Light scattering is one of the simplest optical processes, which

commonly appear in the fields of optics and spectroscopy. In

general, it is classified into inelastic and elastic ones, according

to whether the incident light loses/gains its energy or not. In the

former, Rayleigh, Brillouin, and Raman scatterings are commonly known, whose differences are dependent on the amount of energy

exchanged with the scatterers. In general, it is called Rayleigh

scattering when light illuminating on particles floating in a fluid

or a density and/or thermal fluctuation in fluid and solid causes

inelastic scattering. Its frequency range is normally below MHz.

To measure the spectrum due to Rayleigh scattering, photon

correlation or optical beat spectroscopy is often employed. It is

sometimes called dynamical light scattering to characterize the

fluctuations of particles in liquid and those of a medium itself.