An echo is heard after 5 seconds of the production of sound which moves with a speed of 340 m/s. What Is the distance of the mountain from the source of sound which produced the echo?

To calculate the distance of the mountain, we must account for the fact that an echo involves sound traveling to the obstacle and back to the source [1]. The total distance covered by the sound wave is the product of its speed and the time taken [1]. Given the speed of sound is 340 m/s and the time delay is 5 seconds, the total round-trip distance is 340 m/s × 5 s = 1700 meters. Since this distance represents the path to the mountain and back, the actual distance to the mountain is half of the total distance: 1700 m / 2 = 850 meters. Converting this value into kilometers (850 m / 1000), we get 0.85 km. This principle of using time delay to find distance is a fundamental application of wave reflection.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > 8.3 Speed > p. 113