Fluorescent Lights
The light that comes from glowing interstellar clouds is, quite literally, otherworldly. but the same principles that explain how such clouds emit light are also at the heart of light phenomena that we see here on earth.
A fluorescent lamp produces light in a manner not too different from an emission nebula (H II region). In both cases, the physical effect is called fluorescence: High-energy ultraviolet photons are absorbed, and the absorbed energy is reradiated as lower-energy photons of visible light.
Within the glass tube of a fluorescent lamp is a small amount of the element mercury. When you turn on the lamp, an electric current passes through the tube, vaporizing the mercury and exciting its atoms. This excited mercury vapor radiates light with an emission-line spectrum, including lines in the ultraviolet. The white fluorescent coating on the inside of the glass tube absorbs these ultraviolet photons, exciting electrons in the coating’s molecules to high energy levels.
The electrons in the molecules then cascade down through a number of lower levels before reaching the ground state. During this cascade, visible-light photons of many different wavelengths are emitted, giving an essentially continuous spectrum and a very white light. (By comparison, the hydrogen atoms in an H II region emit at only certain discrete wavelengths, because the spectrum of hydrogen is much simpler than that of the fluorescent coating’s molecules. Another difference is that the molecules in the fluorescent tube never become ionized.)
Many common materials display fluorescence. Among them are teeth, fingernails, and certain minerals. When illuminated with ultraviolet light, these materials glow with a blue or green color. (Most natural history museums and science museums have an exhibit showing fluorescent minerals.) Laundry detergent also contains fluorescent material. After washing, your laundry absorbs ultraviolet light from the Sun and glows faintly, making it appear “whiter than white.”