The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This phenomenon occurs because the perceived frequency is altered when the relative motion between the source and the observer changes the rate at which wave crests are encountered. For sound waves, the change manifests as a shift in pitch; when the source approaches, the frequency increases (higher pitch), and when it recedes, the frequency decreases (lower pitch). In electromagnetism, the same principle applies to light, resulting in a redshift when the source moves away from the observer and a blueshift when the source approaches. The effect is utilized in various scientific applications, including radar technology and astrophysical measurements to determine the velocity and relative motion of celestial objects.