Question

The echo will be heard if the original sound reflected by an obstacle reaches our ears after
A. 10s
B. 5s
C. 1s
D. 0.1s

Answer 1

Hint: Sound is a transverse wave. It is formed when the molecules of the air vibrate. Since, sound is a wave it shows that phenomenon of reflection when it hits an obstacle . When we hear a sound twice due to reflection, we call it an echo. However, not all echoes can be heard by the human ears.

Complete step by step answer:
Sound is a wave that is produced due the vibrations of air molecules. Since sound is wave, it can travel through different mediums.
When a wave passes from one medium to another medium, it is partially absorbed, partially reflected back at a specific angle and the rest transmitted through the medium. This rate of absorption, reflection and transmission depends on the material (medium) of the body on which the wave is incident.
There exist materials that are good reflectors of sound and there also exit materials that are good absorbers of sound.
Therefore, when a sound wave emitted by a source of sound hits an obstacle it reflects. As a result, the same sound is heard twice. And when we hear the sound twice, we call it an echo of the sound. For example, we hear echoes when we shout in front of some mountains at some distances.
However, a human ear cannot sense every reflection of sound or echo. For the echo to be heard by a human ear, the sound from the source must reach to back with reflection after 0.1 seconds.
Suppose we shout out our name in front of a wall and if the sound reflected back at the wall reaches within 0.1 seconds, we will not hear the echo of our name.

Note:
For the sound to come back after 0.1 seconds the obstacle must be at least 17 m far. Now, we can understand why we do not hear echo sounds everywhere. The obstacles that are within a radius of 17 m do not produce the sound. In the mountainous region, the mountains are very far. Hence, echoes are produced.