Electromagnetic radiation: 7. Infra-red
Infra-red radiation, the first non-visible electromagnetic radiation to be discovered, was discovered by William Herschel in 1800.
It has wavelengths below 1mm, down to where they start to become visible (about 700 nm). Thus the frequency range is from 300 GHz up to about 430 THz. Shorter wavelengths are near infra-red; middle ones are mid infra-red; longer ones are far infra-red.
The longest wavelengths (0.1-1mm, 300 GHz-3THz) are sometimes regarded as a distinct band called Submillimetre radiation (from their wavelength) or Terahertz radiation (from their frequency). They can penetrate through some objects opaque to visible light, but can be used to produce images, so are a safe alternative to some uses of X-rays.