It should be noted that the Doppler
Effect is an effect observed using sound waves.
However an analogous effect can be observed using e-m
radiations such as light, radio waves etc.
The analysis of the "Doppler Effect"
in e-m radiations is different from that shown above
because the velocity of light is thought to be the same
for all observers (see
Measurements of the relative Doppler shift of radio
waves are often used to measure relative velocities.
Examples include Doppler Radar used to guide aircraft
during landing and radar "speed traps" used by police.