Concept:A change in the observed frequency of a wave occurs whenever the source and the listener are in relative motion.
Explanation:When a moving source of sound approaches a stationary listener, the sound waves in front of the source are compressed.
This compression reduces the wavelength
λ of the sound.
Since the speed of sound in air remains constant, a smaller wavelength produces a higher frequency
f.
The listener therefore hears a higher pitch than the actual frequency of the source.
When the source moves away from the listener, the waves are stretched out.
The wavelength becomes larger, causing the observed frequency to decrease.
As a result, the listener hears a lower pitch.
This apparent change in frequency due to the relative motion between a source and an observer is called the Doppler effect.
A common example is the sudden drop in pitch of an ambulance siren as the vehicle passes a stationary observer.
Answer:A. Doppler effect.