What is the relationship between the electric and magnetic fields of an electromagnetic wave?
Why An electromagnetic wave consists of an electric field and a magnetic field that regenerate each other as the wave moves. The two fields are oriented perpendicular (90 degrees) to each other, and both are perpendicular to the direction the wave travels. That geometry is why antenna polarization is defined by the electric field orientation: a vertical antenna radiates a vertical E field with the magnetic field lying horizontally.
Watch out Saying they are in parallel is the common trap, but parallel fields would not sustain each other; and both fields travel together at the same speed, the speed of light, so nothing lags behind.
E and H at 90 degrees to each other and to the direction of travel: think of the three edges of a corner of a box.