When an object such as an airplane approaches you, what happens to the frequency waves as the source moves away?

When an object, like an airplane, moves away from an observer, the frequency of the sound waves it produces decreases. This phenomenon is known as the Doppler effect. As the airplane moves away, the sound waves get stretched out, meaning they are more spaced apart compared to when the plane approached. This increase in distance between the waves results in a lower frequency and pitch of the sound that the observer hears. Thus, the correct answer indicates that the waves are further apart as the source moves away.

The other options do not accurately reflect this phenomenon; for instance, the waves do not remain the same distance as when the airplane was approaching or as they are at the source. The concept of waves being "closest together" would suggest an increase in frequency, which contradicts the observed outcome when the object is receding.