The oscillations of a longitudinal wave occur in the same direction as the direction of energy transfer.
The displacement of the medium through which a longitudinal wave travels is either the same as or the opposite as the direction that the wave travels. This causes the wave to appear to move in a “back and forth” motion. As a result, compressions and rarefactions are present in longitudinal waves. The wavelength of a longitudinal wave is the distance between two consecutive compressions, or the distance between two consecutive rarefactions.
Another type of wave is a transverse wave. In transverse waves, the motion of the wave through the medium in which is travels is perpendicular to the direction of displacement. Thus, the wave moves in an “up and down” motion. As a result, crests and troughs are present on a transverse wave. The wavelength of a transverse wave is the distance between two consecutive crests, or the distance between two consecutive troughs.