Greg Hansen, age 8, of White Bear Lake, Minn., for his question:

WHAT IS AN ULTRAVIOLET RAY?

Ultraviolet rays are waves similar to light that lie just beyond the violet end of the visible light spectrum. They are sometimes known as invisible light or black light because they cannot be seen by the human eye.

Sunlight, or white light, is a mixture of light of all colors, or wavelengths. The sun is a strong, natural source of ultraviolet rays, but the air absorbs much of this radiation. As a result, most of the shortest ultraviolet rays do not reach the earth's surface.

In fluorescent lamps, an electric current passing through mercury vapor produces ultraviolet light. A coating on the inside of the glass tube absorbs the ultraviolet radiation and changes it into visible light.