Define absolute refractive index. Absolute refractive indices of medium $A$ and medium $B$ are $n_{a}$ and $n_{b}$ respectively. What is the refractive index of medium $B$ with respect to medium $A$? How does the velocity of light vary with change in the optical density of the media?

(N/A) The absolute refractive index of a medium is defined as the ratio of the velocity of light in a vacuum to the velocity of light in that medium. If $c$ is the velocity of light in a vacuum and $v$ is the velocity in the medium,then $n = c / v$.
If $n_{a}$ and $n_{b}$ are the absolute refractive indices of media $A$ and $B$ respectively,then $n_{a} = c / v_{a}$ and $n_{b} = c / v_{b}$.
The refractive index of medium $B$ with respect to medium $A$ $(_{a}n_{b})$ is given by:
$_{a}n_{b} = \frac{\text{Velocity of light in } A}{\text{Velocity of light in } B} = \frac{v_{a}}{v_{b}} = \frac{c/n_{a}}{c/n_{b}} = \frac{n_{b}}{n_{a}}$.
Regarding the velocity of light: The velocity of light is inversely proportional to the optical density of the medium. As the optical density of a medium increases,the velocity of light in that medium decreases,and vice versa.