The electric fields of two harmonic waves of angular frequency w, and w2₂ are given by E₁ = E₁,ocos (k₁y+wit)i and E₂ = E2,0cos (k₁y-w₂t)î. Find (a) the instantaneous Poynting vector for the resultant wave motion and (b) the time-average Poynting vector. If the direction of propagation of the second wave is reversed, find (c) the instantaneous Poynting vector for the resultant wave motion and (d) the time -average Poynting vector.