Doppler shift

Doppler shift is the change in frequency or pitch of a wave, like sound or light, caused by the relative motion between the source and the observer. If the source moves toward you, the waves get compressed, increasing their frequency and resulting in a higher pitch (for sound) or a shift toward blue in light. If it moves away, the waves stretch out, decreasing their frequency and pitch, or shifting toward red. This phenomenon helps us determine the speed and direction of moving objects, such as sirens approaching or receding and galaxies moving in space.