For my own curiosity's sake, I am attempting to find the gas that at 15 °C and 1 atmosphere of pressure in pure form has the highest refractive index on average and overall, for visible light. Assume visible light means having a wavelength in the range of 380 to 750 nanometers.

Please keep in mind that I am not looking for the densest gas whether active or inert. Density does not necessarily correlate with a materials refractive index.

If for some reason volume needs to be specified, please assume a 1x1x1 kilometer region.

I am being very specific in what I am seeking and have specified wavelengths, pressure, temperature, purity, and even volume.

If I must for whatever reason, I must be any more specific then please assume that the gravity is the same as on Earth at sea level on the equator though I do not actually require that level of precision.

Sea level conditions are commonly used as a common standard for predicting the behaviors of different materials here on Earth and a common standard of that nature was exactly what I needed.

The reasons for the described conditions specificity is to provide a decent set of common standard conditions for measurement and avoid the mistakes of a similar previous question, see: What gas has the highest refractive index? https://en.wikipedia.org/wiki/Standard_sea-level_conditions

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    $\begingroup$ Get hold of Landolt-Bornstein handbook. Small alkanes like n-propane and n-butane will have reasonable refractive index in the visible region. $\endgroup$
    – AChem
    Jan 29, 2023 at 20:14
  • $\begingroup$ The full reference is Landoldt-Bernstein (1962) Zahlenwerte und Funktionen. Eig- enschaften der Materie in ihren Aggregatzustanden. 8. Teil (edited by Hellwege K. H. und Hellwege A. M.), pp. 871-877. $\endgroup$
    – AChem
    Jan 29, 2023 at 20:42

1 Answer 1


The Engineering Toolbox gives indices of refraction for various gases, presumably at one atmosphere pressure(and thus requiring temperatures above 15°C in many cases). Among substances that are actually gases at 15°C and one atmosphere, (di)methyl ether at 1.000891 tops this list. This makes dimethyl ether about three times as strongly refractive against a vacuum as air.

But that list is not exhaustive. The refractive index of uranium hexafluoride as a function of temperature and pressure is given in Ref. 1 yields 1.001455 at 632.8 nm and 1.001496 at 457.9 nm, both at 15°C and 760 torr (1 atmosphere). Uranium hexafluoride is thus over 60% more refractive against a vacuum than dimethyl ether, and more than five times as refractive as air.


  1. Wright, S.P. (Aug 1982). Index-of-refraction measurement of uranium hexafluoride vapor at 6328 and 4579 nanometers (K/AIS--5003). United States

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