Two particles $\mathrm{P} and \mathrm{Q} performs S.H.M. of same amplitude and frequency along the same straight line. At a particular instant, maximum distance between two particles is \sqrt{2} a. The initial phase difference between them is
\left[\sin ^{-1}\left(\frac{1}{\sqrt{2}}\right)=\cos ^{-1}\left(\frac{1}{\sqrt{2}}\right)=\frac{\pi}{4}\right]