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The Doppler effect refers to the change in frequency (and subsequently the wavelength) of a wave in relation to an observer who is moving relative to the wave source. This phenomenon is most commonly associated with sound waves, where the frequency of the sound alters depending on the motion of both the source and the observer. When the source of a sound wave approaches the observer, the waves compress, leading to a higher frequency or pitch. Conversely, as the source moves away, the waves stretch out, resulting in a lower frequency or pitch.

This concept is integral in various fields such as astrophysics, radar, and medical imaging, where it helps in understanding the motion and speed of objects. The other options do not accurately capture the essence of the Doppler effect, which specifically deals with frequency changes due to relative motion rather than changes in volume or the characteristics of light and sound wave reflection.