A uniform plane wave traveling in free space and having the electric field
$$\overrightarrow{E} =\left ( \sqrt{2}\:\hat{a}_{x} -\hat{a}_{z}\right )cos\left [ 6\sqrt{3}\: \pi \times 10^{8}t-2\:\pi \left ( x+\sqrt{2} \:z\right )\right ]V/m$$
is incident on a dielectric medium (relative permittivity $>1$, relative permeability $=1$) as shown in the figure and there is no reflected wave.
The relative permittivity (correct to two decimal places) of the dielectric medium is __________.