Ultraviolet radiation (UV) is a specific range of light on the electromagnetic spectrum. It is invisible to humans because its wavelength range is beyond the limit of human perception. UVs are known to cause sunburn, but they also have beneficial effects on human health. The sun emits much ultraviolet radiation, but most of this is blocked from reaching Earth's surface by the ozone layer. Applications of UV technology are widespread in modern developed countries.
The German physicist Johann Wilhelm Ritter discovered ultraviolet radiation in 1801 by observing sunlight's effects on silver salts. He noticed that light with a wavelength just beyond that of visible violet light induced a chemical reaction in certain compounds. Electromagnetic radiation is divided into groups based on a property called wavelength, which is related to the radiation's energy content. The term "chemical rays" was soon adopted to distinguish this new form of light ray. Ultraviolet radiation is the preferred term today, and is defined to have a wavelength range of 10 to 400 nanometers.
It is known that UV rays cause sunburn and some forms of skin cancer. This occurs when excessive ultraviolet radiation is absorbed by DNA molecules, the molecules that contain genetic instructions in living things. This can cause mutations, or permanent changes in the genetic code. Healthy levels of UV exposure differ for different skin pigments, with darker skin capable of absorbing more UV radiation.
A majority of scientists agree that sunscreen can prevent sunburn in humans. For healthy UV exposure, people should use use sunscreen with an SPF rating of 30 or above, and reapply it every few hours. Babies and young children should be protected from the sun. Sunlight is most intense from 10am to 3pm, so exposure during these hours should be limited. People who spend a lot of time outside should consider wearing tightly-woven fabrics for extra protection.
Despite its detrimental effects, some level of UV exposure is believed to be beneficial. UV rays that strike the skin can induce the production of vitamin D, which has an important regulatory function for the nervous system. Vitamin D is also believed to aid in bone maintenance and growth.
Ultraviolet radiation can be produced artificially, just like visible light can. Tanning beds use UV rays to produce the same effect on skin as natural sunlight. High levels of UV can be used to sterilize, or disinfect, surfaces in hospitals and scientific laboratories. In astronomy, UV detection can give clues to an astronomical object's temperature and composition.