In Young's double slit arrangement, let βbe the fringe width and let Io be the intensity at the central bright fringe. At a distance x from the central bright fringe, the intensity will be

Path difference = x (d /D ) where  d is the separation between two sources and D is the distance of screen from two sources. Phase differenceϕ=2πλ×xdD ⇒φ2=πxβIntensity at a distance x from the central bright fringe is given by I = I0 cos2 φ2=I0 cos2 πxβ