The Doppler shift is a change in the frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. When the source moves toward the observer, the waves are compressed, resulting in a higher frequency, known as a blue shift. Conversely, when the source moves away, the waves are stretched, leading to a lower frequency, called a red shift. This phenomenon is commonly observed in sound and light waves. For example, as a police siren approaches, the sound seems higher in pitch, and as it moves away, the pitch lowers. Similarly, astronomers use the Doppler shift to determine the movement of stars and galaxies in the universe.