9(a)]
for which
S2Q –S1Q = 2l
The waves emanating from S1 will arrive exactly two cycles earlier
than the waves from S2 and will again be in phase [Fig 10 9(a)] Thus, if
the displacement produced by  S1  is given by
y1 = a cos wt
then the displacement produced by  S2  will be given by
y2 = a cos (wt – 4p)  =  a cos wt
where we have used the fact that a path difference of 2l corresponds to a
phase difference of 4p