This is
referred to as destructive interference To summarise: If we have two coherent sources S1 and S2 vibrating
in phase, then for an arbitrary point P whenever the path difference,
S1P  ~ S2P =  nl     (n = 0, 1, 2, 3, )
(10 9)
we will have constructive interference and the resultant intensity will be
4I0; the sign ~ between  S1P and S2 P represents the difference between
S1P and S2 P