Electromagnetic waves travel in a medium with a speed of  $0.75\times {10}^{8}m/s.$The relative permeability of the medium  is 1. Find the relative permittivity.

$$\begin{gathered} we know that velocity em wave in a medium =V=\frac{C}{\sqrt{{\mu }_r{\in }_r}} \\ here V= Velocity of electromagnetic wave in medium \\ C=Velocity of electromagnetic wave in vaccum \\ {\mu }_r= relative permeability of the medium \\ {\in }_r= relative permittivity of the medium \\ \Rightarrow {\in }_r=\frac{{{\displaystyle C}}^{{ }_2}}{V^2{\mu }_r}=\frac{3\times {10}^8\times 3\times {10}^8}{0.75\times 0.75\times {10}^8\times {10}^8\times 1}=16 \end{gathered}$$