Backscatter

In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, although specular backscattering can occur at normal incidence with a surface. Backscattering has important applications in astronomy, photography, and medical ultrasonography. The opposite effect is forward scatter, e.g. when a translucent material like a cloud diffuses sunlight, giving soft light. Backscattering can occur in quite different physical situations, where the incoming waves or particles are deflected from their original direction by different mechanisms: Diffuse reflection from large particles and Mie scattering, causing alpenglow and gegenschein, and showing up in weather radar; Inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering), important in fiber optics, see below; Elastic collisions between accelerated ions and a sample (Rutherford backscattering) Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy); Compton scattering, used in Backscatter X-ray imaging. Stimulated backscatter, observed in non-linear optics, and described by a class of solutions to the three-wave equation. Sometimes, the scattering is more or less isotropic, i.e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In these cases, the term "backscattering" just designates the detector location chosen for some practical reasons: in X-ray imaging, backscattering means just the opposite of transmission imaging; in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution; in astronomy, backscattered light is that which is reflected with a phase angle of less than 90°. In other cases, the scattering intensity is enhanced in backward direction.