Transverse and longitudinal waves

Transverse waves: motion across the direction of travel

Ripples on a water surface are an example of a transverse wave. As a ripple travels horizontally across the surface, a small floating object rises and falls. Its up-and-down movement is perpendicular (at right angles) to the direction in which the ripple travels. A series of ripples can pass the object while it remains in roughly the same place on the water, rather than travelling across the pond with them. This is observable evidence that the disturbance travels, not the water itself.

Longitudinal waves: motion along the direction of travel

Sound travelling through air is a longitudinal wave. A vibrating source, such as a loudspeaker, pushes nearby air particles closer together and then allows them to spread further apart. This produces alternating compressions and rarefactions. Each particle moves back and forth parallel to the direction in which the sound travels and affects neighbouring particles. The pattern of compressions and rarefactions moves onwards through the air.

You can hear a sound some distance from its source, showing that the disturbance has reached you. Yet the air at the source has not travelled all the way to your ear: a lightweight object beside a vibrating loudspeaker can move back and forth without being carried across the room. As with the floating object on water, the medium moves locally while the wave transfers energy onwards.