An electromagnetic wave is propagating towards the west. At a certain moment the direction of the magnetic field vector associat
ed with this wave points vertically up. The direction of the electric field vector of this wave options: vertical and pointing down.
vertical and pointing up.
horizontal and pointing north.
horizontal and pointing south.
horizontal and pointing east.
The Poynting vector points in the direction of propagation (if the wave is a sinusoidal linearly polarized plane wave of fixed frequency, as it is in our case), and it's given by (all vectors):
where points in the same direction as , the magnetic field. This means that must point south, because the vectorial product of a vector pointing south by a vector pointing up gives a vector pointing west. This can be deduced by the right-hand rule: if our index finger is and our middle finger is , then the thumb is .