paraxial ray partow-e parâse-yi Fr.: rayon paraxial A ray that lies close to and almost parallel to the optical axis and behaves according to paraxial equations. |
primary cosmic rays partowhÃ¢-ye keyhÃ¢ni-ye naxostÃ¢n Fr.: rayons cosmiques primaires The → cosmic rays which arrive on the Earth's → atmosphere from the outer space. The primary cosmic rays are very high energy → protons and to a lesser extent heavier nuclei which rain upon the Earth from all diretions in the outer space. They contain about 90% protons, 7% → alpha particles and about 1% still heavier nuclei of amost all the atoms from Li to Ni ( → mass number< 60). See also: → secondary cosmic rays. |
rainbow ray partow-e rangin-kamân Fr.: rayon d'arc-en-ciel The sunlight incident on a tiny spherical droplet of water. |
ray partow (#) Fr.: rayon 1) Any of the lines or streams in which light appears to radiate from a
luminous body. → light ray. M.E. raie, raye, from O.Fr. rai "ray, spoke," from L. → radius "ray, spoke, staff, rod." Partow, ultimately from Proto-Iranian *pari-tap- "to shine around, radiate away." The first component *pari- "around, about;" cf. Mod.Pers. par-, pirâ- "around, about," from Mid.Pers. pêrâ; O.Pers. pariy "around, about," Av. pairi "around, over," per- "to pass over, beyond;" Skt. pari; Gk. peri "around, about, beyond;" L. per "through;" PIE base *per- "through, across, beyond." The second component *tap- "to shine, radiate;" cf. Mod.Pers. tâbidan, variants tâftan "to shine," tafsidan "to become hot;" Mid.Pers. tâftan "to heat, burn, shine;" taftan "to become hot;" Parthian t'b "to shine;" Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth;" cf. Skt. tap- "; to heat, be/become hot; to spoil, injure, damage; to suffer," tapati "burns;" L. tepere "to be warm," tepidus "warm;" PIE base *tep- "to be warm." |
ray of class 1 partw-e rade-ye 1 Fr.: rayon de classe 1 In → rainbows, a sun ray directly reflected from the surface of a water → droplet. |
ray of class 2 partw-e rade-ye 2 Fr.: rayon de classe 2 In → rainbows, a sun ray transmitted directly through a water → droplet. |
ray of class 3 partw-e rade-ye 3 Fr.: rayon de classe 3 In → rainbows, a sun ray that emerges from a water → droplet after one internal reflection. Rays of class 3 give rise to the → primary rainbow. |
ray of class 4 partw-e rade-ye 4 Fr.: rayon de classe 4 In → rainbows, a sun ray that emerges from a water → droplet after two internal reflections. Rays of class 4 give rise to the → secondary rainbow. |
rayleigh rayleigh (#) Fr.: rayleigh A c.g.s. unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh (R) represents the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second; or, in SI units, 795.775 x 10^{6} photons per square meter per steradian (m^{-2}·sr^{-1}). A dark night sky has a light intensity of roughly 250 R. In honor of the English mathematician and physicist Lord Rayleigh (1842-1919), surname of John William Strutt, Third Baron Rayleigh, whose research ranged over several fields of physics. |
Rayleigh line xatt-e Rayleigh Fr.: 1) droite de Rayleigh; 2) raie de Rayleigh 1) A straight line that connects the points corresponding to the initial and final states
on a graph of pressure versus specific volume for a substance subjected to a
→ shock wave. The slope of the Rayleigh line is proportional
to the square of shock speed. Steeper Rayleigh lines correspond to higher
shock speeds. See also → Hugoniot curve. |
Rayleigh number (Ra) adad-e Rayleigh Fr.: nombre de Rayleigh The ratio of the buoyancy force to the viscous force in a medium. This dimensionless number is used to estimate when convection commences in a fluid. It depends on the density and depth of the fluid, the coefficient of thermal expansion, the gravitational field, the temperature gradient, the thermal diffusivity, and the kinematic viscosity. Convection usually starts when Ra is 1000 or more, while heat transfer is entirely by conduction when Ra is less than 10. |
Rayleigh scattering parâkaneš-e Rayleigh Fr.: diffusion Rayleigh The scattering of light by → particles
of size small compared with the → wavelength of
light. The intensity of the light scattered by unit volume of the medium at an
angle θ to the direction of propagation of the incident light is:
I_{θ} = 8 π^{4}α^{2} N I_{0}
(1 + cos^{2}θ)/(R^{2}λ^{4}),
where α is the → molecular polarizability,
N is the number of scattering molecules,
I_{0} is intensity of the incident light, λ is the wavelength, and
R is the distance from the scatterer.
The fourth power dependence on wavelength means that blue light is
much more strongly scattered than red light from a medium containing very fine particles.
The air molecules, mostly → nitrogen (78%) and
→ oxygen (21%) are some 1,000 times larger than
→ visible light wavelengths.
This accounts for the bluish appearance of smoke and of clear sky when the observation is not
along the direction of illumination. The setting Sun, seen through a considerable
thickness of atmosphere appears reddish because long wave radiation predominates in
the transmitted light. → rayleigh; → scattering. |
Rayleigh's criterion sanjidâr-e Rayleigh Fr.: critère de Rayleigh A criterion for the instability of a basic swirling flow with an arbitrary dependence of angular velocity Ω(r) on the distance r from the axis of rotation. This states that in → inviscid fluids: Ω(r) < 0 for instability, where Ω = (1/r^{3}) (d/dr)(r^{4}Ω^{4}). |
Rayleigh-Jeans law qânun-e Rayleigh-Jeans(#) Fr.: loi de Rayleigh-Jeans A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT/λ^{4}). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe. |
Rayleigh-Jeans spectrum binâb-e Rayleigh-Jeans Fr.: spectre Rayleigh-Jeans The part of → electromagnetic spectrum approximated by the → Rayleigh-Jeans law. |
Rayleigh-Taylor instability nâpâydâri-ye Rayleigh-Taylor Fr.: instabilité Rayleigh-Taylor A type of hydrodynamical instability between two fluids of different densities, which occurs when the heavy fluid lies above the lighter fluid in a gravitational field. More generally a material interface is said to be Rayleigh-Taylor unstable whenever the fluid acceleration has an opposite direction to the density gradient. → rayleigh; → Taylor number; → instability. |
reflected ray partow-e bâztâbidé (#) Fr.: rayon réfléchi A → light ray that is reflected from a surface. |
refracted ray partow-e šekasté (#) Fr.: rayon réfracté A → light ray that undergoes a change of velocity and direction, as a result of interaction with the material medium in which it travels. |
secondary cosmic rays partowhâ-ye keyhâni-ye dovomân Fr.: rayons cosmiques secondaires A burst of secondary charged and neutral particles arising when → primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The collision produces mostly → pions (π), along with some → kaons (K), → antiprotons, and → antineutrons. Neutral pions very quickly decay, usually into two → gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a → muon and a → neutrino. The fragments of the incoming nucleus also interact again, also producing new particles. |
soft gamma-ray repeater (SGR) bâzgaršgar-e partowhâ-ye gâmmâ-ye narm Fr.: répéteur des rayons gamma mous Same as → soft gamma repeater (SGR). |