Which of the following is not a mechanical wave?

1. Analyze the question

The question asks us to identify which of the listed options is not a mechanical wave. To answer this, we must distinguish between mechanical waves and electromagnetic waves.

2. Explain the concept

• Mechanical Waves: These are waves that require a material medium (solid, liquid, or gas) to propagate. They involve the oscillation of particles in the medium. Examples include sound waves, water waves, and waves in strings or pipes.
• Electromagnetic (EM) Waves: These are waves that do not require a material medium to propagate. They can travel through a vacuum (empty space). They consist of oscillating electric and magnetic fields. Examples include radio waves, light, X-rays, and gamma rays.

3. Show the solution

• Option A (Wave in a closed pipe): This is a sound wave trapped in a medium (air). It is a mechanical wave.
• Option B (Radio waves): These are part of the electromagnetic spectrum. They travel at the speed of light and do not need a medium to travel; they can pass through the vacuum of space. Therefore, they are not mechanical waves.
• Option C (Water waves): These require water (a medium) to travel. They are mechanical waves.
• Option D (Sound waves): These require air, water, or solids to travel. They are mechanical waves.

4. Identify the answer

Comparing the options, radio waves are the only ones that are electromagnetic, not mechanical.

5. Tip

To remember the difference, use the acronym "RMIVUXG" for the electromagnetic spectrum (Radio, Microwave, Infrared, Visible light, Ultraviolet, X-ray, Gamma). None of these require a medium to travel, so if you see any of these in an exam question asking for "non-mechanical" waves, you know exactly what to pick!