Question

Which of the following is an elastic wave?
A. Sound wave
B. Light wave
C. X – Ray
D. Radio wave

Answer 1

Elastic waves always need a medium to travel. We need to have knowledge about different waves that collectively make electromagnetic waves which are transverse in nature and don't require a medium to travel. Compare each option to check which category they belong to and find out which one is elastic.

Complete step-by-step answer:
Waves generally transfer energy from one place to another. Some waves need medium to transfer energy from one place to another, whereas other kinds of waves don’t need medium like Electro-Magnetic waves.

Thus, on the basis, we divide waves into two groups -
1. Elastic waves – These waves require medium to travel. When energy is transferred to one particle, it vibrates and gets displaced, a force proportional to the displacement acts on the particles to restore them to their original position. In this process energy is transferred from one particle to another.
For example – Sound waves are transmitted through a gaseous medium.

2. Inelastic waves – These waves do not require medium to travel. For example, Electro-Magnetic wave which shows dual nature (particle as well as wave like) travels through an empty space.

Light wave, X-Ray, and Radio waves form different parts of Electro-Magnetic waves, so they don’t need a medium to travel.

So, the correct answer is A. Sound wave, which does require a medium to travel and thus is an elastic wave.

Additional information:
Electro-Magnetic waves can travel through both empty space as well as gaseous medium. But their speed is highest in empty space which is 3,00,000 km/sec, whereas their speed reduces in liquid or gaseous medium.
Waves act as a vehicle to transfer energy.

Note: Electromagnetic waves do not need a medium. This phenomenon can be understood by Maxwell’s equation, which will be taught in higher education, that the electric and magnetic parts of the wave induce one another at a 90-degree phase difference. A change in electric field causes a change in the magnetic field. So light can propagate without a medium.