The electromagnetic spectrum

A microwave oven runs at 2.45 GHz, so its waves are about 12 cm long; red laser light is 633 nm, about 190 000 times shorter.

Light is one kind of electromagnetic wave

In the 1860s James Clerk Maxwell worked out that changing electric and magnetic fields should spread out as waves, and calculated their speed. It came out equal to the measured speed of light, and he concluded that light is an electromagnetic wave with wavelengths the eye can detect. His theory also allowed other wavelengths. OpenStax describes how Heinrich Hertz, starting in 1887, made and detected such waves in his laboratory with sparking circuits, measured their wavelength from interference patterns and showed they travel at the speed of light. Radio had been found in a lab notebook before anyone had a use for it.

All electromagnetic waves travel through a vacuum at the same speed, c = 299 792 458 m/s, a value that NIST lists as exact because the metre has been defined from it since 1983. For these waves the wave equation becomes c = fλ, and OpenStax draws the key consequence: the greater the frequency, the shorter the wavelength. Arranged by wavelength, from long radio waves to very short gamma rays, they form the electromagnetic spectrum.

From radio to gamma rays

Radio waves have the longest wavelengths, from about a metre up to kilometres. OpenStax gives the bands used for broadcasting: AM radio at 540 to 1600 kHz, with wavelengths of hundreds of metres that bend around hills and buildings, and FM radio at 88 to 108 MHz, about 3 m long. Microwaves come next, from about a millimetre to tens of centimetres; they carry mobile phone and satellite signals, and a microwave oven uses 2.45 GHz waves that are absorbed by water in food. Infrared is given off by anything warm and is what thermal cameras see.

Visible light is a narrow band. NASA gives it as about 380 to 700 nanometres, from violet to red; OpenStax uses 400 to 750 nm; the edges depend on the person. Ultraviolet runs from 400 nm down to about 10 nm. OpenStax divides solar UV into UV-A (320 to 400 nm), UV-B (290 to 320 nm) and UV-C (220 to 290 nm). Ozone high in the atmosphere absorbs most UV-B and all UV-C, so about 99% of the solar UV reaching the ground is UV-A. X-rays are shorter still, down to less than 0.01 nm, and are made when fast electrons hit a target. Gamma rays come from nuclear decay.

Three rules of thumb

OpenStax sums up the spectrum with three rules, each with exceptions. A higher frequency means more energy per photon, but penetration depends on the material and wavelength. X-rays can pass through soft tissue, while UV absorbed by skin can damage it. High-frequency waves can carry more information per second, which is why optical fibres carry far more data than copper wires. And the shorter the wavelength used to probe something, the finer the detail it can reveal, which is why X-ray diffraction can map the arrangement of atoms in a crystal. One more fact matters everywhere: the atmosphere lets some bands through, such as visible light and many radio waves, and blocks others, so telescopes for X-rays and gamma rays have to work from space, as NASA explains.