Refractive Index of Gaseous and Liquid Hydrogen

The refractive index of gaseous and liquid hydrogen has been measured by an interferometric method at temperatures between 15° and 298.15°K and at pressures up to 230 atm. The measurements have been analyzed in terms of the density and temperature dependence of the Lorentz–Lorenz function, $${\mathrm{L–L\hspace{0.17em}\equiv \hspace{0.17em}[(n}}_{\lambda }^2{\text{\hspace{0.17em}−\hspace{0.17em}1)\hspace{0.17em}/\hspace{0.17em}(n}}_{\lambda }^2{\text{\hspace{0.17em}+\hspace{0.17em}2)]ρ}}^{\text{−1}}$$ , where $n_{\lambda }$ is the refractive index at $\text{λ\hspace{0.17em}=\hspace{0.17em}5462 Å}$ and $\rho$ is the fluid density in grams per cubic centimeter. The precision and reproducibility of L–L is better than 0.05% in most cases. L–L for gaseous parahydrogen first increases with increasing density to a maximum and then decreases to values below the low‐density limit. L–L is also slightly temperature dependent; the low‐density limit increases with increasing temperature; the maximum on the L–L isotherms decreases with increasing temperature. L–L for saturated liquid parahydrogen decreases with increasing density by about 0.1% at temperatures between 15° and 32°K. The difference in L–L for normal and parahydrogen is consistent with previous theoretical and experimental estimates of the molecular‐polarizability difference.