Question

The minimum distance between the source and the reflector in air. So that an echo is heard is approximately equal to ______.

The minimum distance between the source and the reflector in air, to hear an echo, is approximately ______.

विकल्प

• 10 m

• 17 m

• 34 m 

• 50 m

Answer 1

The minimum distance between the source and the reflector in air. So that an echo is heard is approximately equal to 17 m.

Explanation:

An echo is heard distinctly if it reaches the ear at least 0.1 s after the original sound.

If d is the distance between the observer and the obstacle and V is the speed of sound, then the total distance travelled by the sound to reach the obstacle and then to come back is 2d and the time taken is,

t = `"Total distance travelled"/"Speed of sound" = "2d"/V`

or, d = `(V t)/2`

Putting t = 0.1 s and V = 340 m/s in air at ordinary temperature, we get:

d = `(340 xx 0.1)/2`

d = 17 m

Thus, to hear an echo distinctly, the minimum distance between the source and the reflector in air is 17 m.

Answer 2

The minimum distance between the source and the reflector in air, to hear an echo, is approximately 17 m.