Scattering

Scattering is the deflection or redistribution of a particle, photon, or wave as it interacts with a target. The outcome is described statistically by cross sections and angular distributions. Scattering can be elastic, with energy conserved, or inelastic, with energy transferred between the incident and the target.

In physics, common types include elastic scattering, such as Rutherford scattering and Thomson scattering, where kinetic energy is conserved; and inelastic scattering, such as Compton scattering, where photon energy changes. Scattering also depends on the wavelength relative to the target size: Rayleigh scattering for particles smaller than the wavelength, Mie scattering for comparable sizes, and geometric scattering for much larger objects.

Theoretical descriptions use scattering theory. The differential cross section dσ/dΩ encodes the probability of scattering into a given solid angle. In quantum mechanics the scattering amplitude f(θ) relates to the cross section by dσ/dΩ = |f(θ)|^2. Methods include the Born approximation and partial-wave analysis; the optical theorem relates forward scattering to the total cross section. In many contexts, coherent versus incoherent scattering distinguishes phase relationships.

Applications span measurements of material structure in physics experiments, medical imaging and therapy, atmospheric science (such as the color of the sky and halos), astronomy (dust scattering and polarization), and remote sensing. In particle physics, scattering experiments probe fundamental interactions and the properties of constituents.

Historically, Rutherford's alpha-scattering experiments revealed the atomic nucleus; Compton demonstrated photon–electron scattering; and Rayleigh explained why the sky is blue. Scattering theory remains central to quantum mechanics and related fields.