JEE Main · 2023 · 25 Jan 2023 · Shift 1 · EM Waves

For a plane electromagnetic wave the energy flows along $\vec{S}=\vec{E}\times \vec{B}$, so $\hat{E}\times \hat{B}$ points along the direction of propagation. Here the energy travels along $-\hat{k}$ while $\vec{E}$ lies along $+\hat{j}$, so $\hat{j}\times \hat{B}=-\hat{k}$. Since $\hat{j}\times \hat{i}=-\hat{k}$, the magnetic field must point along $+\hat{i}$. The magnetic field at that instant lies along the positive $x$ direction.