[alancalverd]

Hey...  Does anyone have any comments on my back-of-the-envelope calculation of air being 99.85% vacuum?

It depends on how you measured the refractive index of liquid nitrogen! Most of us would start with a light ray in air and apply a 0.03% correction, which is a bit circular to say the least.

[evan_au]

A small tweak to the calculation:
Solid nitrogen, at 1038 g/l has even less space between the atoms than liquid nitrogen.
https://en.wikipedia.org/wiki/Solid_nitrogen#Bulk_properties

- This gives 0.12% matter in nitrogen gas at STP
- The refractive index of solid nitrogen is 1.25
- So the calculated refractive index  (assuming linear change with density) is 1 + 0.25 x 0.0012 = 1.0003
- Which is a little closer to the measured value of 1.00027
https://en.wikipedia.org/wiki/List_of_refractive_indices#List

I agree with BC that graphing the value at various attainable pressures would give an accurate trend that you could extrapolate to a perfect vacuum. This would also account for possible non-linear effects.

[alancalverd]

This gives 0.12% matter in nitrogen gas at STP

By mass, certainly, but maybe even less by volume as the N₂ molecules are tightly bound and presumably take up a bit less space than two N atoms.

Do you have data for solid argon? And if so, I wonder what impelled anyone to measure it?

[Bored chemist]

Do you have data for solid argon?

https://www.sciencedirect.com/science/article/abs/pii/S0031891463801688

I wonder what impelled anyone to measure it?

One possible reason is that solid argon is used as a "container" for reactive chemicals.
The chemicals are created by photolysis and then studied by spectroscopy while "dissolved" in frozen argon.

If you are doing spectroscopy, you need to know how much light is lost by reflection etc and knowing the refractive index would help calculate that.