Your retina can sense ultraviolet light; your eye's lens simply blocks it
Humans are not truly blind to ultraviolet. Our retinal photoreceptors respond quite well to UVA, but the lens filters most of it out and cannot focus what gets through. People born without a lens, or who lose it, report seeing UVA as a whitish blue or violet glow.
Ultraviolet covers wavelengths from 100 to 400 nanometres, shorter than violet light but longer than X-rays, and the name itself means beyond violet. Johann Wilhelm Ritter found it in February 1801 by noticing that invisible rays past the violet end of the spectrum darkened silver chloride paper faster than violet light did. For much of the 19th century they were called chemical rays, a counterpart to the heat rays discovered a year earlier at the other end of the spectrum. The now-familiar split into UVA, UVB and UVC was agreed unanimously at a congress in Copenhagen on 17 August 1932.
Its photons carry about 3.1 to 12 electron volts, near the threshold for knocking electrons off atoms. Long-wave UV is not considered ionising but can still drive chemical reactions and make many materials fluoresce. Short-wave UV is ionising, damaging DNA and sterilising surfaces. Scientists showed it could kill bacteria in 1878, found the most effective wavelengths lay near 250 nm by 1903, and pinned down its effect on DNA in 1960.
Sunlight above the atmosphere is about 10 per cent ultraviolet. Oxygen and ozone swallow most wavelengths shorter than about 300 nm, which is essential, since land life could not otherwise survive. With the Sun overhead, the atmosphere removes roughly 77 per cent of solar UV; more than 95 per cent of what reaches the ground is UVA, and almost no UVC gets through. UVB still does real work, helping most land vertebrates make vitamin D and influencing plant hormones, alongside the familiar tans, burns and higher skin cancer risk.
Many animals see more of it than we do. Insects, birds and some mammals detect near-UV, and small birds, with a fourth colour receptor, have genuine ultraviolet vision. People manage exposure through sunscreens, which use either organic absorbers or minerals like zinc oxide, and through clothing: an ordinary summer fabric has a protection factor around 6, letting about 20 per cent of UV through.
Source: Ultraviolet
