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Visibility

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Ultraviolet rays are invisible to most humans. The lens of the human eye blocks most radiation in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. Humans also lack color receptor adaptations for ultraviolet rays. Nevertheless, the photoreceptors of the retina are sensitive to near-UV, and people lacking a lens (a condition known as aphakia) perceive near-UV as whitish-blue or whitish-violet. Under some conditions, children and young adults can see ultraviolet down to wavelengths around 310 nm. Near-UV radiation is visible to insects, some mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays; this gives birds "true" UV vision.
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Subtypes

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The electromagnetic spectrum of ultraviolet radiation (UVR), defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: Name Abbreviation Wavelength (nm) Photon energy (eV, aJ) Notes/alternative names Ultraviolet C UVC 100–280 4.43–12.4, 0.710–1.987 Short-wave, germicidal, completely absorbed by the ozone layer and atmosphere: hard UV. Ultraviolet B UVB 280–315 3.94–4.43, 0.631–0.710 Medium-wave, mostly absorbed by the ozone layer: intermediate UV; Dorno  de radiation. Ultraviolet A UVA 315–400 3.10–3.94, 0.497–0.631 Long-wave, black light, not absorbed by the ozone layer: soft UV. Hydrogen Lyman-alpha H Lyman-α 121–122 10.16–10.25, 1.628–1.642 Spectral line at 121.6 nm, 10.20   eV. Ionizing radiation at shorter wavelengths. Far ultraviolet FUV 122–200 6.20–10.16, 0.993–1.628 Middle ultraviolet MUV 200–300 4.13–6.20, 0.662–0.993 Near ultraviolet NUV 300–400 3.10–4.1...
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Discovery

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"Ultraviolet" means "beyond violet" (from Latin ultra , "beyond"), violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency (thus a shorter wavelength) than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them "(de-)oxidizing rays" (German: de-oxidierende Strahlen ) to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, and remained popular throughout the 19th century, although some said that this radiation was entirely different from light (notably John William Draper, who named them "tithonic rays"). The te...
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