Alas, extrapolation is not going to work, as it is not a simple exponential or power function, though it looks similar. If you dont use the correct extrapolation method, then the extrapolation is meaningless. At the moment the only way to calculate the refractive index is to use the actual equation that Mathematica calculator uses, and I do not know what it is, particularly since there are different algorithms that can be used (eg the Edlen and Ciddor equations, which have their own limitations).
Personally though, I feel it is a moot point, given that there is no conceivable way that they could have computed or had the equipment to measure the refractive indices using non-visible wavelengths (unless you invoke advanced technology of which there is no archaeological evidence for).
This is all I can do on this for the time being.
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