Determinate the phase difference between tow waves overlaping on an infinitely large screen placed at distance D from two point sources A and B as a function of the coordinate y where the waves overla

Determinate the phase difference between tow waves overlaping on an infinitely large screen placed at distance D from two point sources A and B as a function of the coordinate y where the waves overlap. The sources are in a vertical plane, separated by a distance 2y0. All considerations are taken in the small indcidence angle regime.