Which of the following types of waves needs a medium for propagation?

Step 1: Analyze the question The question asks which wave type needs a material medium (solid, liquid, or gas) to travel. This is the difference between mechanical waves and electromagnetic waves.

Step 2: The concept

• Mechanical waves need a medium because they travel by making particles of the medium vibrate and pass energy to neighbouring particles. Examples are sound waves, water waves, and waves on a string.
• Electromagnetic waves do not need a medium. They are oscillating electric and magnetic fields and can travel through a vacuum. This is how sunlight reaches us through space. Examples are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Step 3: Check each option

• A. Sound waves: These are longitudinal mechanical waves made of compressions and rarefactions of particles. They cannot travel in a vacuum, so they need a medium. ✔
• B. Light waves: These are electromagnetic and travel through a vacuum. ✘
• C. Radio waves: These are electromagnetic and travel through a vacuum. ✘
• D. X-rays: These are electromagnetic and travel through a vacuum. ✘

Step 4: Answer Only sound waves need a medium for propagation, so the answer is A.

Exam tip: If the options include sound and the rest are on the electromagnetic spectrum (light, radio, X-rays, gamma, and so on), the answer is almost always sound. Remember the phrase: "Sound needs ground (a medium); EM waves roam free."