Editing Backscatter (section)

== Backscatter of waves in physical space ==
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 collision]]s between electromagnetic waves and the transmitting medium ([[Brillouin scattering]] and [[Raman scattering]]), important in fiber optics, see below;
*[[Elastic collision]]s between accelerated ions and a sample ([[Rutherford backscattering]])
*[[Bragg's law|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 (astronomy)|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.{{Needs verification|date=June 2023}}
*[[Coherent backscattering]] is observed in random media; for visible light most typically in [[Suspension (chemistry)|suspension]]s like milk. Due to [[weak localization]], enhanced multiple scattering is observed in back direction.
** The [[Back Scattering Alignment]] (BSA) coordinate system is often used in [[radar]] applications
** The [[Forward Scattering Alignment]] (FSA) coordinate system is primarily used in optical applications

Backscattering properties of a target are wavelength dependent and can also be polarization dependent. Sensor systems using multiple wavelengths or polarizations can thus be used to infer additional information about target properties.

=== Radar, especially weather radar ===
Backscattering is the principle behind [[radar]] systems. In [[weather radar]], backscattering is proportional to the 6th power of the diameter of the target multiplied by its inherent reflective properties, provided the wavelength is larger than the particle diameter ([[Rayleigh scattering]]). 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, but non-Rayleigh ([[Mie scattering]]) effects can confuse interpretation. Another strong return is from melting [[snow]] or wet [[Rain and snow mixed|sleet]], as they combine size and water reflectivity. They often show up as much higher [[Rate (mathematics)|rates]] of [[precipitation (meteorology)|precipitation]] than actually occurring in what is called a ''[[Weather radar#Bright band|brightband]].'' [[Rain]] is a moderate backscatter, being stronger with large drops (such as from a [[thunderstorm]]) and much weaker with small [[droplet]]s (such as [[mist]] or [[drizzle]]). [[Snow]] has rather weak backscatter. Dual polarization weather radars measure backscatter at horizontal and vertical polarizations to infer shape information from the ratio of the vertical and horizontal signals.