Electric field of a plane electromagnetic wave propagating through a non-magnetic medium is given by \( E = 20 \, \sec(2 \times 10^{10}(t - 200x)) \, \text{V}/\text{m} \). The dielectric constant of the medium is equal to: (Take \( \mu_r = 1 \))
\( 2 \)
\( 9 \)
\( \tfrac{1}{3} \)
\( 3 \)
