Hi all, I have a small misunderstanding about the energy conservation in diffraction from 2 slits. First, I understand the energy conservation of interference from 2 slits. If intensity from each slit is I, then I have intensity of 2I after slits plane. Interference is given by: So at bright fringes I get cos^2=1, so intensity is 4I. And in dark fringes I get cos^2=0, so intensity is 0. Since interference just distributes the intensity over screen, the cos^2 pattern with 4I maximum and 0 minimum, on average, results in a 2I intensity, as just after the slits plane. Now I add the diffraction, i.e. multiply by sinc^2. This leads to a lower intensity pattern compared to the case of pure interference, so averaged intensity will be lower then 2I. Where the energy is lost?