# Refractive Index Base Wavelength

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?

• Yellow I think. – Utkarsh futous Apr 8 '17 at 9:38
• Normally this would be stated stated along with the actual number, i.e. the refractive index of material X and wavelength Y is Z. – ZeroTheHero Apr 8 '17 at 10:29

"Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of $589$ nanometers"