It is clear that refractive index (RI) is roughly proportional to the square root of the products of medium electrical and magnetic polarizability (e.g., permittivity and permeability). However, the permittivity is essentially the dielectric constant (Kd) of a material. The dielectric constant for water is ~88 while that for cyclohexane is ~. 2. So I am trying to understand, for example, why the RI of cyclohexane (1.46) is greater that that of water (1.33) at 589nm. For the permeability the relationship is The relative permeability Km is equal to magnetic permeability (mu) divided by the, permeability of free space (muo) This is related to the and magnetic susceptibility Xm by Km = to 1- Xm For both water and cyclohexane: Xm is very small making km essentially one for both fluids. Can someone explain why the RI of cyclohexane is greater that that of water even though the dielectric constant is much smaller? What have I overlooked in the relationship between permittivity, permeability and refractive index?