A sinusoidal electromagnetic wave in a vacuum is propagating in the positive z-direction. At a certain point in the wave at a certain instant in time, the electric field points in the negative x-direction. At the same point and at the same instant, the magnetic field points in the

A. positive y-direction.
B. negative y-direction.
C. positive z-direction.
D. negative z-direction.
E. none of the above

An electromagnetic wave is created by the fluctuation of the electric and magnetic fields, which are perpendicular to each other, the direction of propagation is perpendicular to these two fields.

Let's apply the above to our problem, the wave propagates in the positive direction of the z-axis so that the electric and magnetic fields must be in the xy plane

They express that the electric field is in the negative direction of the x-axis, for the direction of the other field, we can make the vectoial product E x B with the result must be in positive z the only way is that the magnetic field (B) is in the negative direction of the y axis

The correct answer is B negative y-direction