The part of each secondary wave, originating from these small elements and travelling normal to the plane of slit will be focussed at P0 while those travelling at an angle will reach P (Fig.2b). Considering first the wavelet emitted by the element ds situated at the center of slit (let us call it origin). Its amplitude will be directly proportional to length ds and inversely proportional to the distance x (it will produce spherical wave front). At P, it will produce an infinitesimal amplitude (electric field) which is expressed as