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

$\sqrt{\mu_{r} \epsilon_{r}}$

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}$