Gas densities and optical properties

For an ideal gas, properties such as density and refractive index scale as p/T. This approximation is valid if atomic spacing is large compared with atomic size and the gas temperature is far above its boiling point.
This may not be the case, for example, for CO2 or acetylene at room temperature.
Tables of gas properties must be used with caution, since they vary in reference temperature and pressure: In these pages, we follow the early density-effect papers of Sternheimer, Barger, and Seltzer, who tabulate the gas density at NTP, (20°, 1 atm). Note that the Handbook of Chemistry and Physics reference conditions are 25°, 1 atm.

Indices of refraction are taken from where they are given for dry gases at 0° and 1 atm, evaluated at the mean of the Na D doublet, 589.3 nm.
A calculator for the index of air can be found at, where the wavelength, temperature, pressure, humidity, and CO2 content can also be varied.
Index and dispersion for some gases can be found at, The reference pressure is usually not given (assumed to be 1 atm?). T is usually (but not always) 0° C. In some case, e.g. Kr, only UV data are given.

Written 2014 April 1
web pages as of Apr 2014