârâmun-e Fried Fr.: 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 r_{0}, becomes smaller. The Fried parameter is wavelength dependent: r_{0} ∝ λ^{6/5}. 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 hamugeš-e Friedmann Fr.: 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 H^{2}(t) = (8πG)/(3c^{2})ε(t) - (kc^{2})/R^{2}(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 giti-ye Friedmann-Lemaître Fr.: 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 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 fariz (#) Fr.: frange 1) One of the alternating bright or dark bands produced by
→ interference or → diffraction. 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." |
front ru, pišân Fr.: face, front 1) The part or side of anything that faces forward.
→ ionization front. From O.Fr. front "forehead, brow," from L. frontem "forehead," perhaps lit. "that which projects," from PIE *bhront-, from base *bhren- "to project, stand out." Pišân, from pišâni "front, forehead," from piš
"before; in front," from Mid.Pers. pêš "before, earlier;"
O.Pers. paišiya "before; in the presence of" + -ân suffix
of place and time. |
front-end piš-tah Fr.: A device containing a radio-frequency amplifier and associated cryogenic systems, routers, and converters (mixers), whose input is the voltage from a receptor and whose output is an intermediate-frequency signal. → back-end. → front + end, from O.E. ende, from P.Gmc. *andja, originally "the opposite side," from PIE *antjo "end, boundary," from base *anta-/*anti- "opposite, in front of, before." Piš-tah, from piš, → front, + tah "end;" Mid.Pers. tah "bottom." The origin of this term is not clear. It may be related to Gk. tenagos "bottom, swamp," Latvian tigas "depth;" PIE *tenegos "water bottom." |
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