A source emitting electromagnetic signals with proper frequency \(\omega_{0}=3.0 \cdot 10^{10} \mathrm{~s}^{-1}\) moves at a constant velocity \(v\) \(=0.80 c\) along a straight line separated from a stationary observer \(P\) by a distance \(l\) (Fig. 5.37 ). Find the frequency of the signals perceived by the observer at the moment when (a) the source is at the point \(O\); (b) the observer sees it at the point \(O\).