 First, we choose \(K'\) so that \(B\) emits the two bursts of light along \(z'\)-axis in opposite directions. We then choose \(K\) so that \(K\) moves with velocity \(v\) in the negative direction along the \(x'\)-axis. Using standard conventions, this means that an observer at rest in \(K'\) (Alice) judges that the light emitted by \(B\) travels up and down the page. For an observer at rest in \(K\) (Bob), the atom \(B\) moves to the right with velocity \(v\) and the light is emitted toward the right making an angle \(\theta\) with the \(x\)-axis. 