Piezo- and elasto-optic properties of liquids under high pressure. II. Refractive index vs density

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Abstract
In Paper I of this series it was shown that high pressure interferometric measurements on a number of liquids reveal that the refractive index of liquids increases linearly with Eulerian strain even though the volume strain involved is as high as 33%. Further, it was also found that this linearity criterion can be used to discriminate between the various equations of state for liquids and that Keane’s equation appears to be universally valid for all liquids. Having thus established the P–V relationship of liquids, the next problem of the refractive index–density relationship of liquids is considered in this article. The various equations prevalent in the literature—namely, Lorentz–Lorenz, Drude, Eykman, Onsager–Bottcher, Kirkwood–Brown, and Omini equations— are analyzed. The last two equations have been derived using a detailed statistical mechanical approach. It is found that none of these equations can predict the elasto‐optic behavior of liquids at high pressure quantitatively even though the latter two equations appear to yield values in reasonably good agreement with experimentally observed values.