光在空氣的速度是多少???

Since refractive index of a medium n is the ratio of the speed of light in vacuum c to that in the medium v, i.e.
n = c/v
thus, v = c/n
The speed of light in vacuum c is 299 792 458 m/s
and the refractive index of air is 1.0003
thus, the speed of light in air v(air) = 299792458/1.0003 m/s = 299 702 547 m/s
The following link gives the refractive indices for a number of substances from which the speed of light in these substances can be calculated:
http://www.glenbrook.k12.il.us/gbssci/Phys/Class/refrn/u14l1d.html