Structure of an EM wave — the E and B fields

What just happened?

An EM wave is coupled oscillations of the electric field $E$ and the magnetic field $B$. The E field oscillates in one plane (here: vertically), the B field in a plane perpendicular to it (into the screen), and both are perpendicular to the wave’s direction of motion.

Field orientation:

The E and B vectors and the direction of motion $\vec{v}$ form a mutually perpendicular triad. That is why an EM wave is a transverse wave — the fields oscillate across the direction in which the wave travels.

In phase and a constant ratio:

The E and B fields reach their maxima and zeros at the same moments — they oscillate in phase. Their ratio is constant and equal to the speed of light $c \approx 3\cdot 10^8\,\text{m/s}$, so when E grows, B grows in the same proportion.

Does an EM wave need a medium?

No. Unlike sound, an electromagnetic wave does not need a medium — a changing E field creates a B field and vice versa, so the wave “drives itself” and travels even through a vacuum (e.g. sunlight from the Sun).

Why you need this for the exam:

Classic questions: how E and B are oriented (perpendicular, in phase), whether the wave is transverse (yes), whether it needs a medium (no), and the relation $E/B=c$. Trick: draw the triad $E\perp B\perp \vec{v}$ and remember the in-phase oscillation.

Remember: in an EM wave E ⊥ B ⊥ direction of motion, both fields oscillate in phase, and E/B = c. It is a transverse wave that travels even through a vacuum.