Electric field of plane electromagnetic wave propagating through a non-magnetic medium is given by $\text{E}=20\cos \left(2\times {10}^{10}\text{t}-200\text{x}\right)\frac{\mathrm{V}}{\mathrm{m}}$ . The dielectric constant of the medium is equal to 

$\left({\mathrm{\mu }}_{\mathrm{r}}=1\right)$