Backscatter is the reflection of waves, particles, or signals back to the direction they came from. The term is used in astronomy and several fields of physics, as well as in photography and medical ultrasonography.

Backscatter of waves in physical space

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

• Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, 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.

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 optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;
• 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. This can have different reasons:

• In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering;
• in gegenschein, constructive interference might play a role (this needs verification);
• in multiple scattering from suspensions like milk, the enhancement of backscattering is connected with weak localization.

Radar, especially weather radar

Backscattering is the principle behind radar systems.

In weather radar, backscattering is proportional to the 6^th power of the diameter of the target multiplied by its inherent reflective properties. Water is almost 4 times more reflective than ice but droplets are much smaller than snow flakes or hail stones. So the backscattering is dependent on a mix of these two factors. The strongest backscatter comes from hail and large graupel (solid ice) due to their sizes. Another strong return is from melting snow or wet sleet, as they combine size and water reflectivity. They often show up as much higher rates of precipitation than actually occurring in what is called a brightband. Rain is a moderate backscatter, being stronger with large drops (such as from a thunderstorm) and much weaker with small droplets (such as mist or drizzle). Snow has rather weak backscatter.