Van Cittert-Zernike theorem
van farbin-e Cittert-Zernike
Fr.: théorème de Cittert-Zernike
In → Young's experiment of → interference with double apertures, if a monochromatic source is a considerable distance from the → aperture plane and aperture separation is small, → fringe visibility from an extended source is proportional to the → Fourier transform of the source's spatial distribution. The transform variable is the angular separation of the aperture-plane sampling points divided by the wavelength. The van Cittert-Zernike Theorem is at the heart of → aperture synthesis.
Developed independently by Dutch physicists Pieter Hendrick van Cittert (1889-1959) in 1934 and Frits Zernike (1888-1966) in 1939; → theorem.
van den Bergh catalogue (vdB)
kâtâlog-e van den Bergh
Fr.: catalogue de van den Bergh
A catalog of → reflection nebulae containing 158 objects. The catalog gives information for all BD and CD stars north of δ = -33 deg which are surrounded by reflection nebulosity visible on both the blue and red prints of the → Palomar Observatory Sky Survey .
van den Bergh, S., 1966, AJ, 71, 990; → catalog
van der Waals equation
hamugeš-e van der Waals
Fr.: équation de van der Waals
An → equation of state that satisfactorily describes the behavior of → real gass over a wide range of temperatures and pressures. It is derived from considerations based on kinetic theory, taking into account to a first approximation the size of a molecule and the cohesive forces between molecules: (P + a / V2) (V - b) = RT, where P, V, and T are pressure, volume, and temperature and R the gas constant. a and b are characteristic constants for a given substance. For a = b = 0, the van der Waals equation reduces to the characteristic equation of an → ideal gas. See also → Dieterici equation.
Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → equation.
van der Waals force
niru-ye van der Waals
Fr.: force de van der Waals
A weak attractive force between neutral atoms and molecules arising from polarization induced in each particle by the presence of other particles. All molecules contain electrically charged particles, and even though the molecule as a whole is electrically neutral there do exist between molecules van der Waals attractive forces of electric origin.
Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → force.
A soft, ductile, silver-grey metal; symbol V. → Atomic number 23; → atomic weight 50.9415; → melting point about 1,890°C; → boiling point 3,380°C; → specific gravity about 6 at 20°C; and → valence +2,+3, +4, or +5. → It is used in various alloys to increase its shock resistance.
Named 1830 by Swedish chemist Nils Gabriel Sefström (1787-1845), from Old Norse Vanadis, epithet of the goddess Freya, + → -ium.
1, 2) parré (#); 3) bâdnemâ (#)
Fr.: 1) aile, pale; 2) barbe; 3) girouette
M.E., from O.E. fana "flag;" cognate with Ger. Fahne "flag," Gothic fana "piece of cloth."
Parré, from parr, → feather.
Fr.: 1, 2) disparaître, se volatiser; 3) s'en aller
1) To disappear from sight, especially quickly; become invisible.
M.E., from O.Fr. esvanir "disappear; cause to disappear," from L. evanescere "disappear, pass away, die out," from → ex- "out" + vanescere "to vanish," from vanus "empty," cognate with O.E. wanian "to lessen," wan "deficient;" O.N. vanta "to lack;" L. vacare "to be empty," vastus "empty, waste;" see below for Iranian cognates.
Venidan, from Kurd. ven "missing; away; absent," cf. Pers. vang "empty, empty-handed, poor;" related to Av. ūna- "deficient, empty," ū- "to be incomplete;" akin to Skt. vā- "to disappear, diminish," ūná- "deficient;" L. vanus "empty," as above.
1) To → conquer or subdue by superior force,
as in battle.
O.Fr. venquis-, extended stem of veintre "to defeat," from L. vincere "to overcome, conquer."
Vânidan, from Mid.Pers. vânidan "to vanquish, to conquer;" cf. Av. van- "to vanquish, to defeat;" Skt. van- "to win, conquer;" O.E. winnan, E. win, PIE *uen- "to win, conquer."
The gaseous form of substances that are normally in liquid or solid form.
M.E. vapour, L. vapor "exhalation, steam, heat," of unknown origin.
Boxâr, loan from Ar. buxâr.
Fr.: pression de vapeur
The equilibrium pressure of a vapor above its condensed phases (liquid or solid). In other words, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. The vapor pressure varies with the temperature. As the temperature increases its the vapor pressure also increases, and conversely. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the → boiling point. For example, at 20 °C, water would boil at a pressure of about 0.023 atm, or about a fortieth of atmospheric pressure.
Fr.: vaporisation, évaporation
The act of vaporizing. The state of being vaporized.
Verbal noun of → vaporize.
1) boxâridan; 2) boxârândan
Fr.: 1) s'évaporer; 2) évaporer
1) To become converted into vapor.
The quality, state, or degree of being variable or changeable.
Quality noun from → variable.
Fr.: indice de variabilité
A measure of variability of a star (such as Welch-Stetson variability index) which combines the information from two passbands assuming that changes in the star's luminosity occur nearly simultaneously at all optical wavelengths.
Something that may or does vary. → variable star.
M.E., from O.Fr. variable, from L. variabilis "changeable," from variare "to change," → vary.
Vartandé agent noun from vartidan "to change," → vary.
Fr.: étoile variable
A star whose luminosity changes over periods of time; there are many reasons and many types. → cataclysmic variable; → Cepheid; → pulsating star; → nova; → long-period variable; → short-period variable; → Luminous Blue Variable (LBV); etc.
variable star designation
nâmgozini-ye setâre-ye vartandé
Fr.: designation des étoiles variables
A set of conventions used for naming → variable stars. Stars with existing → Bayer designations are not given new designations. Alternatively, the letters R through Z are used followed by the Latin genitive of the name of the hosting constellation. Otherwise, two letters of alphabet are used (334 combinations) with the Latin genitive of the name of the constellation. Finally, the letter V (variable) is used followed by numbers 335, 336, and so on. Some examples are: → P Cygni, → T Tauri, → FU Orionis, → EX Lupi, and → V2052 Oph.
A measure of the scatter of the values of a random variable (X) about its mean (μ). Var(X) = E(X -μ)2. The positive square root of the variance is called the → standard deviation.
M.E., from O.Fr. variance, from L. variantia, from variare "to change," → vary.
Vartâyi quality noun from vartâ agent noun from vartidan, → vary.
1) Liable to or displaying variation.
Fr.: variable aléatoire
A quantity which may take any of the values of a specified set with a specified relative frequency or probability. The variate is therefore often known as a → random variable.