An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 12 Search : dispersion
angular dispersion
  پاشش ِ زاویه‌ای   
pâšeš-e zâvie-yi

Fr.: dispersion angulaire   

The rate of change of the angles of emergence θ of various wavelengths from a dispersing prism: dθ/dλ.

angular; → dispersion.

anomalous dispersion
  پاشش ِ ناسان   
pâšeš-e nâsân

Fr.: dispesrion anormale   

The phenomenon whereby the → refractive index of light in a medium changes rapidly with wavelength in the vicinity of an → absorption band. Hence the → dispersion curve of the substance shows marked deviations from → Cauchy's equation, in contrast with the behavior of → normal dispersion. On the shorter λ side of the absorption band the refractive index falls off more rapidly than required by Cauchy's equation representing values of n for visible light. On the long λ side of the absorption band the index is very high, decreasing at first rapidly and then more slowly as one goes beyond the absorption band.

anomalous; → dispersion.

atmospheric dispersion
  پاشش ِ جوی   
pâšeš-e javvi

Fr.: dispersion atmosphérique   

The splitting of starlight into a spectrum in the atmosphere because the atmosphere acts as a refracting prism. This phenomenon brings about a practical problem for spectroscopic observations using a slit. → differential refraction; → atmospheric refraction.

atmospheric; → dispersion.

cross dispersion
  پاشش چلیپایی، ~ خاجی   
pâšeš-e calipâyi, ~ xâji

Fr.: dispersion croisée   

Dispersion of a light beam by using two dispersing elements (grating, grism), one for separating spectral orders, the other for resolving spectral features within an order.

cross; → dispersion.

dispersion
  پاشش   
pâšeš (#)

Fr.: dispersion   

1) The resolution of white light into its component wavelengths, either by → refraction or by → diffraction. Dispersion is actually an effect in which radiations having → different wavelengths travel at different speeds in the medium. Since the → angle of refraction of each radiation vary as a function of its → wavelength, the component waves deviate from each other.
2) The rate of change of → refractive index with wavelength: dn/dλ. → Cauchy's equation shows that dispersion varies approximately as the inverse cube of the wavelength.
3) The selective → retardation of → radio waves when they propagate through a → plasma. As a result, higher → frequency waves from a → pulsar will arrive at the Earth before the lower frequencies due to the presence of → intervening → ionized gas in the → interstellar medium. → dispersion measure.
4) Statistics: The spread of values of a → variable around the → mean or → median of a → distribution.

Verbal noun of → disperse.

dispersion curve
  خم ِ پاشش   
xam-e pâšeš

Fr.: courbe de dispersion   

A graph displaying the variation of the → refractive index of a substance against the wavelength of the electromagnetic wave passing through the substance.

dispersion; → curve.

dispersion equation
  هموگش ِ پاشش   
hamugeš-e pâšeš

Fr.: équation de dispersion   

An equation representing the variation of → refractive index as a function of → wavelength; for example → Cauchy's equation and → Sellmeier's equation.

dispersion; → equation.

dispersion measure
  اندازه‌ی ِ پاشش   
andâze-ye pâšeš

Fr.: mesure de dispersion   

A parameter used in radio astronomy which describes the amount of dispersion in a radio signal due to its passage through an intervening plasma. It is proportional to the product of the interstellar electron density and the distance to the source.

dispersion; → measure.

dispersion relation
  بازانش ِ پاشش   
bâzâneš-e pâšeš

Fr.: relation de dispersion   

An equation that describes how the → angular frequency, ω, of a wave depends on its → wave number, k. For the simplest of waves, where the speed of propagation, c, is a constant, ω(k) = ck. If the → phase velocity depends on k, that is for a dispersive medium, the function ω(k) is nonlinear.

dispersion; → relation.

normal dispersion
  پاشش ِ هنجارمند   
pâšeš-e hanjârmand

Fr.: dispersion normale   

The dispersion in which a shorter wavelength is associated with a higher → refractive index. Contrasted with the → anomalous dispersion.

normal; → dispersion.

pre-dispersion
  پیش-پاشش   
piš-pâšeš

Fr.: pré-dispersion   

A technique in spectroscopy which uses a combination of several dispersive elements (prisms in series or a grism) before focusing the light on the primary disperser, usually a grating, in order to achieve high spectral resolutions.

pre-; → dispersion.

velocity dispersion
  پاشش ِ تندا   
pâšeš-e tondâ

Fr.: dispersion de vitesses   

The → standard deviation of a velocity → distribution. It indicates how objects of the sample move relative to one another. Objects with similar velocities have a small velocity dispersion, whereas objects with very different velocities have a large velocity dispersion.

velocity; → dispersion.