Question

Which of the following waves does not travel in vacuum?
A) Seismic waves
B) X-rays
C) Light
D) Radio waves

Answer 1

Hint: Mechanical waves require a medium to travel as they propagate through the disturbance produced in the medium by the vibration and oscillation of the particles of the medium. In the absence of particles in the medium, they cannot travel.

Complete step-by-step answer:
Mechanical waves are waves which require a medium to travel as they propagate by constantly stretching and compressing the medium in which they travel. They cause a disturbance in the medium which is pushed forward by the vibrating particles of the medium passing their energy to their neighboring particles and making them oscillate. Hence, without a medium which has material particles in it, there will be nothing that can oscillate and pass the disturbance, that is, the wave forward.
On the other hand, electromagnetic waves travel as a combination of changing electric and magnetic fields. These fields do not depend upon the presence of particles to propagate and hence, can even travel without a medium. Hence, electromagnetic waves can travel in vacuum.
Seismic waves are mechanical waves whereas X-rays, light and radio waves are electromagnetic waves. Hence, seismic waves cannot travel in vacuum.
Therefore, the correct option is A) Seismic waves.

Note: Students might be tempted to think that X-rays travel only in a material medium since they must know that X-rays are used to see the human skeleton. They might think that therefore, X-rays can only travel in the human body. However, they must remember that X-rays are electromagnetic waves and can even travel through vacuum. A good way of remembering which waves are electromagnetic waves is to keep in mind the electromagnetic spectrum.