An electromagnetic wave is propagating along the positive x-axis. At a given instant and position, the electric field vector points along the negative y-axis. The direction of the magnetic field vector at that instant is:

Step-by-step solution

The direction of propagation of an electromagnetic wave is given by the vector c = E B . Given that the wave propagates along the positive x-axis, c = i . The electric field is along the negative y-axis, so E = - j . We know that c , E , and B form a mutually perpendicular right-handed triad. Therefore, the direction of the magnetic field can be found using B = c E . Substituting the given unit vectors: B = i (- j ) = -( i j ) = - k Thus, the magnetic field is along the negative z-axis. Answer: negative z-axis