  This work considers the cospectral and arbitrary light emission of a moving
source. The observed wavelengths of the emitted photons are described in term
of kinematic and dynamical Doppler shifts in which the mass-energy relation
plays a fundamental role. The presentation is an alternative way of emphasizing
the importance of the concept of proper mass as a conserved quantity and the
implications of the mass-energy relation when a source emits radiation. The
physical contexts in which the source changes velocity after emission are
discussed and a set of additional problems is presented.
