question on refractive index........

Light travels fastest in vacuum which is 3x10^8m/s. The refractive index of vacuum is 1. Air has a refractive index that is very close to 1 but it is assumed to be approximately equal to 1.

So if light travels in any medium, its speed must decrease. To find the change in speed use the formula

refractive index = speed in vacuum/ speed in medium.

Try solving the question and write in if you don't get the answer. Don't forget to poste the steps that you have worked out.