Past exam of the physics 2 paper of the University of Oxford / 2014 / 7 / a / 1 / Solution

Let $I_s$ be the intensity that either narrow slit would produce alone, and let $y$ measure screen displacement from the perpendicular bisector. The optical path difference is approximately $\delta =b\sin \theta \simeq by/D$. Equal coherent field amplitudes then giveThe maxima are at $y_m=m\lambda D/b$; the minima are halfway between them. HereThe expression uses the paraxial approximation and neglects the slowly varying single-slit diffraction envelope because the slit widths are unspecified.