Figure 3: Volume effect on refractive indices.
From: The refractive index and electronic gap of water and ice increase with increasing pressure

This figure shows G = (
1(0)−1)/ρe as a function of pressure at 0, 1,000 and 2,000 K, where
1(0) is the real part of the electronic dielectric constant at zero frequency and ρe the valence electron density. The error bars show the s.d. values of the data for water. Lines are drawn to guide the eyes only.