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 In the above analysis we have assumed that the distances S1Q and S2Q
are much greater than d (which represents the distance between S1 and
S2) so that although S1Q and S2Q are not equal, the amplitudes of the
displacement produced by each wave are very nearly the same We next consider a point R [Fig