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It was Johann Ritter who, in 1801, for the first time proved the existence of ultraviolet radiation. It was known in those days that silver chloride turned black under the influence of light and that it reacted more violently on blue light than that it did on red light. Ritter refracted the light with help of a prism and he noticed that the silver chloride still reacted strongly in the invisible area just beyond the last visible blue light. He called this 'chemical radiation', later referred to as ultraviolet radiation. UV-rays can be generated by heating
Sunlight consists of ultraviolet radiation for only a small 4 percent. The wavelength of UV-radiation ranges from 10 to 380 nanometers and is sub-divided into near- and vacuum ultraviolet. Vacuum UV is
harmful for the human body and it may easily damage the eyes. The ozone layer (O3) that surrounds the earth on an altitude between 20 and 50 km protects us largely from this radiation. UV-radiation with wavelengths shorter than 315 nanometers (UV-B and -C) is also for a great deal blocked by glass.
UV-radiation with wavelengths between 400 and 315 nanometers (UV-A) is responsible for the tanning of pigment that is already
available in our skin. UV-B on the other hand, with wavelengths between 315 and 280 nm, stimulates the production of new pigment and of vitamin-D. UV-radiation with wavelengths around 365 nm is frequently used in insect traps. Ultraviolet radiation with wavelengths from 300 up to 185 nanometers (UV-C) is used to kill bacteria in hospitals, for sterilisation of
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up the filament of an incandescent lamp to higher temperatures than required for lighting purposes but ultraviolet radiators based on gas discharge have proven to be much more efficient (see ultraviolet emission).
the area with wavelengths below 200 nanometers and it is to a large extent absorbed by air so that it can freely propagate through vacuum only. Beside this sub-division into near- and vacuum ultraviolet a sub-division into UV-A, -B end -C is also commonly used (see electromagnetic spectrum). UV-radiation between 300 and 185 nanometers can be
food and for the purification of drinking water. UV-A and UV-B is employed for artificial tanning of the human skin but its positive influence on our health reaches furthers than that.