General properties of waves
Light travels as waves. Waves can be described by their amplitude, wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength.
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 wavesseismic waves: Shock waves travelling through the Earth, usually caused by an earthquake. are like this. They must travel through a medium, and 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, microwavesmicrowaves: Electromagnetic radiation with a frequency between that of visible light and radio waves. and other types of electromagnetic radiationelectromagnetic 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.
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