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The driver of a car travelling with speed 30 m/sec towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is 330 m/s, the frequency of reflected sound as heard by driver is:

1. 555.5 Hz
2. 720 Hz
3. 500 Hz
4. 550 Hz

Correct answer: 720 Hz

Solution

The Doppler effect applies twice here: first, the hill acts as a stationary observer for the approaching car, and then as a source reflecting the sound back to the driver. Using the Doppler formula for both cases, the final frequency heard by the driver is 720 Hz.

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