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In an EM wave travelling along the $+z$ direction with $\vec{E}$ along $+x$, the magnetic field $\vec{B}$ is along:
A$+z$, the same as the direction of wave travel
B$-x$, opposite to the electric field direction
C$+y$, perpendicular to both E and the propagation direction
DRandom direction, since B does not align fixed in space
Answer & Solution
Correct answer: C. $+y$, perpendicular to both E and the propagation direction
EM wave: $\vec{E}\times\vec{B}$ points along propagation. $\hat x\times\hat y = \hat z$, so $\vec{B}$ is along $+y$.