The optical density is defined as the property that explains how a light wave propagates while passing through a material. It also determines the amount of lightwave getting slowed down while passing through a medium.
The refractive index refers to the ratio between the speed of light in a Vaccum to the speed of light in any medium.
This refractive index is a direct quantification of optical density and they are both proportional to each other.
For example, the optical density, as well as the refractive index of a diamond, is the highest. On the other hand, the optical density and refractive index of air is the lowest.
