Longitudinal waves

In a longitudinal wave, particles of the medium oscillate back and forth parallel to the direction in which the wave transfers energy. Each particle oscillates about its equilibrium position rather than travelling along with the wave.

As particles move, they affect neighbouring particles, passing the disturbance and its energy through the medium. The wave can therefore transfer energy without the medium as a whole travelling with it.

Sound in air

Sound in air is a longitudinal wave. As air particles oscillate, they produce alternating regions called compressions and rarefactions:

• In a compression, particles are closer together, so particle density and air pressure are higher than their equilibrium values.
• In a rarefaction, particles are farther apart, so particle density and air pressure are lower than their equilibrium values.

These regions travel through the air as the sound wave transfers energy, while the air particles oscillate back and forth about their equilibrium positions.

Transverse waves

In a transverse wave, the vibrations are at right angles to the direction in which the wave travels, or propagates. For example, if a wave travels horizontally, its vibrations may be vertical. The vibrations do not travel along with the wave; they occur across its direction of travel.

The following waves can be modelled as transverse:

• Electromagnetic radiation: electric and magnetic fields oscillate at right angles to the direction of propagation.
• Water waves: the up-and-down movement of the water surface can be modelled as a vibration perpendicular to the direction the wave travels.
• Seismic S-waves, or secondary waves: the material through which the wave travels vibrates at right angles to the direction of propagation.