The ratio of speed of light in vacuum to the speed of light in a medium is :
Where $\mu_{r}=$ relative permeability of the medium
$\epsilon_{r}=$ relative permittivity of the medium
Answer & explanation
Correct answer: option 1
Speed of light in vacuum is $ c = \frac{1}{\sqrt{\mu_0\epsilon_0}}$
Speed of light in medium is $ v = \frac{1}{\sqrt{\mu\epsilon}}$
$ \frac{c}{v} = \frac{\sqrt{\mu\epsilon}}{\sqrt{\mu_0\epsilon_0}}= \sqrt{\mu_r\epsilon_r}$