Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet.
UV light is found in sunlight and is emitted by electric arcs and specialized lights such as black lights. As an ionizing radiation it can cause chemical reactions, and causes many substances to glow or fluoresce. Moreover, the UV spectrum has many other effects, both beneficial and damaging, on human health.
The sun emits ultraviolet radiation in the UVA, UVB, and UVC bands. The Earth's ozone layer blocks 97-99% of this UV radiation from penetrating through the atmosphere. 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA.
UVA (ultraviolet A, long wave, black light), 400nm-315nm
UVB (ultraviolet B, medium wave), 315nm-280nm
UVC (ultraviolet C, short wave, germicidal) 280nm-100nm