How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium with respect to another in terms of the speed of light in these two media.

(N/A) The refractive index $(n)$ of a medium is defined as the ratio of the speed of light in a vacuum $(c)$ to the speed of light in that medium $(v)$. Mathematically,it is expressed as: $n = \frac{c}{v}$.
To find the refractive index of medium $2$ with respect to medium $1$ $(n_{21})$,we take the ratio of the speed of light in medium $1$ $(v_1)$ to the speed of light in medium $2$ $(v_2)$. The expression is: $n_{21} = \frac{v_1}{v_2}$.