<P> Ultraviolet (UV) is electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays . UV radiation is present in sunlight constituting about 10% of the total light output of the Sun . It is also produced by electric arcs and specialized lights, such as mercury - vapor lamps, tanning lamps, and black lights . 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 . 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 . </P> <P> Suntan and sunburn are familiar effects of over-exposure of the skin to UV, 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 (specifically, UVB). The UV spectrum thus has effects both beneficial and harmful to human health . </P>

What is meant by the wavelength of uv light
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