Answer:
the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the source of the wave. When the observer is moving towards the source, the waves are compressed and the frequency increases (called a blue shift). Conversely, when the observer is moving away from the source, the waves are stretched and the frequency decreases (called a red shift). This phenomenon can be observed in sound waves, light waves, and other types of waves.