Compressions and rarefactions are characteristic features of which type of mechanical wave?

Introduction / Context:
Mechanical waves transfer energy through a medium by the oscillatory motion of particles. There are two basic types of mechanical waves: longitudinal and transverse. Understanding how the medium particles move in each case helps you identify which features belong to which wave type. This question asks about compressions and rarefactions, which are key concepts in sound and other longitudinal waves.

Given Data / Assumptions:

• We consider mechanical waves propagating through a medium such as air, a spring or a rope.
• In longitudinal waves, particle displacement is parallel to the direction of wave travel.
• In transverse waves, particle displacement is perpendicular to the direction of wave travel.
• The terms compressions and rarefactions refer to regions of higher and lower density or pressure.

Concept / Approach:
In a longitudinal wave, particles of the medium move back and forth along the direction of propagation. This motion creates regions where particles are closer together (compressions) and regions where they are farther apart (rarefactions). Sound waves in air are the standard example: compressions correspond to high pressure regions and rarefactions to low pressure regions. Transverse waves, such as waves on a rope, have crests and troughs instead, where the displacement is up and down relative to the direction of travel. They do not create alternating compressions and rarefactions along the direction of propagation.

Step-by-Step Solution:
Step 1: Define a longitudinal wave as one where particle motion is parallel to the direction of wave propagation. Step 2: Recognise that as particles move back and forth, they bunch together in some regions and spread out in others. Step 3: Identify regions of high particle density and pressure as compressions. Step 4: Identify regions of low particle density and pressure as rarefactions. Step 5: Contrast this with a transverse wave, where particles move perpendicular to the direction of travel, creating crests and troughs instead. Step 6: Conclude that compressions and rarefactions are characteristic of longitudinal waves, not transverse waves.

Verification / Alternative check:
When a toy spring or slinky is compressed and released, you can see regions where coils are close together (compressions) moving along the spring, followed by regions where coils are farther apart (rarefactions). This clearly illustrates a longitudinal wave. By contrast, when you flick a rope up and down, you see peaks and valleys moving along the rope, with the rope segments moving perpendicular to the wave direction, which indicates a transverse wave. Sound wave diagrams in textbooks likewise show alternating high and low pressure bands, consistent with longitudinal compressions and rarefactions.

Why Other Options Are Wrong:
Transverse waves: These waves are described by crests and troughs, not compressions and rarefactions along the direction of travel.

Both longitudinal and transverse waves equally: This is incorrect because the specific terminology of compressions and rarefactions is reserved for longitudinal waves.

Neither longitudinal nor transverse waves: This would deny a fundamental description of sound waves and other longitudinal wave phenomena.

Common Pitfalls:
Students sometimes memorise the terms crest and trough and then incorrectly attach them to all types of waves, or they may use compressions and rarefactions when talking about water waves, which are actually surface waves with transverse and longitudinal components. To avoid confusion, remember that compressions and rarefactions describe variations in density and pressure along the direction of propagation and are therefore specific to longitudinal waves like sound in fluids.

Final Answer:
Compressions and rarefactions are characteristic of longitudinal waves, such as sound waves in air.