Question

The ratio of speed of light in vacuum to that in a medium is called ___________ (refractive index/focal length).

Answer 1

1. The cause of refraction is the change in the speed of light as it moves from one medium to another.
2. The refraction of light in a given medium can be expressed in terms of a factor called the refractive index of the medium.
3. Refractive index of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.
4. It is also called the index of refraction. It is denoted by the Greek letter$\mu$.
   $$\begin{gathered} Refractiveindex=\frac{Speedoflightinvacuum}{Speedoflightinthemedium} \\ \mu =\frac{C}{V} \end{gathered}$$.

Thus, the ratio of the speed of light in vacuum to that in a medium is called the refractive index.