What is the main characteristic of longitudinal waves?

The main characteristic of longitudinal waves is that particles in the medium through which the wave travels move parallel to the direction of wave propagation. In longitudinal waves, compressions and rarefactions are formed as the particles are pushed together and then pulled apart, effectively resulting in the wave moving forward. This parallel motion is fundamental in understanding how sound waves, and other longitudinal waves, transmit energy. By moving in the same direction as the wave, the particles create areas of high and low pressure that allow the wave to propagate through the medium.

This characteristic distinguishes longitudinal waves from transverse waves, where particles move perpendicular to the wave direction, explaining the nature of the motion and energy transfer in different types of waves. The other options do not accurately describe the behavior of longitudinal waves, thus emphasizing the importance of understanding particle movement in wave dynamics.