The two displacements are now out of phase
and the two displacements will cancel out to give zero intensity 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