in the given arrangement S₁ and S₂  are coherent sources [Fig.(1)]. the point P is a point of 
 

The path difference at P is
$$\begin{gathered} {\mathrm{S}}_1\mathrm{P}-{\mathrm{S}}_2\mathrm{P}=2\mathrm{\lambda }\text{ (even multiple of }\mathrm{\lambda }\text{ ) } \\ \mathrm{Hence} \mathrm{P} \mathrm{is} \mathrm{a} \mathrm{point} \mathrm{of} \mathrm{bright} \mathrm{fringe} \end{gathered}$$