(a) Electric field(b) Magnetic fieldIn a plane electromagnetic wave, the electric and the magnetic fields oscillate sinusoidally. For an electromagnetic wave propagating in the z-direction, the electric and magnetic fields are given by.Ex​=E0​sin(kz−ωt) By​=B0​sin(kz−ωt) These are sinusoidal functions. Therefore, for a fixed value of z, the average value of the electric and magnetic fields are zero.