Hence phase difference between two particles is \(\Delta \phi = \frac{{2\pi }}{\lambda }\Delta x\)
==> \(\left( {\phi - \frac{\pi }{3}} \right) = \frac{{2\pi }}{{60}} \times 15\)
\( \Rightarrow \phi - \frac{\pi }{3} = \frac{\pi }{2} \Rightarrow \phi = \frac{{5\pi }}{6}\)