Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / B7 / a / Solution

The relative slit-centre phase is $2\pi d\sin \theta /\lambda$. Near the optical axis, $x\simeq L\theta$, so adjacent interference maxima have spacingSlow diffraction-envelope variation can slightly shift the exact local maxima.