Ultraviolet Radiation, electromagnetic radiation with wavelengths between those of visible violet light and those of X rays. It is called ultraviolet (meaning "beyond violet") because, having a shorter wavelength than visible violet light, it has a higher frequency. Devices used to detect and measure ultraviolet waves include phototubes, photovoltaic cells, and radiometers. The eyes of bees and certain other animals, unlike those of humans, can detect ultraviolet radiation.
As with most forms of electromagnetic radiation, the principal sources of ultraviolet waves are the stars, including the sun. Most of the ultraviolet radiation that strikes the earth's atmosphere originates in the sun. The action of ultraviolet waves on oxygen in the atmosphere creates a layer of ozone some 9 to 19 miles (15 to 30 km) above sea level. (Ozone is a molecular form of oxygen containing three atoms of oxygen instead of the two atoms found in ordinary oxygen.) The ozone layer absorbs nearly all of the shorterwave ultraviolet radiation before it reaches the earth's surface, where it could cause severe damage or death to many forms of life, including humans.
The longer ultraviolet waves that penetrate the ozone layer are responsible for sunburn and suntan. They are also essential to good health, since they cause vitamin D to be formed in the body, and help in the accumulation of calcium and phosphorus. All of these substances are vital to the formation and maintenance of healthy teeth and bones.
Artificial sources of ultraviolet radiation include several types of electric lamps. Because ordinary glass prevents the passage of ultraviolet waves, special glass or quartz must be used for ultraviolet lamps. The ordinary fluorescent lamp used in homes and offices, while not an emitter of ultraviolet radiation, operates because ultraviolet radiation produced inside the tube causes a fluorescent coating in the tube to glow. Black light is a term used for long-wave ultraviolet radiation emitted from specially filtered or coated electric lamps. Fluorescent pigments and objects glow when struck by the black light, creating dramatic effects that are sometimes used for decorative and display purposes.
Sunlamps emit both ultraviolet and infrared radiation to simulate the tanning and warming effects of the sun. Certain wavelengths of ultraviolet have the ability to kill or weaken many kinds of bacteria, and germicidal lamps are often used in hospitals and other locations to prevent the spread of infection and disease.