A Level Physics Practice Exam

In a longitudinal wave, points on the wave move in a back-and-forth motion along the same direction as the wave travels. This is characteristic of longitudinal waves, where compressions and rarefactions occur in the medium through which the wave travels. When the wave propagates, particles of the medium oscillate parallel to the direction of wave travel, resulting in a series of compressions (where particles are close together) and rarefactions (where particles are spread apart).

This oscillatory movement along the axis of the wave's direction is what distinguishes longitudinal waves from transverse waves, in which particles move perpendicular to the direction of wave travel. Thus, the correct choice highlights this essential relationship between the particle movement and the direction of the wave, reinforcing the fundamental nature of longitudinal waves in physics.