  We present an in depth analysis and a new derivation of the Doppler factor in
the Lienard-Wiechert potentials, based on geometrical considerations in
Minkowski space. We argue that, contrary to a common assumption, the methods
used for deriving the Doppler factor in the case of an electrically charged
extended particle are not applicable in the case of a point particle. A
geometrical interpretation of the Doppler factor is nonetheless found in the
later case, based on the electromagnetic interaction model of Fokker. In this
model the interaction takes place between infinitesimal worldline segments with
end points connected by light signals, just like the points where light pulses
are emitted and detected in general. This analogy reveals that the relativistic
Doppler effect is the missing link between the classical Doppler effect and the
Doppler factor in the Lienard-Wiechert potentials.
