^Diffraction & Huygens’ theory
Diffraction can be explained using Huygens’ theory. According to this theory all parts of the slit AB will become source of secondary wavelets, which all start in the same phase at that position. The wavefronts from any two corresponding points such as (1, 13), (2, 12), (3, 11) etc. from the two halves of the slit travel identical distances to reach O thus have zero path difference, hence they add constructively to produce a bright fringe at point O, centre called central maxima or central bright fringe.