Electromagnetic Waves

An electromagnetic wave is a self-sustaining disturbance of electric and magnetic fields that travels through space, carrying energy and information without needing a material medium. The two fields oscillate at right angles to each other and to the direction of travel, which makes the wave transverse. James Clerk Maxwell's equations predicted such waves and showed that they all move through vacuum at the same speed, the speed of light, about 300 million metres per second.

All electromagnetic waves are the same physical phenomenon differing only in frequency and wavelength, and together they form the electromagnetic spectrum. Ordered from low to high frequency this runs through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The frequency and wavelength of a wave are linked by the speed of light: as one increases the other decreases. A wave's frequency also sets the energy of its photons, which is why high-frequency radiation such as X-rays is far more penetrating and energetic than radio waves.

Several properties characterise a wave beyond its frequency. Polarisation describes the orientation of the oscillating electric field, and waves reflect, refract, diffract and interfere in ways that depend on their wavelength relative to the objects they meet. Antennas and transmission lines are practical means of generating and guiding these waves. Animations help make the subject tangible by showing the crossed electric and magnetic fields advancing together, the wave spreading from a source, and how changing the frequency alters wavelength and behaviour.

Frequently asked questions

How fast do electromagnetic waves travel?

In a vacuum all electromagnetic waves travel at the speed of light, roughly 300 million metres per second. They move more slowly through transparent materials.

What is the electromagnetic spectrum?

It is the full range of electromagnetic waves ordered by frequency, from radio waves and microwaves through visible light to X-rays and gamma rays, all the same phenomenon.

How are frequency and wavelength related?

They are inversely proportional through the speed of light: multiplying frequency by wavelength gives that speed, so a higher frequency means a shorter wavelength.