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By 1903, the most effective wavelengths were known to be around 250 nm. In 1878, the sterilizing effect of short-wavelength light by killing bacteria was discovered. The terms "chemical rays" and "heat rays" were eventually dropped in favor of ultraviolet and infrared radiation, respectively. 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" ). 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. 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. Ultraviolet has a higher frequency (thus a shorter wavelength) than violet light. "Ultraviolet" means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the highest frequencies of visible light. Birds have a fourth color receptor for ultraviolet rays this, coupled with eye structures that transmit more UV gives smaller birds "true" UV vision. Near-UV radiation is visible to insects, some mammals, and some birds. Under some conditions, children and young adults can see ultraviolet down to wavelengths around 310 nm.
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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. Humans also lack color receptor adaptations for ultraviolet rays. The lens of the human eye blocks most radiation in the wavelength range of 300–400 nm shorter wavelengths are blocked by the cornea.
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Ultraviolet rays are invisible to most humans. 9.2 Electrical and electronics industry.8 Degradation of polymers, pigments and dyes.
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UV radiation is also produced by electric arcs.