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photon escape time zamân-e goriz-e foton Fr.: temps d'échappement des photons The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10^{4} years for a constant density Sun. |
photon hardening saxteš-e foton Fr.: durcissement des photons An effect occurring in the outer zones of → H II regions where the number of high-energy ultraviolet photons with energies well above the → ionization potential of hydrogen increases with respect to the number of → Lyman continuum photons. The effect is due to stronger absorption of weaker photons. |
photon noise nufe-ye foton Fr.: bruit de photons An intrinsic noise caused by the quantum nature of light. Same as → quantum noise. |
photon sphere sepehr-e foton Fr.: sphère de photons A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light. However, only photons can have such a velocity; hence the term "photon sphere." For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c^{2} = 3 R_{S}/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_{S} is the → Schwarzschild radius. For a rotating, → Kerr black hole, the situation is much more complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction. |
photon tiring limit hadd-e xastegi-ye foton Fr.: limite par fatigue du photon The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: L_{wind} = Mdot (v_{∞}^{2}/2 + GM/R) = Mdot (v_{∞}^{2}/2 + v_{esc}^{2}/2). For L_{wind} = L_{*}, the mass loss rate is Mdot_{max} = 2L_{*}/(v_{∞}^{2} + v_{esc}^{2}). Following Owoki & Gayly (1997), Mdot_{tir} is the maximum mass loss rate when the wind just escapes the gravitational potential, with v_{∞} tending toward zero. Mdot_{tir} is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10^{-4} Msun yr^{-1} in even the most luminous stars. → photon; tiring, from tire "to weary; become weary," → tired; → limit. |
photon-baryon plasma plâsmâ foton-bâriyon Fr.: plasma photon-baryon The plasma filling space before the → recombination epoch that mainly consisted of → cosmic microwave background radiation photons, electrons, protons, and → light elements. |
photonics fotonik Fr.: photonique The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. |
physical adsorption baršam-e fiziki Fr.: adsorption physique Same as → physisorption. → physical; → adsorption. |
physical condition butâr-e fiziki Fr.: condition physique The state of a → physical system regarding its temperature, density, pressure, etc. at a given time. |
physical constant pâyâ-ye fiziki (#) Fr.: constante physique A fundamental → physical quantity that is generally believed to be both universal in nature and constant in time. |
physical dimension vâmun-e fiziki Fr.: dimension physique Any of basic physical quantities, such as mass, length, time, electric charge, and temperature in terms of which all other kinds of quantity can be expressed. |
physical libration halâzân-e fiziki, roxgard-e ~ Fr.: libration physique A real periodic variation in the rotation rate of a celestial object, as distinct from a → geometrical libration. In particular, slight oscillations in the → Moon's rotation caused by the → gravitational attraction of the Earth on the → equatorial bulge of the Moon's near side. The Moon's physical libration is about 0.03° in longitude and about 0.04° in latitude. |
physical phenomenon padide-ye fiziki (#) Fr.: phénomène physique A natural → phenomenon that can be explained by → physical laws. → physical; → phenomenon. |
physisorption fizi-šameš Fr.: physisorption A kind of → adsorption in which the forces involved are → intermolecular → van der Waals forces. Same as → physical adsorption. See also → chemisorption. |
piecewise continuous function karyâ-ye peyvaste-ye tekke-yi Fr.: fonction continue par morceaux A function f(x) in an interval if :1) the interval can be divided into a finite number of pieces in each of which f(x) is continuous, and 2) the limits of f(x) as x approaches the boundary point of each piece are finite. In other words, a piecewise continuous function is one that is made up of a finite number of continuous pieces. → piecewise; → continuous; → function. |
pincushion distortion cowlegi-ye bâleštaki Fr.: distorsion en coussinet An → aberration of a → lens → system in which → magnification increases with → distance from the → optical axis, whereby → horizontal and → vertical lines bend inward toward the → center of the → field. Opposite of → barrel distortion. Pincushion, from pin, from M.E. pinne, O.E. pinn "peg;" cf. D. pin, Ger. Pinne; perhaps from L. pinna "feather, quill" + cushion, M.E. cuisshin, O.Fr. coissin (Fr. coussin) a variant of V.L. *coxinum, either from L. coxa "hip, thigh," or from L. culcita "mattress;" → distortion. Cowlegi, → distortion; bâleštaki, adj. of bâleštak, diminutive of bâlešt, variant bâleš "cushion, pillow," Mid.Pers. bâlišn, bâlên "cushion, pillow;" Av. barəiš- "pillow, cushion;" cf. Skt. barhis- "straw, a bed or layer of kusa grass strewed over the sacrificial ground." |
pion piyon (#) Fr.: pion An unstable nuclear particle of mass intermediate between that of a proton and an electron; also called π meson. From pi (meson) + → -on. |
piston piston (#) Fr.: piston A disk or cylindrical part tightly fitting and moving within a cylinder, either to compress or move a fluid collected in the cylinder, as air or water, or to transform energy imparted by a fluid entering or expanding inside the cylinder, as compressed air, explosive gases, or steam, into a rectilinear motion usually transformed into rotary motion by means of a connecting rod (Dictionary.com). From Fr. piston, from M.Fr. piston "large pestle," from O.It. pistone "a piston," from pestare "to pound," from L.L. pistare, from pistare "to pound." |
place-value notation nemâdgân-e jâ-arezeši Fr.: notation positionnelle A mathematical notation system in which the → numerals get different values depending on their position relative to the other numerals. Same as → positional notation and → positional number system. |
Planck constant pâyâ-ye Planck (#) Fr.: constante de Planck A physical constant that determines the energy of quantum as a function of its frequency; symbol h. Also called → Planck's constant. On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly 6.626 070 15 × 10^{-34} J s. The → reduced Planck constant, ħ = h / 2π, is also called the → Dirac constant. |
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