Ultraviolet

Ultraviolet (UV) light, sometimes called black light, is a form of non-visible electromagnetic radiation, with wavelengths just shorter than the violet/blue end of visible light. It occupies wavelengths between around 10 and 400 nanometres, or around 1015–1016 Hertz.

Surprisingly, the human retina is sensitive to ultraviolet; however the lens is opaque to it, which means that individuals who have had their lenses removed (such as for severe cataracts) can see in the ultraviolet. Claude Monet's later paintings, following lens removal, underwent a radical change, likely the result of his being able to see patterns in flowers, scenery, etc. that are invisible to the rest of us.

Classification
The ultraviolet spectrum is typically divided into four ranges:


 * UV-A, from 400 – 315 nanometers. This is the "softest" UV, used by black lights and tanning salons.
 * UV-B, from 315 – 280 nm. This is the range that causes sunburns. It's also the range that your skin turns into vitamin D.
 * UV-C, from 280 – 200 nm. This range is germicidal, and can be used in disinfection, including the food irradiation of fresh-pressed apple cider.
 * Vacuum UV, from 200 - 100 nm. This range is absobed by atmospheric oxygen, and thus is only good at propagating through a vacuum.
 * Extreme UV, from about 121 nm down to 10 nm. This is the "hardest" of the ultraviolet frequency ranges. It's scattered in air, and is ionizing radiation. Any radiation of shorter wavelength than 10 nm is considered X-rays, or gamma rays.

Health
Ultraviolet is known to cause damage to skin. For prolonged exposure to the sun, some individuals may receive sunburn, or receive damage to the eyes. Despite the hazardous effects &mdash; or rather, because of them &mdash; it may be used to kill germs in irradiation systems.

Ultraviolet radiation (UV-A, UV-B, and UV-C) causes both skin and eye cancer (melanoma). Remember that the next time you lie down in a tanning booth.

No, it can’t cure coronavirus. Only UV-C is effective for disinfection, and it causes as much skin damage in seconds as sunlight can cause in hours.