For 10 points, name this effect in which the frequency of a wave changes when an observer is moving relative to the wave, exemplified by the change in pitch of a moving siren.
The classical variety of this effect involves a ratio calculated by dividing wave velocity by source velocity plus wave velocity. This effect is utilized in radar guns to measure the speeds of cars and baseballs, and it explains the redshift in the light emitted by receding galaxies. For 10 points, name this effect in which the frequency of a wave changes when an observer is moving relative to the wave, exemplified by the change in pitch of a moving siren.
The classical variety of this effect involves a ratio calculated by dividing wave velocity by source velocity plus wave velocity.
{Doppler} effect [or {Doppler} shift]