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

Verbal form of → diffraction.

→ diffraction.

A wave property of light which allows it to curl around obstacles whose size is about that of the wavelength of the light. As a → wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form a → diffraction pattern.
Related terms: → diffusion; → dispersion; → distribution; → scatter; → scattering.

From Fr. diffraction, from Mod.L. diffractionem, from L. diffrac-, stem of diffringere "break in pieces," from → dis- "apart" + frangere "to break."

Parâš "dispersion, scattering," variant of pâš, pâšidan, → dispersion.

An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.

→ diffraction; → grating.

A series of concentric rings of dark and light color produced by interference.

→ diffraction; → pattern.

Olgu, loanword from Turkish; parâš→ diffraction.

One of several light rays emanating from a bright light source in images taken with → reflecting telescopes. They are artifacts caused by light diffracting around the support or → spider vanes of the → secondary mirror.

→ diffraction; → spike.

The quality of an → optical system that is capable of producing images with angular resolution as small as the theoretical limit of the → Airy disk.

→ diffraction; limited, adj. of → limit.

Karânmand "bounded, limited," from karân→ boundary + -mand possession suffix; parâš→ diffraction.

A minor component of galactic light resulting from the diffusion of starlight by → interstellar dust near the → galactic plane.

→ diffuse; → galactic; → light.

The energy density of a radiation field divided by the equilibrium value for the same color temperature.

→ dilution; → factor.

Formation of two refracted rays of light from a single incident ray; property of certain crystals, notably calcite.

→ double; → refraction.

A → transparent → crystalline substance (such as calcite, quartz, and tourmaline) that is → anisotropic relative to the → speed of light. A ray incident normally on such crystals is broken up into two rays in traversing the crystal, → ordinary ray and → extraordinary ray.

→ doubly; → refracting; → crystal.

Same as → Eddington parameter.

→ Eddington limit; → factor.

In → general relativity, the → action that yields → Einstein's field equations. It is expressed by:
S_{EH = (1/2κ)∫d⁴x (-g)^1/2R + S^m,
where κ ≡ 8πG and S^m is the matter part of the action.}

→ Einstein; → Hilbert space; → action.

A diffraction phenomenon resulting from the passage of electrons through matter, analogous to the diffraction of visible light. This phenomenon is the main evidence for the existence of waves associated with elementary particles; → de Broglie wavelength.

→ electron; → diffraction.

The unified description of two of the four fundamental interactions of nature, → electromagnetism and the → weak interaction which would merge into a single force under conditions of extreme temperature (above 10¹⁶ degrees, 10² GeV) prevalent in the early history of the → Universe.

→ electroweak; → interaction.

1) The time that must be added to the lunar year (12 lunations) to make it coincide with the solar year (about 11 days).
2) The moon's age at the beginning of the calendar year.

From Fr. épacte, from L. epacta, from Gk. epaktos, verbal adj. of epagein "to intercalate, add, bring forward," from epi "on" + ag-, from agein "to bring, to lead;" cf. L. agere "to drive, set in motion," → act.

Barafzâ, from bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + afzâ, afzudan "to add, increase" (Mid.Pers. abzudan "to increase, grow;" O.Pers. abijav- "to increase, add to, promote," from abi-, aiby- "in addition to; to; against" + root jav- "to press forward;" Av. gav- "to hasten, drive;" Sk. jav- "to press forward, impel quickly, excite," javate "hastens").

1) Strictly accurate or correct; precise, as opposed to approximate.
2) The complete solution of an equation. For example, in the equation x² = 7, the positive solution 2.646 is correct to three decimal places. The exact answer is x = 7^1/2.
See also:
→ exact differential, → exact differential equation, → exact science, → accurate, → precise, → rigorous.

From L. exactus, p.p. of exigere, literally "to drive out, thrust out," also "demand, finish, measure," from → ex- "out" + agere "drive, lead, act," → act.

Razin "firm, solid, strong" [Dehxodâ, Steingass], Mid.Pers. razên "firm, strong, secure, solid."

If N(x,y) is a → function of two → independent variables, then dN = (∂N/∂x)dx + (∂N/∂y)dy is the exact differential.

→ exact; → differential.

A → differential equation composed of → continuous  → differentiable functions for which certain conditions are fulfilled. The equation M(x,y)dx + N(x,y)dy = 0 is called exact if M(x,y) and N(x,y) are continuous differentiable functions for which the following relationship is fulfilled: ∂M/∂y = ∂N/∂x, and ∂M/∂y and ∂N/∂x are continuous in some region.

→ exact; → differential; → equation.

A value that is known with complete certainty. Examples of exact numbers are defined numbers, results of counts, certain unit conversions. Some examples: there are exactly 100 centimeters in 1 meter, a full circle is exactly 360°, and the number of students in a class can exactly be 25.

→ exact; → number.

A field of study that admits especially precise predictions and rigorous methods of testing hypotheses, especially reproducible experiments involving quantifiable predictions and measurements.

→ exact; → science.