In a longitudinal wave, particle motion is parallel to the direction of travel.

• A. True
• B. False
• C. Only in solids
• D. It depends on the frequency

Answer: (A)

In a longitudinal wave, the particles of the medium move back and forth along the same line as the wave travels. This oscillation along the direction of propagation creates regions of compression and rarefaction that move through the medium, carrying the wave with it. This is different from transverse waves, where particle motion is perpendicular to the direction of travel, such as a rope wave. The direction of particle motion is not set by the frequency; changing the frequency changes how fast the particles oscillate, not the orientation of their motion. Longitudinal waves also occur in gases and liquids (like sound in air), not just in solids. So the statement is true.