Problem 5.236

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 $O$.