If $\varepsilon_0$ and $\mu_0$ are respectively, the electric permittivity and the magnetic permeability of free space, $\varepsilon$ and $\mu$ the corresponding quantities in a medium, the refractive index of the medium is:

$\sqrt{\frac{\mu \varepsilon}{\mu_0 \varepsilon_0}}$

Refractive index, $\eta=\frac{c}{\mathrm{v}}=\frac{\frac{1}{\sqrt{\mu_0 \varepsilon_0}}}{\frac{1}{\sqrt{\mu \varepsilon}}}=\sqrt{\frac{\mu \varepsilon}{\mu_0 \varepsilon_0}}$