The phenomenon that causes the Doppler effect is the relative motion between a source of sound (or light) and an observer. When the source moves towards the observer, the waves are compressed, leading to a higher observed frequency (pitch) or blue shift in light. Conversely, when the source moves away, the waves are stretched, resulting in a lower observed frequency (pitch) or red shift in light. The Doppler effect is a significant concept in various fields, including astronomy (to measure the velocity of stars) and acoustics(to understand sound waves).