10(b)] 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 + 5p)  = – a cos wt
FIGURE 10 9
(a) Constructive
interference at a
point Q for which the
path difference is 2l (b) Destructive
interference at a
point R for which the
path difference is
2