Two waves are represented by $y_1 = Asinùt$ and $y_2 = aCosùt$. The phase of the first wave, w.r.t. to the second wave is ……….
$ y_1 = a sin \omega t and y_2 = a cos \omega t = a sin ( \omega t + { \pi \over 2 } ) $ $ \therefore 1st wave is lagging behind in phase by { \pi / 2 } $