Question

What is meant by Refractive index?

Answer 1

The refractive index of the second medium with respect to the first medium is given by the ratio of the magnitude of the velocity of light in medium first to that in $2^{nd}$ medium. It is represented by ${}^1{\eta }_2$.
Refractive Index$=\frac{\text{Velocity of light in first medium}}{\text{Velocity of light in second medium}}$
i.e., ${}^1{\eta }_2=\frac{v_1}{v_2}$

and ${}^2{\eta }_1=\frac{v_2}{v_1}$.