A transverse wave is a wave whose oscillations occur in a direction perpendicular to the direction of travel. Longitudinal waves, on the other hand, oscillate in the same direction as the propagation of the wave. The oscillations of a transverse wave can be composed of matter or energy. The wave produced by wiggling a string uses matter to propagate. An electromagnetic (EM) wave is a transverse wave that can use empty space as a medium.
These waves can be contrasted with longitudinal waves, which oscillate in the direction of wave motion. A good example of a longitudinal wave is the wave produced in a Slinky® when someone pushes one end toward the other end. When this happens, the momentum of one part of the Slinky® is transferred along the Slinky® via collisions. Though it is more difficult to imagine, sound waves also work in a similar fashion. Like with the Slinky®, they depend on the longitudinal transfer of momentum through collisions of matter.
In fact, only longitudinal waves can propagate through a gas medium. Particles in gases collide with each other in the same way that larger solid objects do. When this happens with many particles simultaneously, a pressure wave is created. Pressure waves are longitudinal waves that move rapidly outward through a medium in all directions. At the molecular level, however, they consist of nothing more than colliding particles.
A transverse wave can be made with matter as well. When the end of a string is moved side to side, this motion is carried toward the other end of the string. This is because each segment of string is connected firmly to the next segment. In essence, the wave is propagated because pieces of the string get pulled sideways by neighboring pieces.
The same effect can be created in a body of water, such as a pond. When a drop of water lands on the pond’s surface, it pushes some water downward. This downward movement influences adjacent volumes of water because of shear stress. Shear stress results when a liquid is moving at different speeds at different locations.
An EM wave is also a transverse wave. EM waves have an electric and a magnetic component that are both perpendicular to the direction of wave propagation. Unlike many other types of waves, EM waves require no matter to be present for propagation. EM waves move at the speed of light and carry an amount of energy proportional to their frequency. The frequency of a wave refers to the number of oscillations it completes each second.