An interference pattern is produced by four parallel and equally spaced, narrow slits. By drawing appropriate phasor

Question:

An interference pattern is produced by four parallel and equally spaced, narrow slits. By drawing appropriate phasor diagrams, show that there is an interference minimum when the phase difference Φ from adjacent slits is
(a) π/2;
(b) π;
(c) 3π/2 in each case, for which pairs of slits is there totally destructive interference?