What are waves?

Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd. The wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.

Some waves must travel through a substance. The substance is known as the medium, and it can be solid, liquid or gas. Sound waves and seismic waves [seismic waves: vibrations caused by earth movements ] are like this. They must travel through a medium. It is the medium that vibrates as the waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves and other types of electromagnetic radiation [electromagnetic radiation: Energy travelling as waves in the form of changing electrical and magnetic fields. ] are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.