- 1: situated beyond the visible spectrum at its violet end —used of radiation having a wavelength shorter than wavelengths of visible light and longer than those of X-rays
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 most humans except those with aphakia. Near-UV is visible to a number of insects and birds.
UV light is found in sunlight 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 120 nm to 10 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. However, the entire spectrum of ultraviolet radiation has some of the biological features of ionizing radiation: It does 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.
Although ultraviolet radiation is invisible to the human eye, most people are aware of the effect it has on the skin of fair-skinned people, i.e., the suntan and sunburn. Normal human skin responds to exposure to small doses of this kind of radiation by increasing the amount of protective melanin in the skin's outer layers; too much of this radiation in too short a period of time, however, results in cellular damage from radiation burn. In fact, the damaging effects of short-wavelength and mid-wavelength UV means that life on Earth outside of the deep oceans is possible only because the atmosphere, primarily the ozone layer, filters out the vast majority of this light. A small amount of the shorter wavelength ultraviolet reaches the surface, which causes sunburn, long-term skin damage, and skin cancer. Ultraviolet is also responsible for the formation of vitamin D in organisms that make this vitamin (including humans). The UV spectrum thus has many effects, both beneficial and damaging, to human health.