Wendy Bogart, age 12, of Parma Heights, Ohio, for her question:
How does a fluorescent lamp create black light?
This so called black light comes from ultraviolet rays that are invisible to human eyes. However, when they shine on various chemical substances they cause them to glow with an eerie assortment of colors. This is what happens in certain fluorescent tubes and also in ultraviolet lights intended to be used as sunlamps. The ultraviolet rays are called black light because we do not see them but we do see their lighting effects on certain surfaces.
Ultraviolet light is part of the radiant electromagnetic energy that streams out from the sun. The rays of light are assorted wavelengths, all whisking along at 186,000 miles per second. A glass prism bends the wavelengths at different angles and separates the colorless light into the rainbow bands of the spectrum. It reveals the longest wavelengths as a band of red, the next longest in bands of orange, then yellow and green. The shortest wavelengths reveal themselves in a range of blues to dark purple. But where are those ultraviolet rays?
They should be there beyond the blue end of the spectrum but their short wavelengths are invisible to our eyes. The wavelengths of light are measured in angstroms and there are about 250 million of tiny angstrom units in one inch. With normal vision we can see colors with wavelengths ranging from about 3,800 to 7,800 angstroms. The wavelengths of ultraviolet range from 3,800 to 100 times shorter than the shortest visible blue rays. Though invisible to our eyes, they carry forceful electromagnetic energy.
A prism of ordinary glass usually absorbs the ultraviolet rays but a prism made of glassy quartz lets them through. If their energy shines on certain rock samples it causes them to glow with strange eerie colors. It can tan your skin, put marks on film or act as a germicide to sterilize air, food or the operating room in a hospital. This invisible energy also can destroy living cells, create severe sunburns and damage human flesh.
This same invisible energy can be used to create the magic of so called black light. In this case, the ultraviolet rays are created when electric current passes through a fluorescent tube. This excites the molecules or mercury vapor or other gases sealed inside. They emit ultraviolet radiation and the inside of the glass tube is coated with special chemicals that make the best of it. They absorb the ultraviolet and convert it back to visible light. The various colors depend on the sort of chemical coating inside the fluorescent tube.
High flying satellites register the quota of ultraviolet above the atmosphere. An enormous amount of this invisible electromagnetic energy pours from the sun. But apparently most of it filtered out as the solar radiation plunges down through our atmosphere. The full quota would be too much for us. Fortunately our lives are sheltered by the gaseous air with its cloudy moisture and its floating particles of dusty debris.