Re: What is the speed of light as a function of air density?

The speed of light in a vacuum is c = 2.9979 x 10^8 meters/sec.
Air is so diffuse that light passes through it at almost
the same speed. The speed of light 
decreases linearly with increasing air density d:

(speed of light in air) = c * (1 - .00029*(d / d0))

where d0 = 1.29 kg/m^3 is the density of air at 
room temperature and pressure. The equation
is approximately true even for densities 100 times
greater than d0. 

This equation works for visible light only. 
If you're considering other parts of the spectrum 
(infrared, ultraviolet, X-ray, etc.) the 
form of the equation will be the same, but the constant
will be different from .00029.

If you'd like to find more information, search the 
Mad Scientists Network or your local library for 
references to the speed of light or the index of refraction.