  We present an update of the "refractive index of air" experiment commonly
used in optics and undergraduate advanced labs. The refractive index of air is
based on the average molecular polarizability, which we measured from the
period of the phase shift in a Michelson interferometer as a function of
pressure. Our value of the average molecular polarizability of air is
\gamma_mol = 2.133 \pm 0.032 \times 10^{-29} m^3 (95% CI) and from this we find
the refractive index of air at atmospheric pressure to be n = 1.0002651(66),
which is in agreement with the accepted value of n=1.000271375(6).
