It is shown that the refractive index of hydrogen is given by n H 2 = 1 + 3.046 ρ + 2.6 ρ2 where ρ has units of mol/cm3. Here ρ at a given pressure, P, can be obtained from the equation of state, ρ(P). This relation is combined with that for the refractive index of loaded diamond anvils to obtain n H 2/n D vs P. This is found to be in good agreement with the ratio n H 2/nD measured directly at different pressures by Fabry-Perot interferometry. The density variation of the molar refraction is computed.