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In physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles.

In astronomy and optics scattering is deflection of photons that by macroscopic surfaces such as an asteroid or by small particles like 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.

Scattering 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. Also called scatter.

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.

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.