<unk>  <unk>  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.<unk>  radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from 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<unk>  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<unk>  are greater than simple heating effects, and many practical applications of<unk>  radiation derive from its interactions with organic molecules. Short-wave<unk>  light damages DNA and sterilizes surfaces with which it comes into contact. For humans, suntan and sunburn are familiar effects of exposure of the skin to<unk>  light, along with an increased risk of skin cancer. The amount of<unk>  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. More energetic, shorter-wavelength "extreme"<unk>  below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. However,<unk>  light (specifically,<unk> B) is also responsible for the formation of vitamin D in most land vertebrates, including humans. The<unk>  spectrum, thus, has effects both beneficial and harmful to life. The lower wavelength limit of human vision is conventionally taken as 400 nm, so<unk>  rays are invisible to humans, although people can sometimes perceive light at shorter wavelengths than this. Insects, birds, and some mammals can see near-UV (NUV) (i.e., slightly shorter wavelengths than what humans can see).
Ultraviolet
Wart
MedChi
Welt 
Cochrane
Cochrane 
Mohel
Medicine