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Ultraviolet (UV) is an electromagnetic radiation with a wavelength from 10 nm (30 PHz) to 400 nm (750 THz), shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight. It is also produced by electric arcs and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights. Although lacking the energy to ionize atoms, long-wavelength ultraviolet radiation can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules. Suntan, freckling and sunburn are familiar effects of over-exposure, along with higher risk of skin cancer. Living things on dry land would be severely damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere. More-energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has effects both beneficial and harmful to human health. Near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays; this gives birds "true" UV vision. Reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for prey animals to find food in the wild. The males and females of some butterfly species look identical to the human eye but very different to UV-sensitive eyes—the males sport bright patterns in order to attract the females. Most ultraviolet rays are invisible to most humans: the lens on a human eye ordinarily filters out UVB frequencies or lower, and humans lack color receptor adaptations for ultraviolet rays, so humans don’t see many of the "light or colours" certain animals see. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, and people with aphakia (missing lens) or replacement lens can also see some UV wavelengths. People who don't have lenses often report seeing ultraviolet radiation that looks "whitish blue" or "whitish violet". This happens because our three color receptors (red, green and blue) are all sensitive to ultraviolet, so the rays comes in as a mixture of the three receptors, with a slight nod to the blue side of the spectrum.
Source: Wikipedia - Ultraviolet