Electromagnetic waves
A changing electric field creates a changing magnetic field and vice versa, and this self-sustaining pair propagates through space as an electromagnetic wave, including visible light.
Practise this conceptA changing electric field creates a changing magnetic field, and that changing magnetic field creates a changing electric field in turn — each one regenerating the other as they go. This self-sustaining pair propagates through space on its own as an electromagnetic wave, with no medium required to carry it.
That last point sets electromagnetic waves apart from sound and water waves, which genuinely do need a medium to travel through. Light reaching Earth from the Sun has crossed 150 million kilometres of essentially empty vacuum, something a sound wave simply cannot do.
Visible light is just one narrow slice of a much wider electromagnetic spectrum that includes radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays — all fundamentally the same kind of wave, differing only in frequency and wavelength. Every one of them, from the longest radio wave to the shortest gamma ray, travels at exactly the same speed through a vacuum: the speed of light.
Key ideas
Requires: Electromagnetic induction, Wave properties: amplitude, wavelength and frequency
Unlocks: The wave nature of light
Common misconceptions
- Electromagnetic waves need a medium to travel through, just like sound waves.
- All electromagnetic waves travel at different speeds in a vacuum, depending on their frequency.
- Only visible light is an electromagnetic wave; radio waves and X-rays are a different kind of phenomenon.