Two identical sources $\mathrm{S}_{1}$ and $\mathrm{S}_{2},$ separated by distance $d,$
coherently emit light of wavelength $\lambda$ uniformly in all direc-
tions. Defining the $x$ axis with its origin at $S_{1}$ as shown in
Fig. $32,$ find the locations (expressed as multiples of $\lambda )$
where the signals from the two sources are out of phase
along this axis for $x>0$ , if $d=3 \lambda.$