Measurement of refractive index and equation of state in dense He, H2,   H2O, Ne under high pressure in a diamond-anvil cell
  We present an accurate determination of the refractive index of hydrogen, helium, H2O and neon up to 35 GPa at ambient temperature. The experimental method is based on the combination of two interferometric signals of the Fabry-Perot cavity containing the sample in the diamond anvil cell. The data are put in perspective through the variation of the molecular polarisability with density. Significant electronic changes are observed. Interesting possibilities of the method are also illustrated: the high precision to finely probe phase transition; the simultaneous measurement of the volume to obtain the equation of state of transparent media with around 1 % accuracy. 
A. Dewaele, J. Eggert, P. Loubeyre, R. Le Toullec