The refractive index of the medium nm is given by
nm= (Speed of light in air) / (Speed of light in medium)=c/v
Let v1 be the speed of light in medium 1 and v2 be the speed of light in medium 2. The refractive index of medium 2 with respect to medium 1 is given by the ratio of the speed of light in medium 1 and the speed of light in medium 2. This is usually represented by the symbol n21.
n21 =(Speed of light in medium 1)/ (Speed of light in medium 2)=v1/v2