5l corresponds to
a phase difference of 5p 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,