The refractive index is related to the molar refractivity, structure and weight of the monomer.

In general, high molar refractivity and low molar volumes increase the refractive index of the polymer.

Phosphorus-containing groups, such as phosphonates and phosphazenes, often exhibit high molar refractivity and optical transmittance in the visible light region.

The molar refractivity A of a gas is related to its refractive index n by the Lorentz-Lorenz equation:

Molar refractivity, , is a measure of the total polarizability of a mole of a substance and is dependent on the temperature, the index of refraction, and the pressure.

Van der Waals radii may be determined from the mechanical properties of gases (the original method), from the critical point, from measurements of atomic spacing between pairs of unbonded atoms in crystals or from measurements of electrical or optical properties (the polarizability and the molar refractivity).