CBSE Class 10 Science 2019 · Paper 31/1/2/C · Question 5(a)

The refractive index of a medium is inversely related to the speed of light in it: the absolute refractive index is $n_m=\frac{c}{v}$, the speed of light in vacuum divided by the speed of light in the medium, so the higher the refractive index, the slower light travels in that medium. For medium 1 (speed $v_1$) and medium 2 (speed $v_2$): $n_1=\frac{c}{v_1}$ and $n_2=\frac{c}{v_2}$, so the refractive index of medium 2 with respect to medium 1 is $n_{21}=\frac{n_2}{n_1}=\frac{c/v_2}{c/v_1}=\frac{v_1}{v_2}$, that is, the speed of light in medium 1 divided by the speed of light in medium 2.