In astronomy and optics scattering is deflection of photons by either macroscopic surfaces such as an asteroid or by small particles as in the rings of Saturn. No atomic or molecular absorption or emission processes are involved. The sky is blue because molecules in the air preferentially scatter blue light.
Rayleigh and Mie scattering
Scattering, also called scatter, is the process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation.
Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.
One of the most robust interference phenomena that survives multiple scattering is coherent backscattering or weak localization. In weak localization, interference of the direct and reverse paths leads to a net reduction of light transport in the forward direction. This phenomenon is typical of any coherent wave which is multiple scattered. It is typically discussed for light waves, for which it is similar to the weak localization phenomenon for electrons in disordered (semi)conductors and often seen as the precursor to Anderson (or strong) localization of light. Weak localization of light can be detected since it is manifest as an enhancement of light intensity in the backscattering direction. This substantial enhancement is called the cone of coherent backscattering.
Coherent backscattering has its origin in the interference between direct and reverse paths in the backscattering direction. When a multiply scattering medium is illuminated by a laser beam, the scattered intensity results from the interference between the amplitudes associated with the various scattering paths; for a disordered medium, the interference terms are washed out when averaged over many sample configurations, except in a narrow angular range around exact backscattering where the average intensity is enhanced. This phenomenon, is the result of many sinusoidal two-waves interference patterns which add up. The cone is the Fourier transform of the spatial distribution of the intensity of the scattered light on the sample surface, when the latter is illuminated by a point-like source. The enhanced backscattering relies on the constructive interference between reverse paths. One can make an analogy with a Young's interference experiment, where two diffracting slits would be positioned in place of the "input" and "output" scatterers.
Scattering in particle physics
In particle physics, scattering refers to deflection of subatomic particles, a process central to many experiments. In scattering experiments, a target of some material is bombarded with a beam of particles (typically electrons, protons, or neutrons) and the number of particles emerging in various directions is measured. This distribution reveals information about the interaction that takes place between the target and the scattered particle.
A famous scattering experiment of alpha particles off gold nuclei performed by Ernest Rutherford revealed the basic structure of the atom - a tiny nucleus surrounded by electrons. See Rutherford scattering. Scattering has also been done off of nucleons and quarks.
Mathematically, scientists describe scattering by an impact parameter (which describes how close the incident particle would come to the target if it moved in a straight line) and an angle of deflection (which describes the angle at which the particle emerges relative to its original direction). The distribution of deflection angles is described by a function known as the differential cross section, which (roughly) relates a direction in space in which some particles emerge to the amount of the incoming beam (in area) those particles came from.
The abstract mathematics of scattering is developed as scattering theory.
See also S-matrix.