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

The scattering of monochromatic light (visible or ultraviolet) by molecules in which the scattered light differs in wavelength from the incident light. It is caused by the light's interaction with the vibrational or rotational energy of the medium's scattering molecules.

→ Raman effect; → scattering.

The scattering of light by → particles of size small compared with the → wavelength of light. The intensity of the light scattered by unit volume of the medium at an angle θ to the direction of propagation of the incident light is: I_θ = 8 π⁴α² N I₀ (1 + cos²θ)/(R²λ⁴), where α is the → molecular polarizability, N is the number of scattering molecules, I₀ is intensity of the incident light, λ is the wavelength, and R is the distance from the scatterer. The fourth power dependence on wavelength means that blue light is much more strongly scattered than red light from a medium containing very fine particles. The air molecules, mostly → nitrogen (78%) and → oxygen (21%) are some 1,000 times larger than → visible light wavelengths. This accounts for the bluish appearance of smoke and of clear sky when the observation is not along the direction of illumination. The setting Sun, seen through a considerable thickness of atmosphere appears reddish because long wave radiation predominates in the transmitted light.
Historically, John Tyndall first discovered this phenomenon in 1859 (→ Tyndall effect), but Lord Rayleigh studied it in more detail in 1871.

→ rayleigh; → scattering.

The absorption and prompt re-emission of photons of a particular wavelength by an atom. In this process, a photon of exactly the right wavelength (i.e. energy) excites an electron in the atom from one energy level to another. The electron then drops back down to its original energy level more or less immediately, emitting a photon of almost identical energy to the one that was absorbed in the first place, but in some random direction. Resonant scattering applies only to line radiation, unlike other forms of scattering which are of continuous radiation (Oxford Dictionary of Astronomy, 2 ed.).

→ resonant; → scattering.

1) To cause → electromagnetic waves or a → beam of → particles to be irregularly → deflected, → dispersed, or → reflected, or be turned aside in the process of → scattering.
2) The act of → scattering; something that is → scattered.

M.E. scateren, schateren "to disperse, break up, destroy;" cf. M.Du. schaderen "to scatter."

Parâkandan "to scatter, to disperse;" Mid.Pers. parakandan "to scatter" (cf. apakandan "to throw"), from Proto-Iranian *pari-kan-, from *pari, *par- "around" (cf. Pers. pirâ-, variant par- "around, about," from Mid.Pers. pêrâ; O.Pers. pariy "around, about," Av. pairi "around, over," per- "to pass over, beyond;" cf. Skt. pari; PIE base *per- "through, across, beyond;" cf. Gk. peri "around, about, beyond;" L. per "through") + *kan- "to throw, place, put" (cf. Pers. afgandan "to throw; to lay, place;" kandan "to dig; to extract;" Mid.Pers. kan-, kandan "to dig;" O.Pers. kan- "to dig," akaniya- "it was dug;" Av. kan- "to dig," uskən- "to dig out;" cf. Skt. khan- "to dig," khanati "he digs," kha- "cavity, hollow, cave, aperture").

1) Occurring or distributed over widely spaced and irregular intervals in time or space.
2) The quality of a particle that has undergone → scattering.

Past participle of → scatter.

A → particle that causes → scattering of another particle through interaction with it.

→ scatter; → -er.

The process in which the direction of motion of → particles or → waves is changed randomly because of their → interactions (→ collisions) with other particles of the → medium transversed.
Two parameters govern scattering: 1) the wavelength (λ) of the incident radiation, and 2) the size of the scattering particle (r), usually expressed as the nondimensional size parameter, x = 2πr / λ. The size parameter defines three types of scattering:
1) x much less than 1 (or r much smaller than λ), → Rayleigh scattering;
2) x ~ 1 (or rλ), → Mie scattering; and
3) x much larger than 1 (or r much larger than λ), → geometric scattering.
See also: → atmospheric scattering, → backscattering, → Brillouin scattering, → coherent scattering, → Compton scattering, → elastic scattering, → forward scattering, → last scattering, → last scattering surface, → multiple scattering, → noncoherent scattering, → quasi-single-scattering approximation, → Raman scattering, → scattering angle, → scattering coefficient, → scattering of stars, → selective scattering, → single scattering, → spin-flip scattering, → surface of last scattering, → Thomson scattering.
Related terms: → diffraction; → diffusion; → dispersion; → distribution.

Verbal noun of → scatter.

The angle between the → incident radiation on a → particle (such as a water droplet in a rainbow) and the scattered radiation (such as the light ray leaving the droplet). Scattering angle is a function of → impact parameter. In other words, The angle along which the change of direction has taken place, irrespective whether radiation is scattered by particles or reflected (refracted) by a surface.

→ scattering; → angle.

The fraction of light scattered per unit distance in a medium.

→ scattering; → coefficient.

The progressive increase of random motions of → disk stars with increasing stellar → ages. While some initial random motion seems likely in the disturbed conditions of disks when the oldest stars formed, the observation is generally attributed to scattering processes. Both massive gas → clumps and → spiral waves are considered as scattering agents (J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).

→ scattering; → star.

A type of scattering that occurs when certain → particles are more effective at scattering a particular → wavelength of light, as in → Rayleigh scattering.

→ selective; → scattering.

A type of scattering where photons are scattered only once. Single scattering dominates in → optically thin media, since photons have a high probability of exiting the medium (e.g., a thin cloud) before being scattered again.

→ single; → scattering.

Quantum mechanics: The scattering of a particle that reverses the spin direction.

→ spin; flip, from flip-flap; → scattering.

Parâkaneš, → scattering; bâ "with;" vâruni, noun from vârun, → inverse; espin, → spin.

Same as → last scattering surface.

→ surface; → last; → scattering.

The classical, → non-relativistic scattering of photons by free charged particles. When an electromagnetic wave is incident on a charged particle, the electric and magnetic components of the wave exert a force on the particle, setting it into motion. As it accelerates, it in turn radiates in all directions. Such scattering is independent of wavelength and equal numbers of photons are scattered forward and backward. Thomson scattering occurs in stellar atmospheres and in any non-relativistic → plasma. Thomson scattering is normally taken as the minimum → opacity.

→ Thomson; → scattering.