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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?

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  • $\begingroup$ I’ve removed a comment that should have been posted as an answer, and replies to it. $\endgroup$
    – rob ♦
    Commented May 24, 2022 at 17:14

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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.

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    $\begingroup$ Crazy. In 60 years of doing things with optics, I've never seen "optical density" used for "index of refraction". But there it is. $\endgroup$
    – John Doty
    Commented May 24, 2022 at 15:17
  • $\begingroup$ @JohnDoty - I agree, it does not appear in any college or graduate text book or monograph that I have on my shelf. Some vague memories from high school suggest it was used in analogy to mechanical waves and then dropped quickly. Its been a long time since I took physics in high school... $\endgroup$
    – Jon Custer
    Commented May 24, 2022 at 15:21
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    $\begingroup$ Reader beware: this usage of “optical density” is quite nonstandard. (Brittanica is not what it once was — for instance the video at the link above gets the direction of refraction wrong for light entering water.) In optics, the “optical density” usually refers to the absorbance of a material. For example, a “50% neutral density filter” would cut the intensity of transmitted light in half, without favoring one color over another and without making any brighter or dimmer patches. A neutral density filter would make a good lens for untinted sunglasses. $\endgroup$
    – rob ♦
    Commented May 24, 2022 at 17:28
  • $\begingroup$ @rob - No doubt it is nonstandard at, I would say, undergraduate and above. Still it comes up quite easily in Google searches and high school textbooks... Perhaps I should strengthen the intro. $\endgroup$
    – Jon Custer
    Commented May 24, 2022 at 18:27
  • $\begingroup$ Optical density is extremely used in academic contexts, for example in the spectroscopy field where OD (optical density) is the logarithmic light absorbance. Also recall how ND filters in optics are "neutral density" because they display an optical density (in absorption terms) that is roughly the same for a broadband region, and they are also rated by their OD value. Either way, as the refractive index is complex, optical density in this sense is more related to $\kappa$. $\endgroup$
    – José Andrade
    Commented May 25, 2022 at 8:16
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Devil's Advocate Answer. Optical density is the property of a material that affects the speed of light in that material (higher density, slower light speed) and the index of refraction is a measure of that property.

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