Up to this point our discussion has avoided any detailed reference to the underlying physical mechanisms producing scattered waves. One most often thinks of scattering in terms of particles undergoing elastic collision, either with other particles or with macroscopic objects such as walls. In the microscopic domain this view is even extended to light when the photon picture is appropriate. As we have seen, geometric optics permits a similar interpretation wherein rays mimic the scattering behavior of particles.

When one can no longer neglect the wave nature of light, however, this intuitive view of the scattering process is not entirely adequate and we are compelled to look beyond it for physical origins. On a microscopic level an electron, atom, or molecule will couple to an incident electromagnetic wave in an oscillating fashion, such that it re-radiates in all directions, producing a ‘scattered’ wave. It is tempting to amplify this mechanism to macroscopic targets, since they are certainly composed of these constituents, but to do so in detail would be a forbidding problem in many-body physics. A more appropriate macroscopic approach might be to envision the incident fields as inducing electric and magnetic multipoles that oscillate, and hence radiate, while maintaining definite phase relations with the incident wave. When the wavelength of the incident radiation is long compared with the dimensions of the scatterer, only the lowest-order multipoles will be important, and the re-radiation process can be approximated by invoking electric and magnetic dipoles.