5.6-2 Refractive Index of Air. The refractive index of air can be precisely measured with the help of a Michelson interferometer and a tunable light source. At atmospheric pressure and a temperature of 20 C, the refractive index of air differs from unity by n-1= 2.672 x 10-4 at a wavelength of 0.76 m, by n - 1 = 2.669 x 10-4 at a wavelength of 0.81 m, and by n-1= 2.665 x 10-4 at a wavelength of 0.86 m. (a) Using a quadratic fit to these data, determine the wavelength dependence of the group velocity. (b) Obtain an expression for the dispersion coefficient Dx in ps/km-nm and compare your result with that for a silica optical fiber.