When the motion of particles in a medium is parallel to the direction of propagation, what type of wave is generated?

The correct choice is based on the fundamental characteristics of wave motion. In a longitudinal wave, the particles of the medium move back and forth in the same direction as the wave propagates. This type of wave is exemplified by sound waves, where compressions and rarefactions occur as particles are displaced parallel to the direction of energy transfer.

When analyzing wave types, transverse waves, in contrast, feature particle motion that is perpendicular to the direction of wave propagation. This distinction is crucial, as it differentiates waves like light or waves on a string from longitudinal waves. Wavefronts represent surfaces of constant phase in a wave, but they don’t specify the type of wave involved. Surface waves combine characteristics of both transverse and longitudinal wave behaviors, typically observed at the interface between two different media, like water and air. Understanding these definitions helps clarify why a longitudinal wave is characterized by particle movement parallel to propagation, aligning with the definition provided in the correct answer.