My book states that:
When passing from one medium to another, if light slows down, the second medium is said to be optically denser than the first medium, and if light speeds up, then the second medium is said to be optically rarer than the first medium.
It then proceeds to give an example that kerosene oil, being less dense than water, has higher optical density than water, showing that material density and optical density are not interrelated.
What is dense here? Like in material density, its the concentration of molecules in a given space. But what is being crammed together in a given quantity while considering optical density?
Is it a misnomer then?
What exactly does optical density depend upon then? Does it even depend on something or is it intrinsic property of a material?