Ultraviolet rays are part of the electromagnetic spectrum that includes visible light and other forms of natural radiation, such as microwaves and radio waves. Their wavelength is positioned just past violet light on this spectrum; hence their name. Ultraviolet rays are invisible to the unaided human eye, but have a wide range of physical effects. These include chemical reactions and both positive and negative health effects on humans and other organisms. The Earth’s natural ozone layer filters out much harmful ultraviolet (UV) radiation.
Many forms of electromagnetic radiation are emitted by the sun and other natural energy sources. The most familiar form of this radiation is visible light, but other, invisible forms of radiation are emitted by these same sources. The position of this radiation on the electromagnetic spectrum is determined by its wavelength. Long-wavelength light is reddish in color, and just beyond the red end of the visible spectrum is infrared (IR) radiation. At the opposite end of the visible spectrum, with a wavelength slightly shorter than blue and violet light, are ultraviolet rays.
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Ultraviolet rays are themselves divided into several categories, depending on their wavelength and their effects on human health. The ultraviolet radiation closest to visible light is classified as UVA, UVB, and UVC, while that closest to X-ray radiation is called extreme ultraviolet (EUV). In humans, UVB radiation exposure promotes the production of vitamin D, an essential nutrient that aids the immune system. Various forms of UV radiation are used in medical therapy, including the treatment of skin diseases and nutritional deficiencies such as rickets.
Overexposure to ultraviolet rays, however, can lead to serious health problems. UV radiation causes suntanning, as the skin produces melanin, a darkening agent, to filter out UV radiation. Prolonged exposure, however, can increase the risk of medical problems such as skin cancers and cataracts; this is why the producers of sunscreens and sunglasses emphasize their UV filtration properties. Various natural substances also filter out harmful UV radiation, including glass, the Earth’s atmosphere, and the ozone layer. It is believed that reduction of the ozone layer may lead to an increase in UV-created health problems.
Ultraviolet rays have numerous industrial and commercial applications. They can be used to detect chemical signatures, either those placed deliberately, such as on passports and money, or those created naturally by substances like animal waste. Concentrated UV light is an effective germicide, as many harmful microorganisms cannot tolerate this form of radiation. Scientists use ultraviolet rays for analytic purposes in fields ranging from forensics to astronomy. Inks that fluoresce, or glow, under UV rays are used to create “black light” posters, clothing, and makeup.