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.