Consider the following statements regarding waves : 1. If the particles of a medium move along the direction of motion of the wave, the wave is called a longitudinal wave. 2. If the particles of a medium move perpendicular to the direction of motion of the wave, the wave is called a transverse wave. 3. Transverse waves may involve change in the shape of the medium, whereas longitudinal waves involve change in the density of the medium. Which of the statements given above is/are correct ?
- A.1 and 2 only
- B.1 only
- C.2 and 3 only
- D.1, 2 and 3
Correct answer
Explanation
The correct answer is D, 1, 2 and 3. All three statements correctly describe how particles move in the two kinds of mechanical waves.
In a longitudinal wave, such as sound in air, the particles vibrate back and forth along the direction in which the wave travels. This forms compressions and rarefactions, regions of higher and lower density, so the wave travels as a change in density (and pressure). In a transverse wave, such as a wave on a stretched string or ripples on water, the particles move at right angles to the direction of travel, forming crests and troughs, so the medium changes its shape. That is why transverse mechanical waves need a medium with rigidity and cannot travel through the body of a gas, while longitudinal waves pass through solids, liquids and gases.
Option A is wrong because it leaves out the correct statement 3. Option B is wrong because statements 2 and 3 are also correct. Option C is wrong because statement 1 is also correct.
Exam tip: longitudinal = along the wave, compressions and rarefactions (sound); transverse = at right angles, crests and troughs (string, light).
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