A plane electromagnetic wave propagating in free space in incident normally on a large slab of loss-less, non magnetic, dielectric material with $\varepsilon>\varepsilon_{0}$. Maxima and minima are observed when the electric field is measured in front of the slab. The maximum electric field is found to be $5$ times the minimum field. The intrinsic impedance of the medium should be
- $120 \; \pi \Omega$
- $60 \; \pi \Omega$
- $600 \; \pi \Omega$
- $24 \; \pi \Omega$