Question

why the direction along which the electromagnetic wave travels is not the same as the direction along which the changing electric field is applied.

Answer 1

Which of these is the beginning of the life cycle of stars with different initial masses?

red giant

supernova

nebula

blackhole

Answer 2

By using Maxwell's Equations and some obnoxious vector calculus, you can prove Poynting's Theorem, which says the vector S along which electromagnetic energy is changing most rapidly must be given by:

S = (c/4pi) (E x B)

where E is the electric field vector and B is the magnetic field vector. If energy is flowing in only one direction, i.e. we have a plane EM wave propagating along the +z axis, then S points along the +z axis. However, since it's equal to the cross product of the E and B fields, it therefore follows that both those fields must be perpendicular to S, e.g. along the +x and +y axes.