How does a source's motion affect observed frequency?

A source's motion affects observed frequency through a phenomenon known as the Doppler Effect.

The Doppler Effect is a change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It was named after the Austrian physicist, Christian Doppler, who proposed it in 1842. This effect is commonly observed with sound waves - for instance, the change in pitch of a passing siren - but it also applies to light waves and other types of electromagnetic radiation.

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

The amount of frequency shift depends on the relative speed of the source and the observer. The faster the source is moving, the greater the shift. It's important to note that the actual frequency of the wave is not changing; what changes is the frequency at which the observer receives the wave due to the relative motion of the source.

In physics, this concept is crucial in various fields. For example, in astronomy, the Doppler Effect is used to determine the speed and direction of distant galaxies. By observing the red or blue shift of light from these galaxies, astronomers can infer whether they are moving towards or away from us, and at what speed. This has been instrumental in supporting the theory of an expanding universe.

In summary, the motion of a source can significantly affect the observed frequency due to the Doppler Effect. This effect, which results in a shift in observed frequency depending on the relative motion of the source and observer, has profound implications in our understanding of the physical world.