  We show that when the observers are located in a plane electromagnetic wave
it is not compulsory for them to take into account the time dilation and length
contraction effects when the wave is detected from two inertial reference
frames in relative motion. We also illustrate the difference between the
approach that uses Einstein type of observers confronted with time dilation and
length contraction effects and an unconventional approach that uses observers
detecting information from the energy that arrives at their location.
