$\therefore$ Phase, $\phi_{1}=(\omega t+k x+0.57)$
$y_{2}=\operatorname{acos}(\omega t+k x)=a \sin \left(\omega t+k x+\frac{\pi}{2}\right)$
$\therefore$ Phase, $\phi_{2}=\omega t+k x+\frac{\pi}{2}$
Phase difference, $\Delta \phi=\phi_{2}-\phi_{1}$
$=\left(\omega t+k x+\frac{\pi}{2}\right)-(\omega t+k x+0.57)$
$=\frac{\pi}{2}-0.57=1.57-0.57=1 \mathrm{\,radian}$
