Question

Optical density is only for glass right ?

Answer 1

Optical density is just a measure of how fast or how slow light propagates through a medium with respect to initial one.

OPTICAL DENSITY IS NOWHERE RELATED TO MASS DENSITY. This is a common misconception as one might observe that :

Density of vacuum<air<water<glass

And the speed of light decreases as it moves from one material to another, but there are many exceptions to this such as ETHANOL and WATER