A man standing between two cliffs produces a sound and hears two successive echoes at intervals of and respectively.
Why will an echo not be heard, when the distance between the source of sound and the reflecting surface is ?
Step 1: Given data:
Distance between the source of sound and the reflecting surface
Time [persistence of hearing]
Step 2: Calculating the minimum distance between the object and the source of sound for hearing distinct echoes:
The time delay between the original sound and the reflected sound must be at least to hear a definite echo.
Assume that the speed of sound is .
The sound must travel to the obstacle and then reflect back to the listener's ear after at a particular temperature, such as in air.
Since,
Thus, the total distance traveled by sound from its source to the reflecting surface and back should be at least .
Hence, the minimum distance between the object and the source of sound for hearing distinct echoes must be half of this distance, or
Since the given distance between the source of sound and the reflecting surface which is , is less than .
Therefore, as the distance is only 10m, therefore, no echo is produced.