In understanding the basis of density fluctuation scattering in liquids, it is helpful to begin with a consideration of molecular, or Rayleigh, scattering by gases such as air. According to the Rayleigh theory, within any particle, such as an air molecule, in a light field, a dipole is induced by the electrical vector of the field. As the dipole oscillates at the frequency of the exciting radiation, it emits radiation of the same frequency in all directions. It is this radiation that is the scattered light.
The Rayleigh molecular theory of scattering does not apply to liquids: the strong interactions between the molecules make it impermissible to consider the interaction of the radiation with molecules on an individual basis. In any liquid, however, the continual random motion of the molecules leads to localized microscopic fluctuations of density and therefore of dielectric constant and, in the Einstein-Smoluchowski theory, the interaction of the radiation field with these inhomogeneities - each of which can be regarded as a dipole - rather than with the individual molecules is considered. The predicted angular distribution of scattering is similar to that given by the Rayleigh theory for gases, i.e. it is identical in the forward and backward directions (Figs. 2.2 and 4.8). Again, as in Rayleigh scattering by gases, scattering by a pure liquid is predicted to vary inversely with the fourth power of the wavelength.
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