Ultraviolet Rays and Radiation

Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. On the spectrum, ultraviolet light lies between violet light and x rays, with wavelengths ranging from four to 400 nanometers. Although it is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light, for it is this radiation that causes tanning, sunburn, and can lead to skin cancer.

The man credited with the discovery of ultraviolet light is the German physicist Johann Ritter. Ritter had been experimenting with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum—blue, indigo, violet—was a much more efficient catalyst for this reaction. Experimenting further, he discovered that silver chloride broke down most efficiently when exposed to radiation just beyond the blues, radiation that was invisible to the eye. He called this new type of radiation ultraviolet, meaning "beyond the violet." While ultraviolet radiation in large doses is hazardous to humans, a certain amount is required by the body. As it strikes the skin, it activates the chemical processes that produce Vitamin D. In areas that lack adequate sunshine, children are sometimes plagued by rickets. In order to treat these cases, or to supplement natural light in sun-starved communities, ultraviolet lamps are often used in place of natural sources.

There are three varieties of ultraviolet lamps, each producing ultraviolet light of a different intensity. Near-ultraviolet lamps are fluorescent lights whose visible light has been blocked, releasing ultraviolet radiation just beyond the visible spectrum. These lamps are also known as black lights, and are primarily used to make fluorescent paints and dyes "glow" in the dark. This effect is often seen in entertainment, but can also be used by industry to detect flaws in machine parts.

Middle-ultraviolet lamps produce radiation of a slightly shorter wavelength. They generally employ an excited arc of mercury vapor and a specially designed glass bulb. Because middle-ultraviolet radiation is very similar to that produced by the Sun, these lamps are frequently used as sunlamps and are often found in tanning salons and greenhouses. Photochemical lamps generating middle-ultraviolet light are also used in industry, as well as by chemists to induce certain chemical reactions.

Far-ultraviolet lamps produce high-energy, short-wavelength ultraviolet light. Like middle-ultraviolet lamps, they use mercury-vapor tubes; however, far-ultraviolet radiation is easily absorbed by glass, and so the lamp's bulb must be constructed from quartz. Far-ultraviolet light has been found to destroy living organisms such as germs and bacteria; for this reason, these lamps are used to sterilize hospital air and equipment. Far-ultraviolet radiation has also been used to kill bacteria in food and milk, giving perishables a much longer shelf life.

A more passive application of ultraviolet light is in astronomy. Much of the light emitted by stars, particularly very young stars, is in the ultraviolet range. By observing the output of ultraviolet light, astronomers can determine the temperature and composition of stars and interstellar gas, as well as gain insights into the evolution of galaxies. However, most of the ultraviolet light from distant sources is unable to penetrate the Earth's atmosphere; therefore, ultraviolet observations must be made from Earth's orbit by sounding rockets, space probes, or astronomical satellites.

See also Electromagnetic spectrum; Solar illumination: Seasonal and diurnal patterns