"UV" redirects here. For other uses, see UV (disambiguation). For other uses for "ultraviolet", see Ultraviolet (disambiguation).

Portable ultraviolet lamp

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range between 400 nm and 10 nm, corresponding to photon energies from 3 eV to 124 eV. It is so-named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. These frequencies are invisible to humans, but near UV is visible to a number of insects and birds.

UV light is found in sunlight (where it constitutes about 10% of the energy in vacuum) and is emitted by electric arcs and specialized lights such as mercury lamps and black lights. It can cause chemical reactions, and causes many substances to glow or fluoresce. A large fraction of UV, including all that reaches the surface of the Earth, is classified as non-ionizing radiation. The higher energies of the ultraviolet spectrum from wavelengths about 10 nm to 120 nm ('extreme' ultraviolet) are ionizing, but due to this effect, these wavelengths are absorbed by nitrogen and even more strongly by dioxygen, and thus have an extremely short path length through air.^[1] However, the entire spectrum of ultraviolet radiation has some of the biological features of ionizing radiation, in doing far more damage to many molecules in biological systems than is accounted for by simple heating effects (an example is sunburn). These properties derive from the ultraviolet photon's power to alter chemical bonds in molecules, even without having enough energy to ionize atoms.

Electric arcs produce UV light, and arc welders must wear eye protection to prevent welder's flash.

Although ultraviolet radiation is invisible to the human eye, most people are aware of the effects of UV on the skin, called suntan and sunburn. In addition to short-wavelength UV blocked by oxygen, a great deal (>97%) of mid-range ultraviolet (almost all UV above 280 nm and most up to 315 nm) is blocked by the ozone layer, and like ionizing short-wavelength UV, would cause much damage to living organisms if it penetrated the atmosphere. Indeed, the existence of the ozone layer is what allows life to exist on land, outside of the deep oceans. After atmospheric filtering, only about 3% of the total energy of sunlight at the zenith is ultraviolet,^[2] and this fraction decreases at other sun angles. Much of it is near-ultraviolet that does not cause sunburn, but is still capable of causing long-term skin damage and cancer. An even smaller fraction of ultraviolet that reaches the ground is responsible for sunburn and also the formation of vitamin D (peak production occurring between 295 and 297 nm) in all organisms that make this vitamin (including humans). The UV spectrum thus has many effects, both beneficial and damaging, to human health.