A light source moves with constant velocity \(v\) in the frame of inertial observer \(\mathcal{O}\). The source radiates isotropically in its rest frame. Show that in the inertial frame of \(\mathcal{O}\) the light is concentrated in the direction of motion of the source, where half of the photons lie in a cone of semi-angle \(\theta\), where \(\cos \theta=v / c\).