The motion of a wave source has a significant impact on the frequency observed by an observer, a phenomenon known as the Doppler Effect.

The Doppler Effect refers to the change in frequency or wavelength of a wave as perceived by an observer moving relative to the wave source. Named after the Austrian physicist Christian Doppler, who first proposed it in 1842, this effect is most commonly observed with sound waves—such as the change in pitch of a passing siren—but it also applies to light waves and other forms of electromagnetic radiation.

When the wave source moves toward the observer, the waves are compressed, resulting in a higher observed frequency (or shorter wavelength). This phenomenon is commonly referred to as a blue shift in the context of light waves, as the increased frequency shifts the light toward the blue end of the spectrum. Conversely, if the source is moving away from the observer, the waves are stretched, leading to a lower observed frequency (or longer wavelength), which is known as a red shift.

The extent of the frequency shift is directly related to the relative speed of the source and the observer: the faster the source moves, the greater the shift. It is essential to note that while the frequency perceived by the observer changes, the actual frequency of the wave emitted by the source remains constant; the variation occurs due to the relative motion between the source and the observer.

The implications of the Doppler Effect are significant across various fields of physics. For instance, in astronomy, this effect is employed to ascertain the speed and direction of distant galaxies. By examining the red or blue shift of light emitted from these galaxies, astronomers can determine whether the galaxies are moving toward or away from us, as well as their respective speeds. This observation has been crucial in supporting the theory of an expanding universe.

In summary, the motion of a wave source can greatly influence the observed frequency due to the Doppler Effect. This phenomenon, which results in a shift in frequency based on the relative motion of the source and the observer, has profound implications for our understanding of the physical universe.