Temperature dependence of the refractive indices of liquids: Deviation from the Lorentz–Lorenz formula

Abstract

The temperature dependence of the refractive index of liquids is an old problem which has received much attention because experimental results depart significantly from the values inferred from the usual Lorentz–Lorenz formula. Using a sensitive interferometric device, we have very accurately measured the values of (∂n/∂T)p for 15 different molecular liquids, and we give the corresponding deviations from the values predicted by the Lorentz–Lorenz formula. We review the main phenomena able to explain this deviation. Although the Yvon theory gives an expression which well describes our experimental data, it must be considered as an empirical result, since drastic approximations have been made. As both the density correlations and the polarizability anisotropy cannot explain the whole deviation, electronic overlaps and distortions of the molecular framework necessarily affect the refractive index derivatives.