When superposing waves in say double slit interference from two slits, I seem to have come across two approaches: 1. Sum the two waves in complex form to get the resultant amplitude, take the real part, and square to get the intensity, i.e I=[Re(A)]2 2. Sum the two waves in complex form to get the resultant complex amplitude. Then take the modulus squared to get the intensity, i.e I=AA*. What is it that makes these two approaches give the same result. Mathematically they seem to be different to me. Also, where does the second come from? I know I=cε0E2 but this doesn't involve the modulus does it?