A unit of optical measurement that expresses the refractive power of a lens or prism. In a lens or lens system, it is the reciprocal of the focal length in meters.
L. dioptra, from Gk. di-, variant of dia- "passing through, thoroughly, completely" + op- (for opsesthai "to see") + -tra noun suffix of means.
Dioptr loanword from Fr.
An instrument used in antiquity to measure the apparent diameter of the Sun and the Moon. It was a rod with a scale, a sighting hole at one end, and a disk that could be moved along the rod to exactly obscure the Sun or Moon. The Sun was observed directly with the naked eye at sunrise or sunset in order to prevent eye damage. Aristarchus (c.310-230 B.C.), Archimedes (c. 290-212 B.C.), Hipparchus (died after 127 B.C.), and Ptolemy (c.100-170 A.D.) used the dioptra. The instrument could also serve for measurement of angles, land levelling, surveying, and construction of aqueducts and tunnels.
1) Navigation: The angular difference between the visible horizon and
the true horizon. Same as → dip of the horizon.
O.E. dyppan "to immerse," cognate with Ger. taufen "to baptize," and with → deep.
Našib, → depression.
Fr.: angle d'inclinaison
dip of the horizon
Fr.: inclinaison de l'horizon
The angle created by the observer's line of sight to the → apparent horizon and the → true horizon. Neglecting the → atmospheric refraction, dip of the horizon can be expressed by θ (radians) = (2h/R)1/2, where h is the observer's height and R the Earth's radius. An an example, for a height of 1.5m above the sea, and R = 6.4 x 106 m, the dip angle is about 0.00068 radians, or 0.039 degrees, about 2.3 minutes of arc, quite appreciable by the eye. See also → distance to the horizon. Same as → dip angle.
Diphda (β Ceti)
Diphda, from Ar. zafda' (
Mid.Pers. wazaγ, vak; Av. vazaγa- "frog," → tadpole orbit.
Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).
From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos "having two sounds," from → di- "double" + phthongos "sound, voice."
A pathological condition of vision in which a single object appears double because the eyes are not focusing properly. Same as → double vision.
From L. diplo- "double, in pairs," from Gk., combining form of diplos "twofold" + -opia, from Gk. -opia, from ops "eye."
Dobini, from do→ two + bini "vision, seeing," from bin "to see; seer" (present stem of didan; Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see").
1) A combination of two electrically or magnetically charged particles
of opposite signs, which are separated by a very small distance.
Fr.: anisotropie dipolaire
A form of anistropy in the temperature of the → cosmic microwave background radiation, appearing as one hot pole and one cold pole, caused by our motion with respect to the cosmic background radiation. The temperature variations, amounting to 1 part in 1000, yield a velocity of about 600 km/sec for our Galaxy with respect to the background. → cosmic microwave background anisotropy.
ânten-e doqotbé (#)
Fr.: antenne dipôle
One of the simplest kinds of antenna which is connected at the center to a radio-frequency feed line for transmitting or receiving radio frequency energy. It differs from the dish antenna in that it consists of many separate antennas that collect energy by feeding all their weak individual signals into one common receiving set.
gaštâvar-e doqotbé (#)
Fr.: moment dipolaire
1) The product of the strength of either of the charges in an
→ electric dipole and the distance separating the two charges.
It is expressed in → coulomb meters. Dipole moment is a
→ vector quantity.
Its direction is defined as toward the positive charge. In chemistry dipole moment is
a quantitative measure of polarity in a molecule;
the unit is the → debye.
Fr.: rayonnement dipolaire
Paul A. M. Dirac (1902-1984), English theoretical → physicist, one of the founders of → quantum mechanics and → quantum electrodynamics, Nobel Prize for Physics in 1933. → Dirac annihilation, → Dirac equation, → Dirac function, → Dirac's constant, → Fermi-Dirac statistics .
Fr.: annihilation de Dirac
Same as → pair annihilation.
Fr.: équation de Dirac
The equation that describes the behavior of an electron in a way that combines the requirements of quantum mechanics with the requirements of relativity.
Fr.: fonction de Dirac
A function of x defined as being zero for all values of x other than x = x0 and having the definite integral from x = -∞ to x = +∞ equal to unity.
Fr.: constante de Dirac