The intensity of light due to (see figure) interference of two waves (1) and (2) which have path difference of λ is: (where I0 = Intensity of each wave)

Zero I0 2I0 4I0

4I0

Path difference is $\lambda$ $\Rightarrow \phi = 2\pi$ $ I = I_0 + I_0 + 2\sqrt{I_0 I_0} cos{2\pi} = 4\ I_0$