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I saw in my course than when light hit a medium, it makes some dipole oscillating with the same frequency as the one of the light $\omega$. By a classical mechanics reasoning, one can show that the indice $n=\frac{\epsilon}{\epsilon_0}$ depends on the frequency, $$n=n(\omega)$$ A wave is scattered by the medium, and the way it is scattered depends on its wave length. What I don't understand is why do we talk about refraction indices of some medium $n$, since it depends on the nature of the incident wave ? Why for example, do we talk about the indice of refraction of water, or glass ? This is not clear for me. Any help would be appreciated.

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  • $\begingroup$ Does this answer your question? Intuitive explanation for why blue light is refracted more than red light? $\endgroup$ – Semoi Jul 12 at 14:57
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    $\begingroup$ The index does vary with frequency but somewhat slowly over the range of frequencies covering visible light, so one can often take an average value without too much error. $\endgroup$ – ZeroTheHero Jul 12 at 15:05
  • $\begingroup$ Could the downvoter please explain the reason of the downvote ? $\endgroup$ – Dicordi Jul 12 at 16:00
  • $\begingroup$ @ZeroTheHero thank you this answers my question ! $\endgroup$ – Dicordi Jul 12 at 16:01
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Yes, the index of refraction is a function of frequency. I think, though it would depend upon the actual language used by an author, that many discussions of index of refraction revolve around optical (visible) frequencies. For visible frequencies, substances like glass and water have a roughly constant index of refraction.

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