Ultraviolet rays

Ultraviolet (UV) rays were discovered by J. Ritter in 1801. They can be produced by a mercury vapour lamp, an electric spark, a carbon arc lamp, or by striking an electrical discharge in hydrogen or xenon gas tubes. The Sun is by far the most important natural source of ultraviolet radiation, though most of the Sun's UV output is absorbed by the ozone layer in the Earth's atmosphere before it reaches the ground.

Ultraviolet rays are high-energy EM waves; being electrically neutral radiation, they are not deflected by electric or magnetic fields. They produce fluorescence in certain materials called phosphors, and they can cause the photoelectric effect (ejecting electrons from a suitable metal surface). In terms of transmission through materials, UV rays cannot pass through ordinary glass but do pass through quartz, fluorite and rock salt; on skin, they trigger the synthesis of vitamin D. Their practical uses include destroying germs and bacteria (making them useful for sterilising surgical instruments and for water purification), operating burglar alarms and security systems, distinguishing real gems from fakes, analysing the composition of chemical compounds, and detecting forgery. A related everyday example is a fluorescent light bulb: it is coated inside with a powder and filled with a gas, and passing electricity through the gas makes it emit ultraviolet radiation, which then stimulates the tube's inner coating to emit the visible light we actually see; the same electron-excitation idea makes the phosphor pixels of a television or computer screen fluoresce when they are struck by electrons from an electron gun. It is also worth noting that 'visible light' is defined purely by w …