A girl clapped her hands near a cliff and heard the echo after 4 s. What is the distance of the cliff from the girl if the speed of the sound is taken as 320 m s⁻¹?

Correct option is D

The correct answer is (d) 640 m

Explanation:

• In physics, an echo is a distinct acoustic phenomenon produced when travelling sound waves are reflected back to the original listener from a large, rigid and distant surface.

• The problem states that the total time taken for the sound to travel from the girl to the cliff and reflect back to her ears is 4 s.

• Therefore, the time taken for the sound to reach the cliff (one-way distance) is half of the total time:
t = 4 / 2 = 2 s

• The speed of sound in air is given as 320 m/s.

• Using the formula:
Distance = speed × time

• Distance = 320 × 2 = 640 m

Information Booster:

• To hear a clear echo, the minimum distance between the source of sound and the reflecting surface should be about 17.2 m (assuming air temperature around 22°C).

• This is because the sensation of sound remains in the human ear for about 0.1 s, so the reflected sound must arrive after this time to be heard separately.