Fr.: diffusion atmosphérique
The → scattering of → electromagnetic radiation by various particles in the Earth's → atmosphere. The phenomenon is caused by collisions between photons and several scattering agents such as atoms, molecules, → aerosols, and water droplets in clouds. → Rayleigh scattering.
Scattering of radiation or particles through angles greater than 90° with respect to the original direction of motion.
Fr.: diffusion de Brillouin
Scattering of electromagnetic waves in solids and liquids when, as a result of the scattering process, an acoustic → phonon is emitted or absorbed. Brillouin scattering is analogous to → Raman scattering.
Fr.: diffusion cohérente
A scattering process in which the scattered radiation bears the same frequency and phase as the incident radiation.
parâkaneš-e Compton (#)
Fr.: diffusion Compton
double Compton scattering
parâkaneš-e Compton-e dotâyi
Fr.: diffusion Compton double
An electron-photon interaction that can be thought of as a → Compton scattering event associated with the production or destruction of an extra photon.
parâkaneš-e qobâri, ~ pat qobâr
Fr.: diffusion par la poussière
Fr.: diffusion élastique
In a → collision between two → particles,
the reaction in which the total → kinetic energy
of the system, projectile plus target, is the same before the collision as after.
bâl-e parâkaneš-e elektron
A → line broadening phenomenon involving the scattering effect of → free electrons on the → radiation transfer in → stellar atmospheres. The scattering of radiation by free electrons plays an important role in the atmospheres of → hot stars, such as → O-types, early → B-types, and → Wolf-Rayet stars. The first detailed study of electron scattering in Wolf-Rayet stars was by Castor et al. (1970), who used electron scattering to explain the broad emission wings of N IV λ3483 in HD 192163. In → P Cygni stars the explanation of the very extended (almost symmetric) wings on the → Balmer lines as caused by electron scattering was first made by Bernat & Lambert (1978). Hillier (1991) showed that significant reduction in the strength of an electron-scattering wing can be achieved in a model of → clumped wind for a lower mean → mass loss rate. This resulted in a better agreement between observations and theoretical predictions. Electron-scattering wings provide diagnostics regarding the presence of density inhomogeneities in → stellar winds (Münch, 1948, ApJ 108, 116; Hillier, 1991, A&A 247, 455).
Fr.: diffusion en avant
Scattering in which photons emerge from the → scattering medium travelling predominantly in the same direction as they entered. The → halos around the Sun and Moon in wet weather are caused by forward scattering by water droplets in the Earth's atmosphere. → backscattering.
Fr.: diffusion géométrique
A type of scattering in which the wavelength (of the light or the sound) is much smaller than the size of object causing the scattering.
Fr.: diffusion inélastique
Fr.: dernière diffusion
The epoch in the early evolution of the Universe when matter and photons decoupled. Once atoms formed, light and matter stopped constantly interacting with one another, and photons were able to travel freely. As a result, the Universe became transparent. Light from this period is observed today as the → cosmic microwave background radiation. Same as → decoupling era and → recombination era.
last scattering surface
ruye-ye vâpasin parâkaneš
Fr.: surface de dernière diffusion
The set of locations in space corresponding to the → last scattering epoch in the early Universe. It is a spherical surface around the present-day observer from which the → cosmic microwave background radiation appears to emanate.
Fr.: diffusion de Mie
The scattering of → electromagnetic waves by → particles of → size comparable to the radiation → wavelength. Mie scattering depends weakly upon the wavelength, hence the → scattered light spectrum is similar to that of the → incident light. Mie scattering explains the → white color of clouds when scattering is due to → water droplets having a size of few microns. Cloud → droplets with a diameter of around 20 microns or so are large enough to scatter all visible wavelengths more or less equally. Because all wavelengths are scattered, clouds appear to be white. When clouds become very deep, less and less of the incoming solar radiation makes it through to the bottom of the cloud, which gives these clouds a darker appearance.
Named after Gustav Adolf Mie (1868-1957), a German physicist, whose theory of 1908 explains the process; → scattering.
Fr.: diffusion multiple
A process of → radiative transfer in which more than one → scattering event may be of importance before → transmission, → reflection, or → absorption. In → radiation-driven winds photon scattering can take place in different → spectral lines. Each scattering occurs in a different spectral line, and successive scatterings occur at lower energies (longer wavelength). The standard theory of line driving (→ CAK model) assumes that photons can be scattered only once in the wind, which is a reasonable assumption for normal → O stars. In → Wolf-Rayet stars, where photons evolve in an atmosphere with a strong → ionization stratification, multiple scattering is important. Indeed the strength of W-R winds appears to exceed the single scattering limit.
Fr.: diffusion incohérente
The absorption of a photon and its re-emission at a different frequency (in the observer's frame of reference) by scattering atoms.
A model of radiative transfer that ignores forward scattering of photons; assuming forward-scattered light as un-scattered.
parâkaneš-e Raman (#)
Fr.: diffusion Raman
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.
Fr.: diffusion Rayleigh
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:
Iθ = 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.