Backscatter - Backscatter of Waves in Physical Space

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:

• 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.

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).
• Coherent backscattering is observed in random media; for visible light most typically in suspensions 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