The German physicist Johann Wilhelm Ritter, in 1801, found that there exist invisible rays beyond the violet end of the visible spectrum of the Sun that darken silver chloride even more efficiently than visible light. This spectral region between visible light and X-rays was found to be more chemically active than visible light, and named as the ultraviolet region.
Ultra-violet rays can have wavelengths as short as 10-9m. They have high frequencies, though not as much a penetrating power to sufficiently penetrate the skin. They are invisible to human eyes, but can be seen by many insects.
Ultra-violet radiation of very short wavelengths can be very harmful to the skin, and are responsible for causing our sunburns. Tanning and natural body pigments prevent the destruction of skin cells by ultraviolet light to a certain extent. But prolonged exposure can cause skin cancer, cataract and damage to the human immune system.
However, we are protected from the large amounts that the Sun releases by the layer of Ozone gas in the atmosphere, which absorbs a major part of the small-wavelength rays. A small dose of UV rays is though beneficial to the human body, and can be used to treat some skin deseases as they can kill some harmful bacteria. Scientists today, have developed a UV index to help people protect themselves from these harmful ultraviolet waves.
Sources of UV rays include our Sun, synchrotron radiation sources, mercury or xenon arc lamps, and gaseous discharge tubes filled with gas atoms like mercury, deytrerium and hydrogen, whose internal electron energy levels correspond to the photons of ultraviolet light.
When ultraviolet light strikes certain materials, it causes them to fluoresce - emit electromagnetic radiation of lower energy. The spectrum of this fluorescent light reflects the material's composition and thus can be used for screening minerals, detecting bacteria in spoiled food, identifying pigments, detecting forgeries of artworks and other objects. For e.g. the fluorescence emitted by the surface of ancient marble is yellow-green, whereas a freshly cut marble surface emits bright violet fluorescence.
The high energy of ultraviolet light gives rise to certain photochemical reactions. This characteristic is exploited to produce impressions on fabrics and for blueprinting design drawings, and also for photolithography and the processing of the microelectronic circuits. Ionized atomic O2, N2 and NO2 are produced in the upper atmosphere by absorption of solar UV radiation. This ionized region is the Ionosphere. UV wavelenghts are also used in astronomical observations.
Optical instruments for UV rays are made of special materials like quartz, certain silicates, and metal fluorides, which are transparent in the near ultraviolet. Far ultraviolet radiation is absorbed by nearly all gases and materials. and hence requires reflection optics in vacuum chambers.
Ultraviolet radiation is detected by photographic plates and by photoelectric effect in photomultiplier tubes. It can also be converted to visible light by fluorescence before detection.
The layer of ozone (O3) that protects us from the ultra-violet radiation emitted by the Sun, by absorbing the short wavelength ultraviolet rays (between 2000 and 2900 Å), and attenuates those with higher wavelengths, is produced about 10 to 50 kilometres above the Earth's surface by reaction between upward-diffusing molecular oxygen (O2) and downward-diffusing ionized atomic oxygen (O+).
Many scientists today believe that this life-protecting stratospheric ozone layer is being reduced by the chlorofluorocarbon gases released into the atmosphere by different sources on the earth. Many envieonment groups are vehemently protesting against the use of these gases, and their use in many places in the world has been banned. Pollution on the earth has already caused a hole in the ozone layer above the Antarctic.