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.
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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?
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Is it a misnomer then?
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What exactly does optical density depend upon then? Does it even depend on something or is it intrinsic property of a material?
Best Answer
In this particular context, "optical density" refers to the index of refraction of the material. This is not a common usage for physicists, as can be seen from the comments below. So, beware using 'optical density' to mean the index of refraction and be prepared to pretty much forget about using it as such going forward once you've finished this class.
But, current references include places like:
Physics Classroom - "Like any wave, the speed of a light wave is dependent upon the properties of the medium. In the case of an electromagnetic wave, the speed of the wave depends upon the optical density of that material."
Or, even Brittanica.com - "Recall that as light passes from one medium into another, it bends, causing refraction! It bends as a result of changing velocity at the surface between the two media. Just how much a particular medium slows the transmission of light is known as its optical density."
So, yes, the 'optical density' here is the index of refraction of the material. Now $n$ can depend on various things, including temperature, actual material density $\rho$, etc.