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The refractive index of a medium is usually stated as a single "number" rather than something depending on wavelength. However, it does in fact depend on wavelength. This lead to a question I haven't been able to find the answer to:

The refractive index of Borosilicate glass is always stated to be $1.517$. However, is this with respect to some sort of standardised "white light" with known quantities of different wavelengths? Or is it with respect to one specific wavelength of light in the middle of the visible spectrum, around green or yellow?

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    $\begingroup$ Yellow I think. $\endgroup$ – Utkarsh futous Apr 8 '17 at 9:38
  • $\begingroup$ Normally this would be stated stated along with the actual number, i.e. the refractive index of material X and wavelength Y is Z. $\endgroup$ – ZeroTheHero Apr 8 '17 at 10:29
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"Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of $589$ nanometers"

Turns out another 15 minutes of googling was what it took to find an answer!

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    $\begingroup$ Often the index will be subscripted. It's very common nowadays to specify the index at the mercury e line at 541,6nm, especially for European glass and optics manufacturers. The standard ISO10110 favors the mercury e line for index and Abbe number specification. $\endgroup$ – WetSavannaAnimal Apr 8 '17 at 10:02
  • $\begingroup$ @Shanye please have a look at my answer below. $\endgroup$ – Wrichik Basu Apr 8 '17 at 14:48
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Absolute refractive index of any medium is defined as the ratio of the speed of light in vaccum to that of the speed of light on that medium, i.e. $$Refractive \, Index, \, \mu =\frac {speed\, of\, light\, in\, vaccum }{speed\, of\, light\, in\, that \, medium} $$

So, if the Refractive index of Borosilicate glass is $1.517$, it means that the ratio of speed of light in air to that in Borosilicate glass is $1.517$, that is, the speed of light in Borosilicate glass is $\frac {1}{1.517} $ times the speed of light in vaccum.

This light is always taken as "The mean colour of light", that is, approximately green light, as it lies in the middle of the VIBGYOR spectrum, and it's wavelength is approximately the average of the wavelengths of others.

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  • $\begingroup$ Most of this answer is already pre-understood in the OP, and your first paragraph is completely ambiguous with respect to the core question: yes, $n=c/v$, but $v$ depends on the wavelength, so saying "the speed of light in glass is..." without saying at what wavelength is precisely the problem OP points out. Furthermore, your second paragraph is vague ("green"? what wavelength of green?) and in direct conflict with the other specified alternatives, so, can you provide references to sustain it? $\endgroup$ – Emilio Pisanty Jul 26 '17 at 16:06

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