Fr.: image virtuelle
Optics: An image formed inside an instrument at the point where diverging rays would cross if they were extended backward into the instrument. Such an image cannot be obtained on a screen placed at its apparent position, since the rays do not pass through that point. → real image.
Fr.: observatoire virtuel
An international initiative by the astronomical community to allow global electronic access to the available astronomical data archives of space and ground-based observatories. It also aims to enable data analysis techniques through a coordinating entity that provides common standards, wide-network bandwidth, and state-of-the-art analysis tools. The Virtual Observatory is also intended for re-using data for scientific objectives different from the original ones, in order to optimize the science return of astronomical observations. The Virtual Observatory's capabilities are enabled through the use of standard protocols for registering the existence and location of data and for requesting data that satisfies the user's interests. These standards are developed on an international basis through the → IVOA. The cornerstone of the Virtual Observatory is → interoperability.
Fr.: particule virtuelle
A subatomic particle that, according to the uncertainty principle, comes into being out of energy fluctuations of the "vacuum" and lasts for extremely short periods of time. An electron-positron pair can exist only about 4 x 10-21 seconds. The lifetime increases as the mass and energy involved decreases. Virtual particles are real and have measurable effects, but cannot be directly observed, according to the uncertainty principle. → vacuum polarization.
Fr.: travail virtuel
virtual work principle
parvaz-e kâr-e virâgin
Fr.: principe du travail virtuel
In → analytical mechanics, a principle whereby it is necessary and sufficient for the equilibrium of any material system with ideal constraints that the sum of the elements of work, performed by the applied forces acting on the system in any virtual displacement, be equal to zero (if all constraints are bilateral) or less than zero (if some of the constraints are unilateral).
An instrument used to measure the → viscosity of a liquid. Same as viscosimeter.
Same as → viscometer.
The property of a → fluid that resists the force tending to cause the fluid to flow. Viscosity may be thought of as the internal → friction of two fluid layers which flow parallel to each other at different speeds. The cause of viscosity is the transport of → momentum by the molecules from one layer to the other. Viscosity is given by η = φ.u.λ.ρ, where φ is a coefficient which depends on the nature of the interaction between the molecules, u is the average velocity of thermal motion of the molecules, λ is the → mean free path, and ρ the → density of the fluid. Also called → dynamic viscosity or → absolute viscosity. See also → kinematic viscosity.
M.E., from M.Fr. viscous, from L. viscosus "sticky," from viscum "anything sticky; mistletoe."
From vošk "a kind of sticky gum" + -sân suffix of similarity, from sân "way, manner."
Fr.: dissipation visqueuse
šârre-ye vošksân (#)
Fr.: fluide visqueux
A fluid whose viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid.
Fr.: force visqueuse
The force per unit volume or per unit mass arising from the action of tangential stresses in a moving → viscous fluid.
diyâri, padidâri (#)
General: The state or fact of being visible.
karyâ-ye diyâri, ~ padidâri
Fr.: fonction de visibilité
The Fourier transform of a source's brightness distribution, weighted by the characteristics of the interferometer's antennas.
hâmon-e diyâri, ~ padidâri
Fr.: plan de visibilité
In interferometry, the projection of a baseline onto the plane normal to the source direction defining a vector in (u,v) space, measured in wavelength units.
diyâr (#), padidâr (#), peydâ (#)
1) Capable of being seen by, or perceptible to, the human eye.
M.E., from O.Fr. visible, from L. visibilis "that may be seen," from visus, p.p. of videre "to see;" cognate with Pers. bin, present stem of didan "to see" (Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" cf. Skt. veda "I know;" Gk. oida "I know," idein "to see;" PIE base *weid- "to know, to see").
Diyâr "visible" in several dialects, e.g. Âštiyâni, Malâyeri,
Širâzi, Tabari, related to
didan "to see;" Mid.Pers. ditan "to see, regard, catch sight of,
contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)-
"to see," didāti "sees;" cf. Skt. dhī- "to perceive,
think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka
ofoq-e padidâr, ~ diyâr
Fr.: horizon apparent
That line where Earth and sky appear to meet, and the projection of this line upon the celestial sphere. Also known as the apparent horizon.
nur-e diyâr, ~ didani
Fr.: lumière visible
The portion of the → electromagnetic radiation that can be seen by the human → eye. The → wavelengths extend from about 400 nm (violet) to 750 nm (red). The wavelengths of various colors of the visible spectrum are as follows: → violet: 390-455 nm; → blue: 455-492 nm; → green: 492-577; → yellow: 577-597; → orange: 597-622; → red: 622-780 nm.
binâb-e diyâr, ~ didani
Fr.: spectre visible
M.E., from O.Fr. vision, from L. visionem (nominative visio) "act of seeing, sight, thing seen," from p.p. stem of videre "to see," cognate with Pers. bin, present stem of didan "to see" (Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" cf. Skt. veda "I know;" Gk. oida "I know," idein "to see;" PIE base *weid- "to know, to see").
Negâh, "look," from Mid.Pers. nikâh "look, glance, observation;" Proto-Iranian *ni-kas- "to look down," from ni- "down, below," → ni- (PIE), + *kas- "to look, appear;" cf. Av. nikā-, nikāta- (in the name of the 15-th nask) "that which is observed," ākas- "to look;" Mid.Pers. âkâh, Mod.Pers. âgâh "aware, knowing;" Skt. kāś- "to become visible, appear;" Ossetic kast/kaesyn "to look;" did, from didan "to see" (Mid.Pers. ditan "to see, regard, catch sight of, contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)- "to see," didāti "sees;" cf. Skt. dhī- "to perceive, think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka "have seen").