The electromagnetic spectrum

When Maxwell made his prediction, visible light was the only familiar electromagnetic wave; ultraviolet and infrared were barely established. X-rays and gamma rays were discovered by the end of the nineteenth century.

Arranging electromagnetic waves by frequency gives the electromagnetic spectrum. In order of decreasing wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

There is no sharp boundary between one kind and the next, and neighbouring regions overlap. The names follow roughly how each kind is produced or detected.

The spectrum runs, in principle, over an unlimited range; the named regions span about 10⁻¹² m (gamma rays) to 10⁶ m (long radio waves).

Since all of them have the same speed in vacuum, the only basic difference between the types is their wavelength or frequency. That is why they interact with matter so differently.

They act on matter through their electric and magnetic fields, which set the charges in all matter oscillating. How strongly a wave is absorbed or scattered depends on its wavelength and on the atoms and molecules of the medium.

Wavelength ranges (Table 8.1): radio above 0.1 m; microwave 0.1 m to 1 mm; infrared 1 mm to 700 nm; light 700 nm to 400 nm; ultraviolet 400 nm to 1 nm; X-rays 1 nm to 10⁻³ nm; gamma rays below 10⁻³ nm.