You perceive the sound of a siren to have a higher frequency when the ambulance is approaching you than when it is receding from you. Which of the following statements about this observation is true?
a.The difference is a figment of your imagination.
b.The difference is due different intensities of the sound.
c.The difference is caused by the mechanical properties of sound.
d.The difference is unique for sound waves and cannot be observed for light.
e.The difference is known as the Doppler Effect.