What does the Doppler effect describe?

The Doppler effect describes the observed change in frequency or wavelength of a wave due to relative motion between the observer and the source of the wave. When the source of waves moves towards an observer, the waves are compressed, leading to a higher frequency and thus a higher pitch sound in the case of sound waves. Conversely, if the source moves away from the observer, the waves are stretched, resulting in a lower frequency and a lower pitch sound. This phenomenon is not limited to sound waves; it also applies to electromagnetic waves, such as light, which can also shift in frequency based on the relative motion between the source and observer. This effect is commonly observed in everyday life, such as hearing the change in pitch of a passing siren.