Ab observer detects an apparent change in the frequency of sound waves produced by an airplane passing overhead. This phenomenon illustrates
A. the doppler effect
B wave amplitude increase
C the refraction of sound waves
D Wave intensity increase
Reason: The Doppler effect is an increase or decrease in the frequency of sound, light, or waves as the source and observer move to each other. An example would be a firetruck with its siren on driving away from you.
