Change in the Wavelength (and Frequency) of a Wave as a result of the Motion of either the source or receiver of the waves. If the source and the receiver
are approaching each other, the frequency of the wave will increase and the
wavelength will be shortened-sounds will be higher in pitch and Light will be bluer. If the source and receiver are moving apart, sounds will
become lower-pitched, and light will appear redder (referred to as the Red Shift).
This is why a vehicle with a siren has a higher pitch coming toward you than
when going away. Astronomers analyze Doppler shifts of light and Radio waves to measure the velocities and (indirectly) distances of remote objects.