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A driver moves toward a stationary hill while sounding the horn. The frequency of the echo heard by the driver is higher than the emitted frequency mainly because
Athe reflected sound undergoes two Doppler shifts
Bthe speed of sound increases after reflection
Cthe hill acts as a source of lower frequency
Dreflection always doubles the amplitude and frequency
Answer & Solution
Correct answer: A. the reflected sound undergoes two Doppler shifts
First, the hill receives sound from an approaching source, so the frequency at the hill is Doppler shifted upward. Then the reflected sound is heard by the approaching driver, producing another upward Doppler shift. Thus the echo frequency is affected twice, making A correct. Speed of sound in the medium does not increase because of reflection, so B is false.
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