(a) We can regard the total contributions of the wave front LN at some point P on the screen, as the resultant effect of the superposition of its wavelets like LM, , . These have to be superposing taking into account their proper phase differences. We therefore, get maxima and minima, i.e. a diffraction pattern, on the screen.
(b) Conditions for first minimum on the screen
Angular width of the central fringe on the screen
Angular width of first diffraction fringe,
Hence angular width of central fringe is twice the angular width of first fringe.
(c) Maxima become weaker and weaker with increasing n. This is because the effective part of the wave front, contributing to the maxima, becomes smaller and smaller, with increasing n.
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