Ultraviolet rays. Uses of ultraviolet rays. Harmful effects

Ultraviolet rays are an invisible form of light. They lie just beyond the violet end of the visible spectrum. The sun is the major natural source of ultraviolet rays. Lightning, or any other electrical spark in the air, also emits ultraviolet rays. The rays can be produced artificially by passing an electric current through a gas or vapor, such as mercury vapor. The rays can cause sunburn. Overexposure can cause skin cancer. Ultraviolet rays also destroy harmful organisms and have other useful effects.

Ultraviolet rays have shorter wavelengths than visible light has. A wavelength, the distance between the crests of two waves, is often measured in units called nanometers. A nanometer (nm) is a billionth of a meter, or about 1/25,000.000 inch. Wavelengths of visible light range from about 400 to 700 nm. Ultraviolet wavelengths range from about 1 to 400 nm. See Electromagnetic waves (diagram: The electromagnetic spectrum).

The wavelength of ultraviolet rays determines whether a material they shine on absorbs the rays or transmits them. For example, only ultraviolet rays with large wavelengths can pass through ordinary window glass. The glass absorbs rays with shorter wavelengths, though they can pass through other materials.

Uses of ultraviolet rays. Ultraviolet rays with wavelengths shorter than 300 nm are effective in killing bacteria and viruses. Hospitals use germicidal lamps that produce these short rays to sterilize surgical instruments, water, and the air in operating rooms. Many food and drug companies use germicidal lamps to disinfect various types of products and their containers.

Direct exposure to ultraviolet rays with wavelengths shorter than 320 nm produces vitamin D in the body. Physicians once used sun lamps that produced these rays to prevent and treat rickets, a bone disease caused by the lack of vitamin D. The lamps are used today to treat some skin disorders, such as acne and psoriasis.

Some instruments use ultraviolet rays to identify the chemical composition of unknown materials. Medical researchers use such instruments to analyze substances in the human body, including amino acids and proteins. The electronics industry uses ultraviolet rays in manufacturing integrated circuits.

Harmful effects. The sun's shortest ultraviolet rays— those with wavelengths below about 320 nm—are particularly harmful to living things. Overexposure to these rays can cause painful eye irritation or eye inflammation. High-quality sunglasses protect the eyes. Overexposure to ultraviolet rays also can cause a painful burn. Melanin, brown-black pigment in the skin, and sunscreen lotions provide some protection against sunburn.

Exposure to the sun's ultraviolet rays over a long period can cause skin cancer and other changes in human cells. Such exposure also can damage or kill plants. Ozone, a form of oxygen in the earth's upper atmosphere, absorbs most of the sun's ultraviolet radiation. Without the ozone layer, ultraviolet rays would probably destroy most plant and animal life. See Ozone.

Scientific research. Ultraviolet rays originate within the atoms of all elements. Scientists learn about the makeup and energy levels of atoms by studying the rays. Experts also learn about distant stars and galaxies by analyzing the ultraviolet rays that they give off Much research has focused on the role of ultraviolet rays in chemical reactions that break down the earth's protective ozone layer. As the ozone layer breaks down, it becomes less effective as a barrier against harmful ultraviolet rays. Experiments indicate that bees, butterflies, and other insects can see ultraviolet light. The reflection of ultraviolet rays off wings reveals patterns that help insects identify mates.