Most discussions of light scattering by simple (e.g. classical, atomic) fluids have treated only ‘first order’ processes, i.e. those where the incident light is scattered only once and the atomic polarizabilities are undistorted by interaction. Correspondingly the scattered intensity is related by Fourier transform to the time- and space- pair correlations. In this paper we describe instead the ‘second order’ processes of collision-induced scattering (c.i.s.), in which the incident light is scattered only once but the relevant polarizability is that of an interacting cluster of atoms, and multiple light scattering (m.l.s.), in which only undistorted polarizabilities are involved but the incident light is scattered more than once. In both cases the scattered intensity is determined by correlations involving more than two particles. In addition, the c.i.s. experiments provide information about the many-atom polarization while the m.l.s. studies offer new probes of large fluctuations in critical and nucleating fluids. We discuss in particular theoretical and experimental c.i.s. investigations of the two-body polarizability anisotropy induced by collision; it is concluded that the nature and origin of non-point-dipole behaviour has yet to be satisfactorily explained. Similarly, we consider how various depolarization m.l.s. studies suggest improved analyses of pair correlation properties in classical systems.
