the.com/doppler effect

means The phenomenon where a wave's pitch (or frequency) seems to rise as its source approaches and fall as it moves away, because the waves get compressed on approach and stretched on departure.

from Named for Christian Doppler, an Austrian physicist who proposed the principle in 1842 to explain the color of binary stars, arguing that light waves (like sound waves) shift frequency depending on the relative motion of source and observer. He tested the sound version with trumpeters playing on a moving train — proof you could hear before you could see it. The idea took his name permanently, 'effect' simply marking it as an observable consequence of his theory.