1. All waves transfer energy from one place to another without transferring matter overall.
2. Waves are classified as transverse or longitudinal by the direction of oscillation compared with the direction of energy transfer.
3. Ripples on a water surface are transverse: the surface moves up and down while energy travels sideways across the water.
4. In a longitudinal wave, each particle vibrates backwards and forwards about a fixed position, in the same line as the wave travels.

Crests and troughs

1. A transverse wave shows a repeating pattern of high points called crests and low points called troughs.
2. A crest is the highest point of the wave above the undisturbed (rest) position, and a trough is the lowest point below it.
3. Examples of transverse waves include ripples on a water surface, waves on a rope or string and all electromagnetic waves, such as light.
4. Mechanical waves, such as water ripples, need a medium to travel through, whereas electromagnetic waves are also transverse but can travel through a vacuum, so they need no medium.

Compressions and rarefactions

1. A longitudinal wave shows alternating regions called compressions and rarefactions.
2. A compression is a region where the particles are pushed closer together.
3. A rarefaction is a region where the particles are further apart.
4. Sound waves travelling through air are longitudinal.
5. A vibrating loudspeaker cone pushes air particles together and then pulls back, sending a train of compressions and rarefactions through the air.

The wave travels, not the material

1. A wave transfers energy but does not carry the material it passes through along with it.
2. A floating cork on water only bobs up and down as a ripple passes, showing the water does not travel across the surface with the wave.
3. Sound crosses a room without any steady flow of air across it, because the air particles only vibrate about their positions while the compressions and rarefactions travel to the listener.