ABSTRACT
The apparent frequency (or quantum energy) of emitted radiation depends on the relative motion of source and observer. The term Doppler effect embraces all aspects of this phenomenon whereby the observed frequency or wavelength of a periodic disturbance is modified. Thus the relativistic combination of velocities leads, without any extra assumptions, to precisely the result that Fresnel and the other ether theorists had to explain in terms of a partial dragging of the light by the medium. Prior to the development of Einstein’s kinematics, the phenomenon was a mysterious one and the explanations of it were very artificial. The most dramatic manifestation of this form of the Doppler effect, for relative motion of source and observer along the line joining them, is the famous red shift of distant galaxies. The spectrum of a complete galaxy, being a synthesis from all the different radiating objects in it, is close to being a continuous smear.
