Editing Electromagnetic spectrum (section)

=== Ultraviolet radiation ===
{{Main|Ultraviolet}}
[[File:Ozone altitude UV graph.svg|right|thumb|The amount of penetration of UV relative to altitude in Earth's [[ozone layer|ozone]]]]
Next in frequency comes [[ultraviolet]] (UV). In frequency (and thus energy), UV rays sit between the violet end of the [[visible spectrum]] and the X-ray range. The UV wavelength spectrum ranges from 399&nbsp;nm to 10&nbsp;nm and is divided into 3 sections: UVA, UVB, and UVC.

UV is the lowest energy range energetic enough to [[ionization|ionize]] atoms, separating [[electron]]s from them, and thus causing [[chemical reaction]]s. UV, X-rays, and gamma rays are thus collectively called ''[[ionizing radiation]]''; exposure to them can damage living tissue. UV can also cause substances to glow with visible light; this is called ''[[fluorescence]]''. UV fluorescence is used by forensics to detect any evidence like blood and urine, that is produced by a crime scene. Also UV fluorescence is used to detect counterfeit money and IDs, as they are laced with material that can glow under UV.

At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making molecules unusually reactive. [[Sunburn]], for example, is caused by the disruptive effects of middle range UV radiation on [[Human skin|skin]] [[Cell (biology)|cells]], which is the main cause of [[skin cancer]]. UV rays in the middle range can irreparably damage the complex [[DNA]] molecules in the cells producing [[thymine dimers]] making it a very potent [[mutagen]]. Due to skin cancer caused by UV, the sunscreen industry was invented to combat UV damage. Mid UV wavelengths are called UVB and UVB lights such as germicidal lamps are used to kill germs and also to sterilize water.

The Sun emits UV radiation (about 10% of its total power), including extremely short wavelength UV that could potentially destroy most life on land (ocean water would provide some protection for life there). However, most of the Sun's damaging UV wavelengths are absorbed by the atmosphere before they reach the surface. The higher energy (shortest wavelength) ranges of UV (called "vacuum UV") are absorbed by nitrogen and, at longer wavelengths, by simple diatomic [[oxygen]] in the air. Most of the UV in the mid-range of energy is blocked by the ozone layer, which absorbs strongly in the important 200–315&nbsp;nm range, the lower energy part of which is too long for ordinary [[dioxygen]] in air to absorb. This leaves less than 3% of sunlight at sea level in UV, with all of this remainder at the lower energies. The remainder is UV-A, along with some UV-B. The very lowest energy range of UV between 315&nbsp;nm and visible light (called UV-A) is not blocked well by the atmosphere, but does not cause sunburn and does less biological damage. However, it is not harmless and does create oxygen radicals, mutations and skin damage.
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