An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Two integrals that involve quadratic equations in the sine and cosine functions and are defined as: C(x) = ∫ cos (πt²/2) dt and C(y) = ∫ sin (πt²/2) dt, integrated from 0 to x. They are quite frequently used in optics studying → Fresnel diffraction and similar topics. The Fresnel integrals are also used in railway and freeway constructions. These integrals may be evaluated to arbitrary precision using → power series. Alternatively the amplitudes may be found graphically by use of → Cornu's spiral.

→ Fresnel diffraction; → integral.

An optical lens composed of a series of rings of glass so curved that they all have the same focus. It is flat on one side and ridged on the other making it possible that nearly every ray of light from the source be re-directed out on a horizontal path. This design enables the construction of lenses of large aperture and short focal length avoiding thus the large weights and volumes of material which would be required in conventional lenses. The first Fresnel lens, designed for use in a lighthouse on the river Gironde, was installed in France in 1823, and by the 1850s many examples were in use everywhere. Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. They can also be used as magnifiers and projection lenses. Nowadays, Fresnel lenses made of optical plastics are widely used for various applications.

→ Fresnel diffraction; → lens.

A pair of plane mirrors which are slightly inclined to one another. It is used for producing two coherent images in interference experiments.

→ Fresnel diffraction; → mirror.

A piece of special glass in the form of an oblique → parallelepiped so cut that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing two internal reflections. It is a type of → quarter-wave retarder used to produce a → circularly polarized light from a → plane polarized light, or the reverse.

→ Fresnel diffraction; → rhombus.

An optical element consisting of two small angle → prisms, joined together at their bases, used to produce two → coherent sources. The thin double prism refracts the light from a source into two overlapping beams, which produce → interference fringes. With this experiment Fresnel was able to produce interference without relying upon → diffraction to bring the interfering beams together.

→ Fresnel diffraction; → bi-; → prism.

Two plane mirrors, fitted side by side at a small angle, used to create two mutually → coherent sources in a famous → interference experiment first suggested by A. Fresnel. A point source reflected at the mirrors appears as a pair of → virtual light sources, positioned close together, which interfere with each other due to their → coherence. This arrangement removes the problem that two separate light sources do not produce observable interference on account of their incoherence. Same as Fresnel's double mirror. See also → Fresnel's biprism, → Lloyd's mirror.

→ Fresnel diffraction; → mirror.

One of the first → cosmological models to incorporate Einstein's → general relativity, predicting that → galaxies should be → receding from each other due to → cosmic expansion.

→ Friedmann equation; Georges Edouard Lemaître (1894-1966), a Belgian cosmologist and priest who proposed a first sketch of the → Big Bang theory; → universe.

The lowest frequency in a complex wave.

→ fundamental; → frequency.

General: Time that is to be or come hereafter.
In a → space-time diagram, those events that could be influenced by a given element.

M.E. futur, from O.Fr., from L. futurus "about to be," irregular suppletive future participle of esse "to be."

Âyandé "future" agent noun/adjective of âmadan "to come, to occur, to become," from Mid.Pers. âmatan; O.Pers. gam- "to come; to go," Av. gam- "to come; to go," jamaiti "goes;" Proto-Iranian *āgmatani; Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE root *gwem- "to go, come."

The set of all points in a → space-time diagram that are reached by signals travelling from a specified point at the speed of light.

→ future; → light; → cone.

A way of → mapping an → input space to an → output space using → fuzzy logic. FIS uses a collection of fuzzy → membership functions and rules, instead of Boolean logic, to reason about data. Also called → fuzzy logic system.

→ fuzzy; → inference; → system.

A process used in a → fuzzy logic system where the → truth value for the premise of each rule is computed and applied to the conclusion part of each rule. This results in one fuzzy set to be assigned to each output variable for each rule.

→ fuzzy; → inference.

The global shape and the arrangement of the various parts or constituents of a galaxy.

→ galactic; → structure.

Same as → inertial reference frame.

→ Galilean; → reference; → frame.

The principle according to which the fundamental laws of physics are the same in all frames of reference moving with constant velocity with respect to one another (→ inertial reference frames). Same as → Galilean invariance and → Newtonian relativity.
See also: → Galilean transformation, → Einsteinian relativity.

→ Galilean; → relativity.

The direct electric current that flows between metals or conductive nonmetals in a → galvanic couple.

→ galvanism; → current.

An aggregate of several different kinds of gases which do not react chemically under the conditions being considered. A gas mixture constitutes a homogeneous thermodynamical system.

→ gas; → mixture.

The total normal induction over any closed surface drawn in an electric field is equal to 4π times the total charge of electricity inside the closed surface. Gauss's theorem applies also to other vector fields such as magnetic, gravitational, and fluid velocity fields. The theorem can more generally be stated as: the total flux of a vector field through a closed surface is equal to the volume → integral of the vector taken over the enclosed volume. Also known as → divergence theorem, Ostrogradsky's theorem, and Gauss-Ostrogradsky theorem.

→ gauss; → theorem.

The secular motions of the → celestial equator and → ecliptic. In other words, the sum of → lunisolar precession, → planetary precession, and → geodesic precession.

→ general; → precession

The secular displacement of the → equinox on the → ecliptic of date.

→ general; → precession; → longitude.