The basic idea is to divide the slit into much smaller
parts, and add their contributions at P with the proper
phase differences We are treating different parts of the
wavefront at the slit as secondary sources Because the
incoming wavefront is parallel to the plane of the slit, these
sources are in phase It is observed that the intensity has a central
maximum at q = 0 and other secondary maxima at q l
(n+1/2) l/a, which go on becoming weaker and weaker
with increasing n