scattering: one construct, many systems

In optics, scattering is the redirection of light out of its original direction by particles or refractive-index fluctuations, with the angular pattern and strength set by the particle size relative to the wavelength, from Rayleigh scattering by tiny molecules that makes the sky blue to Mie scattering by larger droplets. Van de Hulst's Light scattering by small particles and Bohren and Huffman's Absorption and scattering of light by small particles develop the cross sections and phase functions that quantify it. The construct is the diversion of a wave by an obstacle, captured by a scattering cross section, and it is the same idea that recurs in radar returns and in neutron experiments.[1][2]

Unit

The scattering cross section, an effective area per scatterer, with its angular phase function

Recovery means

Not applicable

Measured by

Quantified by the measured scattering cross section and angular intensity distribution versus wavelength and particle size

In physics, scattering is the deflection of an incident particle or wave by a target, described by a differential cross section giving the probability of being sent into each angle, and by the structure or response it encodes; thermal neutron scattering uses this to map atomic positions and motions in matter. Squires's Introduction to the theory of thermal neutron scattering sets out the cross sections for elastic and inelastic processes. The construct is identical to the optical one, a target diverting an incoming flux and measured by a cross section, here realised with neutrons or other particles rather than visible light.[3]

Unit

The differential and total cross section, an effective area for deflection into a solid angle

Recovery means

Not applicable

Measured by

Determined from the measured differential cross section versus scattering angle and energy transfer

In radar engineering, scattering is how a target intercepts an incident radio wave and re-radiates it in all directions, with the fraction sent back toward the radar captured by the radar cross section, an effective area that depends on the target's size, shape, and material. Skolnik's Introduction to radar systems treats the radar cross section and how it governs detection range. The construct is the same wave-diversion-by-a-target idea as in optics and particle physics, expressed at radio wavelengths and summarised, as there, by a cross section.[4]

Unit

The radar cross section, an effective echoing area in square metres

Recovery means

Not applicable

Measured by

Measured as the radar cross section versus aspect angle, frequency, and polarisation