He called them "(de-)oxidizing rays" ( German: de-oxidierende Strahlen) to emphasize chemical reactivity and to distinguish them from " heat rays", discovered the previous year at the other end of the visible spectrum. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. Ultraviolet has a higher frequency (thus a shorter wavelength) than violet light. "Ultraviolet" means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the highest frequencies of visible light. Insects, birds, and some mammals can see near-UV (NUV), i.e., slightly shorter wavelengths than what humans can see. The lower wavelength limit of the visible spectrum is conventionally taken as 400 nm, so ultraviolet rays are not visible 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. These interactions can involve absorption or adjusting energy states in molecules, but do not necessarily involve heating. Many practical applications, including chemical and biological effects, derive from the way that UV radiation can interact with organic molecules. It is also produced by electric arcs Cherenkov radiation and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights.Īlthough 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. 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 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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