The electric field part of an electromagnetic wave in a medium is represented by $E_x=0$, $E_y=2.5\frac{\mathrm{N}}{\mathrm{C}}\cos \left[\left(2\mathrm{\pi }{\times 10}^6\frac{\mathrm{r}\mathrm{a}\mathrm{d}}{\mathrm{m}}\right)t-\left(\mathrm{\pi }\times {10}^{-2}\frac{\mathrm{r}\mathrm{a}\mathrm{d}}{\mathrm{s}}\right)x\right]$,  $E_z=0$. The wave is