The relation has also been used to calculate refractive index of glass and minerals in optical mineralogy.
The plot of volume or density versus molecular fraction of ethanol in water is a quadratic curve.
In recent optical crystallography, Gladstone–Dale constants for the refractivity of ions were related to the inter-ionic distances and angles of the crystal structure.
The macroscopic values (n) and (V) determined on bulk material are now calculated as a sum of atomic or molecular properties.
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.
This compatibility index is a required calculation for approval as a new mineral species (see IMA guidelines).
[10] The Gladstone–Dale relation requires a particle model of light because the continuous wave-front required by wave theory cannot be maintained if light encounters atoms or molecules that maintain a local electric structure with a characteristic refractivity.
Similarly, the wave theory cannot explain the photoelectric effect or absorption by individual atoms and one requires a local particle of light (see Wave–particle duality).
A local model of light consistent with these electrostatic refraction calculations occurs if the electromagnetic energy is restricted to a finite region of space.
An electric-charge monopole must occur perpendicular to dipole loops of magnetic flux, but if local mechanisms for propagation are required, a periodic oscillatory exchange of electromagnetic energy occurs with transient mass.
This local photon has zero rest mass and no net charge, but has wave properties with spin-1 symmetry on trace over time.
In this modern version of Newton's corpuscular theory of light, the local photon acts as a probe of the molecular or crystal structure.