The Gladstone–Dale relation can be expressed as an equation of state by re-arranging the terms to . 
where n is the index of refraction, D = density and constant = Gladstone-Dale constant.
The macroscopic values (n) and (V) determined on bulk material are now calculated as a sum of atomic or molecular properties. Each molecule has a characteristic mass (due to the atomic weights of the elements) and atomic or molecular volume that contributes to the bulk density, and a characteristic refractivity due to a characteristic electric structure that contributes to the net index of refraction.
The refractivity of a single molecule is the refractive volume k(MW)/N in nm, where MW is the molecular weight and N is the Avogadro constant. To calculate the optical properties of materials using the polarizability or refractivity volumes in nm, the Gladstone–Dale relation competes with the Kramers–Kronig relation and Lorentz–Lorenz relation but differs in optical theory.
The index of refraction (n) is calculated from the change of angle of a collimated monochromatic beam of light from vacuum into liquid using Snells law for refraction. Using the theory of light as an electromagnetic wave, light takes a straight-line path through water at reduced speed (v) and wavelength (λ). The ratio v/λ is a constant equal to the frequency (ν) of the light, as is the quantized (photon) energy using the Planck constant and . Compared to the constant speed of light in vacuum (c'), the index of refraction of water is .
The Gladstone–Dale term  is the non-linear optical path length or time delay. Using Isaac Newton's theory of light as a stream of particles refracted locally by (electric) forces acting between atoms, the optic path length is due to refraction at constant speed by displacement about each atom. For light passing through 1 m of water with , light traveled an extra 0.33 m compared to light that traveled 1 m in a straight line in vacuum. As the speed of light is a ratio (distance per unit time in m/s), light also took an extra 0.33 s to travel through water compared to light traveling 1 s in vacuum.