What is the primary characteristic of a longitudinal wave?

In a longitudinal wave, the primary characteristic is that the particles of the medium through which the wave travels move parallel to the direction of the wave's propagation. This means that as the wave travels, the particles are compressed and rarefied in the same direction as the wave itself is moving. This is commonly observed in sound waves, where areas of compression and rarefaction travel through the air, with air molecules oscillating back and forth along the direction of wave travel.

This behavior contrasts sharply with other wave types. In transverse waves, for instance, particles move perpendicular to the direction of wave motion, which is characteristic of waves like light or waves on a string. The movement of particles in circular paths typically describes surface waves, such as those found in deep water. Lastly, if particles were to remain stationary, no wave would be able to propagate through the medium, as there wouldn't be any oscillation or disturbance to transfer energy.

Therefore, the correct understanding of a longitudinal wave hinges on recognizing the parallel movement of particles with respect to the direction the wave is moving.