ultraviolet

Ultraviolet (UV) is electromagnetic radiation (EMR) with wavelengths in the range of 10-400 nm, situated between visible light and X-ray radiation. Terms for subranges:

• near ultraviolet (NUV, Near-UV) 300-400 nm.
• middle ultraviolet (MUV) 200-300 nm.
• far ultraviolet (FUV) 100-200 nm.
• extreme ultraviolet 10-100 nm (ionizing radiation).

The Sun produces mostly visible light, but also some ultraviolet. Hotter stars produce more, the hotter, the more ultraviolet. Terms used for sunscreen/sunblock:

• UVA 315-400 nm.
• UVB 280-315 nm.
• UVC 100-280 nm.

More than a very small amount of ultraviolet is damaging to human health. Ultraviolet from the Sun is largely blocked from Earth ground level, the short-wavelength end of the visible atmospheric window allowing a fraction of the received UVA and smaller fractions of UVB and very little UVC. At high altitudes, larger fractions are allowed through.

Ultraviolet astronomy uses ultraviolet telescopes, which are generally space-based, such as the International Ultraviolet Explorer (IUE), the Extreme Ultraviolet Explorer (EUVE), the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Galaxy Evolution Explorer (GALEX). Some visible-light telescopes (on the ground as well as Hubble Space Telescope) do cover the NUV range, and EUV is often observed by space X-ray telescopes.

Ultraviolet is a factor in a planet's habitability and the chances of life. Some UV is considered helpful in creating some of the molecules associated with Earth life, but too much UV breaks down vital molecules, and habitability factors include the presence of a UV-protective atmosphere on extra-solar planets as well as the number and strength of the host star's stellar flares that include UV.