The electric field of a plane electromagnetic wave, travelling in an unknown nonmagnetic medium is given by, $$ E_y=20 \sin \left(3 \times 10^6 x-4.5 \times 10^{14} \mathrm{t}\right) \mathrm{V} / \mathrm{m} $$ (where $x, \mathrm{t}$ and other values have S.I. units). The dielectric constant of the medium is $\_\_\_\_$ (speed of light in free space is $3 \times 10^8 \mathrm{~m} / \mathrm{s}$ )