The speed of light in any medium is given by the speed of light in vacuum divided by the relative refractive index of the given material. Now the relative refractive index of vacuum is obviously one. And the relative refractive index of is 1.0003 at STP, and the value of C or the speed of light in vacuum is approximately 300000000 m/s ,so the difference between speed of light in vacuum in air and in vacuum =89973.0080976 m/s , therefore there is quite a lot of difference in the speed of light in air and in vacuum . But this difference does not bring a lot of difference in short distance observations ,so in case of our daily life observations it is not a matter of worry but for a long distance observation it might cause a problem.
nakul
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