1. The problem statement, all variables and given/known data (this is high school level) The pic below shows interference fringes observe in Young's fringes experiment: Explain why wider slits at the same spacing could produce fringes at the outer edges brighter than some of the fringes nearer the centre. Bear in mind there was a question (part a) just before which asked me to explain why fringes at the outer edges are dimmer than fringes nearer the centre. I answered by saying that because these interference fringes have to be contained within the 'envelope' intensity shape of the central diffraction fringe, it therefore follows that intensity decreases away from the centre. 2. Relevant equations fringe spacing of interference fringes = (lamda * D) / slit separation width of central diffraction fringe = (2 * lamda * D) / aperture width D = slit-screen distance 3. The attempt at a solution I know from the second formula above that wider slits will mean that the central diffraction fringe will be smaller in width. But thats where I'm stuck. I don't understand how its possible that fringes (just to make sure they mean interference fringes, right?) at the outer edges could be brighter than some of the fringes nearer to the center just because the slit is made wider. Surely these interference fringes would still follow the 'envelope' shape in intensity variation set out by the diffraction fringe in which they are contained within. and just to confirm something basic, is the picture I've attached above showing only the central diffraction fringe for the red and blue light?