Question

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

Answer 1

Hello There

Answer: The Doppler effect

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.

I hope this helps
-Chris