Compression waves whose particle displacement is parallel to the direction of propagation are called

• A. Longitudinal waves
• B. Rayleigh waves
• C. Lamb waves
• D. Shear waves

Answer: (A)

Compression waves move particles in the same direction that the wave travels, so their displacement is parallel to the propagation direction. This makes them longitudinal waves (often called P-waves or compression waves in UT contexts). The particles oscillate along the travel direction, creating alternating regions of compression and rarefaction as the wave passes.

In contrast, Rayleigh waves are surface waves with motion that mostly rolls along the surface; Lamb waves are guided plate waves with complex motion across the plate thickness; and shear waves have particle motion perpendicular to the direction of travel (transverse). Because the question specifies motion parallel to the direction of travel, the correct term is longitudinal waves.