Optical density

 

Refractive index is a constant that identifies a pure substance. Watch the diagram -

The refractive index of many substances increases as the density increases. Ice - water- (salt water) etc. The observation (by analogy) lead to the term optical density, indicating a link between density and refractive index.

Note: the rule is not universal. Ethyl alcohol has a refractive index of 1.36 and a density of 0.96 g/cc.