Why is it that that poynting vector is independent of distance from the source? Is it because EM waves are plane waves? Furthermore I do not fully understand why EM waves have to be plane waves. I understand that changing magnetic fields give rise to electric fields and vice versa, but does that constitute a constant E and B field over an infinite plane? My book states that the intensity of the wave decreases as 1/r^2, but I don't see this anywhere in the poynting vector, and doesn't that violate the premise of a plane wave?