 Let us now examine the light emission from the perspective of the inertial frame \(K\). Relative to \(K, B\) moves (to the right) with velocity \(v\). Because relative to \(K'\) the bursts of light are collinear, the \(x\)-component of the velocity of the light as measured in \(K\) must be \(v\). However, the velocity of the light also has a vertical component, i.e., a \(z\)-component. This is significant, because we wish to calculate the change in the momentum of \(B\). 