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law of reflection qânun-e bâztâb (#) Fr.: loi de réflexion One of the two laws governing reflection of light from a surface: a) The → incident ray, normal to surface, and reflected ray lie in the same plane. b) The → angle of incidence (with the normal to the surface) is equal to the → angle of reflection. → law; → reflection. |
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. → law; → refraction. |
law of sines qânun-e sinushâ Fr.: loi des sinus In any triangle the sides are proportional to the sines of the opposite angles: a/sin A = b/sin B = c/sin C, where A, B, and C are the three vertices and a, b, and c are the corresponding sides. |
lawrencium lawrensiom (#) Fr.: lawrencium An artificially produced → radioactive→ chemical element; symbol Lr (formerly Lw). → Atomic number 103; → atomic weight of most stable isotope 262; → melting point about 1,627°C; → boiling point and → specific gravity unknown; → valence +3. The longest half-life associated with this unstable element is 3.6 hour ^{262}Lr. Credit for the first synthesis of this element in 1971 is given jointly to American chemists from the University of California laboratory in Berkeley under Albert Ghiorso and the Russian team at the Joint Institute for Nuclear Reactions lab in Dubna, under Georgi N. Flerov. Named the American physicist Ernest 0. Lawrence (1901-1958), who developed the → cyclotron, + → -ium. |
laws of dynamics qânunhâ-ye tavânik Fr.: lois de dynamique The three basic laws of → dynamics which were first formulated by Isaac Newton in his classical work "Mathematical Principles of Natural Philosophy" published in 1687. → Newton's first law of motion; → Newton's second law of motion; → Newton's third law of motion. |
layer lâyé (#) Fr.: couche A thickness of some material laid on or spread over a surface. From M.E. leyer, legger + -er. The first element from layen, leggen "to lay," from O.E. lecgan; cf. Du. leggen; Ger. legen; O.N. legja; Goth. lagjan Lâyé "layer," from lâ, lây "fold" + -é nuance suffix of nouns. |
Layzer-Irvine equation hamugeš-e Layzer-Irvine Fr.: équation de Layzer-Irvine The ordinary Newtonian energy conservation equation when expressed in expanding cosmological coordinates. More specifically, it is the relation between the → kinetic energy per unit mass associated with the motion of matter relative to the general → expansion of the Universe and the → gravitational potential energy per unit mass associated with the departure from a homogeneous mass distribution. In other words, it deals with how the energy of the → Universe is partitioned between kinetic and potential energy. Also known as → cosmic energy equation. In its original form, the Layzer-Irvine equation accounts for the evolution of the energy of a system of → non-relativistic particles, interacting only through gravity, until → virial equilibrium is reached. But it has recently been generalized to account for interaction between → dark matter and a homogeneous → dark energy component. Thus, it describes the dynamics of local dark matter perturbations in an otherwise homogeneous and → isotropic Universe (P. P. Avelino and C. F. V. Gomes, 2013, arXiv:1305.6064). W. M. Irvine, 1961, Ph.D. thesis, Harvard University; D. Layzer, 1963, Astrophys. J. 138, 174; → equation. |
Le Chatelier's Principle parvaz-e Le Chatelier Fr.: principe de Le Chatelier A change in one of the variables (such as temperature, pressure, and concentration of various species) that describe a system at equilibrium produces a shift in the position of the equilibrium that counteracts the effect of this change. Named after the French chemist and engineer Henry Louis Le Chatelier (1850-1936); → principle. |
lead sorb (#) Fr.: plomb A metallic chemical element; symbol Pb (L. plumbum, of unknown origin). Atomic number 82; atomic weight 207.2; melting point 327.502°C; boiling point about 1,740°C. One of the oldest metals used by humanity, because of its relatively low melting point. O.E. lead, from W.Gmc. *loudhom (cf. O.Fris. lad, M.Du. loot "lead," Ger. Lot "weight, plummet"). Sorb, from Mid.Pers. srub "lead;" Av. sru- "lead." |
leader spot lakke-ye pišrow (#) Fr.: tache de tête In a → sunspot group, the first spot to form in the direction of rotation and the last to disappear. It is the largest, the strongest in magnetic intensity, and the closest to the solar equator among the group sunspots. See also → follower spot. From M.E. leder(e), O.E. lædan "cause to go with one, lead," from W.Gmc. *laithjan (cf. O.S. lithan, O.N. liða "to go," O.H.G. ga-lidan "to travel," Goth. ga-leiþan "to go"); → spot. Lakké, → spot; pišrow "leader, forerunner," from piš "in front, forward, before" (Mid.Pers. pêš "before, earlier;" O.Pers. paišiya "before; in the presence of") + row "going," present stem of raftan "to go, elapse, glide by, depart" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack"). |
leaf barg (#) Fr.: feuille One of the organs, usually green and flat, growing from the side of a stem or branch or direct from the root of a tree or plant. A leaf constitutes a unit of the foliage, and functions primarily in food manufacture by → photosynthesis. M.E. leef, lef; O.E. leaf; cf. O.S. lof, O.N. lauf, O.Fris. laf, Du. loof, O.H.G. loub, Ger. Laub. Barg "leaf" (Tabari, Gilaki valg, balg; Kurd. belg, balk, Semnâni valg); Mid.Pers. warg "leaf;" Av. varəka- "leaf;" cf. Skt. valká- "bark, bast, rind;" Russ. volokno "fibre, fine combed flax." |
leap andarheli Fr.: bissextile, intercalaire In a calendar, having an extra day or month inserted. O.E. hleapan "to jump, run, leap" (cf. O.S. hlopan, O.N. hlaupa, O.Fris. hlapa, Du. lopen, Ger. laufen "to run," Goth. us-hlaupan "to jump up"), of uncertain origin, with no known cognates beyond Germanic. The noun is O.E. hlyp (Anglian *hlep). Noun in leap year, so called from its causing fixed festival days to "leap" ahead one day in the week. Andarheli, from andarhel, verbal noun of andarhelidan, andarheštan "to insert," from andar-→ inter- + helidan, heštan "to place, put" from Mid.Pers. hištan, hilidan "to let, set, leave, abandon;" Parthian Mid.Pers. hyrz; O.Pers. hard- "to send forth;" ava.hard- "to abandon;" Av. harəz- "to discharge, send out; to filter," hərəzaiti "releases, shoots;" cf. Skt. srj- "to let go or fly, throw, cast, emit, put forth;" Pali sajati "to let loose, send forth." |
leap day ruz-e andarheli Fr.: jour intercalaire The extra day added to a solar calendar (e.g. Gregorian, Iranian) in a leap year. |
leap month mâh-e andarheli Fr.: mois intercalaire An intercalary month employed in some calendars to preserve a seasonal relationship between the Lunar and Solar cycles. → embolismic month. |
leap second sâniyeh-ye andarheli Fr.: seconde intercalaire A one-second added between 60s and 0s at announced times to keep the Coordinated Universal Time (UTC), counted by atomic clocks, within 0s.90 of mean solar time (UT1). Generally, leap seconds are added at the end of June or December. |
leap year sâl-e andarheli Fr.: année bissextile In solar calendars the year that contains 366 days, instead of 365, in order to keep the calendar in pace with the real solar time. |
leap year rule razan-e sâl-e andarheli Fr.: régle des années bissextiles The three criteria that identify → leap years in the → Gregorian calendar: 1) The year must be evenly divisible by 4; 2) If the year can be evenly divided by 100, it is not a leap year, unless; 3) The year is also evenly divisible by 400. This means that in the Gregorian calendar, the years 1600, 2000, and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years. |
least kucektarin, kamtarin Fr.: moindre, plus petit Smallest in size, amount, degree, etc. From M.E. leest(e), O.E. læst, læsest "smallest" (superlative of læs "smaller, less"). Kucaktarin, kamtarin, superlatives of kucak and
kam, → small. |
least common multiplier (LCM) kucektarin bastâgar-e hamdâr Fr.: plus petit commun multiple Of two or more → integers, the smallest positive number that is divisible by those integers without a remainder. → least; → common; → multiplier. |
least squares kucaktarin cârušhâ Fr.: moindres carrés Any statistical procedure that involves minimizing the sum of squared differences. |
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