It is possible to "see" colorless gases due to their different refractive index with respect to that of air.
By projecting a light with more or less parallel rays through the gas, a series of shadows and bright areas are projected on a white screen due to the difference in refractive index of the gas with respect to air.

https://en.wikipedia.org/wiki/Refract...
https://en.wikipedia.org/wiki/Butane
https://en.wikipedia.org/wiki/Carbon_...
https://en.wikipedia.org/wiki/Helium
https://en.wikipedia.org/wiki/Mirage

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