What is the Doppler effect?

The Doppler effect refers specifically to the change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source. This phenomenon is commonly experienced with sound waves; for example, as a sound-emitting object, like an ambulance with its siren on, approaches an observer, the sound waves get compressed, resulting in a higher frequency or pitch. As the object moves away, the sound waves are stretched, resulting in a lower frequency or pitch. This principle can also apply to electromagnetic waves, such as light, where it describes changes in color based on the relative motion between a light source and an observer, but its most iconic illustration comes from sound. Thus, the description of a change in sound frequency due to motion precisely encapsulates the essence of the Doppler effect.