What describes the Doppler effect?

The Doppler effect is best described as the change in wave frequency due to relative motion between the source of the waves and an observer. This phenomenon occurs when the source of sound waves (or any type of waves) is moving towards or away from a stationary observer—or vice versa.

When the source moves closer to the observer, the waves are compressed, leading to an increase in frequency and a higher pitch of sound. Conversely, if the source is moving away from the observer, the waves are stretched, resulting in a decrease in frequency and a lower pitch. The key aspect of the Doppler effect is that this change in frequency is dependent on the relative velocities of the source and the observer, illustrating how motion influences our perception of wave characteristics.

In contrast, the other choices do not accurately define the Doppler effect. Changes in amplitude with distance pertain more to wave intensity rather than frequency; a constant frequency from a stationary source ignores the dynamic nature of the Doppler effect; and the velocity of sound remaining unchanged does not account for the variations in frequency that can result from motion. Thus, the choice that captures the essence of the Doppler effect is the change in wave frequency due to relative motion.