What physical phenomenon describes the change in frequency of a wave when the source moves relative to the observer?

The Doppler effect describes the change in frequency or wavelength of a wave in relation to an observer who is in motion relative to the wave source. When the source of the wave moves towards the observer, the waves are compressed, resulting in a higher frequency (or a shorter wavelength). Conversely, if the source moves away, the waves are stretched, producing a lower frequency (or a longer wavelength). This phenomenon is commonly observed with sound waves, such as a passing siren, but it also applies to electromagnetic waves, including light.

The other concepts mentioned, such as Rayleigh scattering, involve the scattering of light and its wavelength dependency, the photoelectric effect relates to the emission of electrons from materials when exposed to light, and interference pertains to the superposition of waves leading to patterns of reinforcement or cancellation. These do not pertain to the change in frequency due to relative motion between the source and observer, making the Doppler effect the accurate choice.