Question

Echo is due to
(A) Reflection of sound
(B) Refraction of sound
(C) Both reflection and refraction of sound
(D) Diffraction of sound

Answer 1

Hint Just like light, the sound wave can reflect off surfaces too. Unlike light waves, sound waves have a very long wavelength. Thus, sound waves require a much bigger area to get reflected as compared to the light wave.

Complete step by step answer:


When a sound wave (longitudinal wave) occurs on a flat and hard surface, it reflects back. This phenomenon of reflection of sound is called echo. For this phenomenon, it is required that the reflecting surface should be much bigger as compared to the wavelength of the sound wave.
A reflected sound can be heard separately and distinctly from the source only if the source is very close to the listener and the reflecting surface is comparatively far from the listener.
Practically, this condition can be realized by the example that we often experience echoes near the mountain (where reflecting surface i.e. the mountains are very far) but if you stand in front of a wall and speak loudly, you don’t observe this phenomenon.

Additional Information
In the case of a numerical, you should keep in mind that the distance travelled by sound waves is double the distance between the source and reflecting surface. So, the velocity of sound can be given as
 \[v = \dfrac{{2d}}{t}\]
Where, $d$ is the distance between the source and reflecting surface.
$t$ is the time taken to cover that distance.

Note
Keep in mind that any wave can undergo reflection, refraction, and diffraction. However, the conditions for these to happen depends on the wave. So if a light wave is diffracted by an object, it doesn’t necessarily mean that a sound wave will be diffracted too.