Which of the following waves requires a material medium for its propagation?

Step 1: Analyze the question The question asks which wave needs a material medium (solid, liquid, or gas) to travel. In other words, which wave cannot travel through a vacuum?

Step 2: Explain the concept Waves fall into two groups:

• Mechanical waves are produced by vibrating particles. They need a material medium because the vibrations are passed from particle to particle. Examples are sound waves, water waves, and waves on a string.
• Electromagnetic waves are oscillating electric and magnetic fields. They need no medium and can travel through a vacuum. This is how sunlight reaches Earth through space. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Step 3: Apply this to each option

• A. Radio waves: electromagnetic, so no medium is needed.
• B. Light waves: electromagnetic, so no medium is needed. Light travels from the Sun through space.
• C. Sound waves: mechanical (longitudinal), so a medium is required. Sound cannot travel in a vacuum.
• D. X-rays: electromagnetic, so no medium is needed.

Step 4: Answer Only sound waves need a material medium, so the answer is C. Sound waves.

Exam tip: Remember that "sound needs something to shake." If a question asks which wave needs a medium, look for the one that is not part of the electromagnetic spectrum. A classic related experiment is the bell jar: as air is pumped out, the ringing bell becomes fainter and fainter, which shows sound cannot travel in a vacuum.