An E.M. wave is propagating along the +z-axis. The electric and the magnetic field in it can be represented by

The electromagnetic wave is propagating along the +z axis. Since the electric and magnetic fields are perpendicular to each other and also perpendicular to the direction of propagation of the wave.

Also, $\overrightarrow{E}\times \overrightarrow{B}$ gives the direction of wave propagation.

$\begin{array}{l}\overrightarrow{\mathrm{E}}={\mathrm{E}}_0\widehat{\mathrm{i}},\mathrm{B}={\mathrm{B}}_0\widehat{\mathrm{j}}\\ (\because \widehat{\mathrm{i}}\times \widehat{\mathrm{j}}=\widehat{\mathrm{k}})\end{array}$