The speed of energy propagation = e/n wave is the speed of the wavefront plane wave that travel in the k v wae direction (wavefronts normal to k ). but k is not, in general, the direction in which energy is transported. exb instead, energy propagates along the direction of the poynting vector s s this speed is given by, ray velocity in the sdirection v ray,š 1 where ss exb energy transported by the em wave per unit area and per unit time energy density of the electromagnetic field 2 exb 4.a show that or equivalently, u . -od hint: use b = k x e b (cxa) a (bxc) c(axb) 1 c2-1/() and h v wavek 4.b show that ray, i cosd s where is the angle between k and s