Consider two electromagnetic?plane waves eq.?

one with amplitude E^a₀ and wave vector k^a and the other with amplitude E^b₀ and wave vector k^b. These waves are said to add coherently if the average energy density u in the resulting wave is proportional to |E^a₀ + E^b₀|² or incoherently if the average energy density is proportional to |E^a₀|² + |E^b₀|². Show that two electromagnetic plane waves are coherent only if they are propagating in the same direction with the same frequency and the same polarization.

E1) = Eo sin(kx wt), B(1) = Eo sin(kx wt). E2) = Eo sin(k.x wt), B(2) - Eo sin (k.x ot)