Question

An electromagnetic wave is traveling straight down toward the center of the Earth. At a certain moment in the time the electric field ponts east. In which direction does the magnetic field point at this moment? Explain your reasoning.

Answer 1

Remember the basic property of EM waves: the E field, the B field and the velocity v, all are perpendicular to each other.

E is the electric field vector, and B is the magnetic field vector of the EM wave. For electromagnetic waves E and B are always perpendicular to each other and perpendicular to the direction of propagation. The direction of propagation is the direction of E x B.

So, as the E field is to the right(east), v is down, therefore B must point out of the page.