In a medium the speed of light wave decreases to 0.2 times to its speed in free space The ratio of relative permittivity to the refractive index of the medium is $x_i$. 1 . The value of x is $\_\_\_\_$ . (Given speed of light in free space $=3 \times 10^8 \mathrm{~m} \mathrm{~s}^{-1}$ and for the given medium $\mu_r=1$ )