JEE Main · 2023 · 24 Jan 2023 · Shift 1 · EM Waves

In a plane electromagnetic wave the magnetic field is perpendicular to both the electric field $\vec{E}$ and the direction of travel $\vec{K}$, pointing along $\vec{K}\times \vec{E}$, with magnitude $B=E/c$. Since $c=\omega /K$, the magnitude becomes $B=\frac{K}{\omega }E$, and uniting size with direction gives $\vec{B}=\frac{1}{\omega }\left(\vec{K}\times \vec{E}\right)$.