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 The two displacements are once again in phase
and the intensity will again be 4 I0 giving rise to constructive interference