 We wish now to examine this emission of light by \(B\) from two different inertial frames. The first inertial frame we consider is simply the inertial frame in which \(B\) is at rest. Since we will not be performing many calculations using quantities defined relative to this inertial frame, we will label it \(K'\). The direction of motion of the second inertial frame, which we will label \(K\), is carefully chosen to simplify the calculations. 