The lower wavelength limit of human vision is conventionally taken as 400 nm, so ultraviolet rays are invisible to humans, although people can sometimes perceive light at shorter wavelengths than this. The UV spectrum, thus, has effects both beneficial and harmful to life. However, ultraviolet light (specifically, UVB) is also responsible for the formation of vitamin D in most land vertebrates, including humans.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. The amount of UV light produced by the Sun means that the Earth would not be able to sustain life on dry land if most of that light were not filtered out by the atmosphere. For humans, suntan and sunburn are familiar effects of exposure of the skin to UV light, along with an increased risk of skin cancer. Short-wave ultraviolet light damages DNA and sterilizes surfaces with which it comes into contact. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun. Ultraviolet ( UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. Arc welders must wear eye protection and cover their skin to prevent photokeratitis and serious sunburn. UV radiation is also produced by electric arcs.
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