Luminescence 368 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS Curie noted that calcium fluoride glows when exposed to a radioactive material known as radium. Curie—who also coined the term “radioac- tivity”—helped spark a revolution in science and technology. As a result of her work and the dis- coveries of others who followed, interest in lumi- nescence and luminescent devices grew. Today, luminescence is applied in a number of devices around the household, most notably in television screens and fluorescent lights. Fluorescence As indicated in the introduction to this essay, the difference between the two principal types of luminescence relates to the timing of their reac- tions to electromagnetic radiation. Fluorescence is a type of luminescence whereby a substance absorbs radiation and almost instantly begins to re-emit the radiation. (Actually, the delay is 10 -6 seconds, or a millionth of a second.) Fluorescent luminescence stops within 10 -5 seconds after the energy source is removed; thus, it comes to an end almost as quickly as it begins. Usually, the wavelength of the re-emitted radiation is longer than the wavelength of the radiation the substance absorbed. British mathe- matician and physicist George Gabriel Stokes (1819-1903), who coined the term “fluores- cence,” first discovered this difference in wave- length. However, in a special type of fluorescence known as resonance fluorescence, the wave- lengths are the same. Applications of resonance include its use in analyzing the flow of gases in a wind tunnel. BLACK LIGHTS AND FLUO- RESCENCE. A “black light,” so called because it emits an eerie bluish-purple glow, is actually an ultraviolet lamp, and it brings out vibrant colors in fluorescent materials. For this reason, it is useful in detecting art forgeries: newer paint tends to fluoresce when exposed to ultraviolet light, whereas older paint does not. Thus, if a forger is trying to pass off a painting as the work of an Old Master, the ultraviolet lamp will prove whether the artwork is genuine or not. Another example of ultraviolet light and flu- orescent materials is the “black-light” poster, commonly associated with the psychedelic rock music of the late 1960s and early 1970s. Under ordinary visible light, a black-light poster does not look particularly remarkable, but when exposed to ultraviolet light in an environment in which visible light rays are not propagated (that is, a darkened room), it presents a dazzling array LIKE MANY MARINE CREATURES , JELLYFISH PRODUCE THEIR OWN LIGHT THROUGH PHOSPHORESCENCE. (Photograph by Mark A. Johnson. The Stock Market. Reproduced by permission.) set_vol2_sec9 9/13/01 1:16 PM Page 368 Luminescence of colors. Yet, because they are fluorescent, the moment the black light is turned off, the colors of the poster cease to glow. Thus, the poster, like the light itself, can be turned “on” and “off,” sim- ply by activating or deactivating the ultraviolet lamp. RUBIES AND LASERS. Fluores- cence has applications far beyond catching art forgers or enhancing the experience of hearing a Jimi Hendrix album. In 1960, American physicist Theodore Harold Maiman developed the first laser using a ruby, a gem that exhibits fluorescent characteristics. A laser is a very narrow, highly focused, and extremely powerful beam of light used for everything from etching data on a sur- face to performing eye surgery. Crystalline in structure, a ruby is a solid that includes the element chromium, which gives the gem its characteristic reddish color. A ruby exposed to blue light will absorb the radiation and go into an excited state. After losing some of the absorbed energy to internal vibrations, the ruby passes through a state known as metastable before dropping to what is known as the ground state, the lowest energy level for an atom or mol- ecule. At that point, it begins emitting radiation on the red end of the spectrum. The ratio between the intensity of a ruby’s emitted fluorescence and that of its absorbed radiation is very high, and, thus, a ruby is described as having a high level of fluorescent efficiency. This made it an ideal material for Maiman’s purposes. In building his laser, he used a ruby cylinder which emitted radiation that was both coherent, or all in a single direction, and monochromatic, or all of a single wavelength. The laser beam, as Maiman discovered, could travel for thousands of miles with very little dis- persion—and its intensity could be concentrated on a small, highly energized pinpoint of space. FLUORESCENT LIGHTS. By far the most common application of fluorescence in daily life is in the fluorescent light bulb, of which there are more than 1.5 billion operating in the United States. Fluorescent light stands in contrast to incandescent, or heat-producing, electrical light. First developed successfully by Thomas Edison (1847-1931) in 1879, the incandescent lamp quite literally transformed human life, making possible a degree of activity after dark that would have been impractical in the age of gas lamps. Yet, incandescent lighting is highly inefficient compared to fluorescent light: in an incandescent bulb, fully 90% of the energy out- put is wasted on heat, which comes through the infrared region. A fluorescent bulb consuming the same amount of power as an incandescent bulb will produce three to five times more light, and it does this by using a phosphor, a chemical that glows when exposed to electromagnetic energy. (The term “phosphor” should not be confused with phosphorescence: phosphors are used in both fluorescent and phosphorescent applica- tions.) The phosphor, which coats the inside sur- face of a fluorescent lamp, absorbs ultraviolet light emitted by excited mercury atoms. It then re-emits the ultraviolet light, but at longer wave- lengths—as visible light. Thanks to the phos- phor, a fluorescent lamp gives off much more light than an incandescent one, and does so with- out producing heat. PHOSPHORESCENCE. In contrast to the nearly instantaneous “on-off” of fluores- cence, phosphorescence involves a delayed emis- sion of radiation following absorption. The delay may take as much as several minutes, but phos- phorescence continues to appear after the energy source has been removed. The hands and num- bers of a watch that glows in the dark, as well as any number of other items, are coated with phos- phorescent materials. Television tubes also use phosphorescence. The tube itself is coated with phosphor, and a narrow beam of electrons causes excitation in a small portion of the phosphor. The phosphor then emits red, green, or blue light—the primary colors of light—and continues to do so even after the electron beam has moved on to another region of phosphor on the tube. As it scans across the tube, the electron beam is turned rapidly on and off, creating an image made up of thousands of glowing, colored dots. PHOSPHORESCENCE IN SEA CREATURES. As noted above, one of the first examples of luminescence ever observed was the phosphorescent effect sometimes visible on the surface of the ocean at night—an effect that scientists now know is caused by materials in the bodies of organisms known as dinoflagellates. Inside the body of a dinoflagellate are the sub- stances luciferase and luciferin, which chemically react with oxygen in the air above the water to produce light with minimal heat levels. Though 369 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:16 PM Page 369 Luminescence 370 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS dinoflagellates are microscopic creatures, in large numbers they produce a visible glow. Nor are dinoflagellates the only biolumines- cent organisms in the ocean. Jellyfish, as well as various species of worms, shrimp, and squid, all produce their own light through phosphores- cence. This is particularly useful for creatures liv- ing in what is known as the mesopelagic zone, a range of depth from about 650 to 3,000 ft (200- 1,000 m) below the ocean surface, where little light can penetrate. One interesting bioluminescent sea creature is the cypridina. Resembling a clam, the cypridi- na mixes its luciferin and luciferase with sea water to create a bright bluish glow. When dried to a powder, a dead cypridina can continue to produce light, if mixed with water. Japanese sol- diers in World War II used the powder of cyprid- ina to illuminate maps at night, providing them- selves with sufficient reading light without exposing themselves to enemy fire. Processes that Create Luminescence The phenomenon of bioluminescence actually goes beyond the frontiers of physics, into chem- istry and biology. In fact, it is a subset of chemi- luminescence, or luminescence produced by chemical reactions. Chemiluminescence is, in ABSORPTION: The result of any process wherein the energy transmitted to a system via electromagnetic radiation is added to the internal energy of that system. Each material has a unique absorption spectrum, which makes it possible to iden- tify that material using a device called a spectrometer. (Compare absorption to emission.) ELECTROMAGNETIC SPECTRUM: The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wave- length, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electro- magnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. ELECTROMAGNETIC WAVE: A transverse wave with electric and magnetic fields that emanate from it. These waves are propagated by means of radiation. EMISSION: The result of a process that occurs when internal energy from one sys- tem is transformed into energy that is car- ried away from it by electromagnetic radi- ation. An emission spectrum for any given system shows the range of electromagnetic radiation it emits. (Compare emission to absorption.) EXCITATION: The transfer of energy to an atom, either by collisions or due to radiation. FLUORESCENCE: A type of lumines- cence whereby a substance absorbs radia- tion and begins to re-emit the radiation 10 -6 seconds after absorption. Usually the wavelength of emission is longer than the wavelength of the radiation the substance absorbed. Fluorescent luminescence stops within 10 -5 seconds after the energy source is removed. FREQUENCY: The number of waves passing through a given point during the interval of one second. The higher the fre- quency, the shorter the wavelength. KEY TERMS set_vol2_sec9 9/13/01 1:16 PM Page 370 Luminescence 371 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS turn, one of several processes that can create luminescence. Many of the types of luminescence discussed above are described under the heading of elec- troluminescence, or luminescence involving elec- tromagnetic energy. Another process is tribolu- minescence, in which friction creates light. Though this type of friction can produce a fire, it is not to be confused with the heat-causing fric- tion that occurs when flint and steel are struck together. Yet another physical process used to create luminescence is sonoluminescence, in which light is produced from the energy transmitted by sound waves. Sonoluminescence is one of the fields at the cutting edge in physics today, and research in this area reveals that extremely high levels of energy may be produced in small areas for very short periods of time. WHERE TO LEARN MORE Birch, Beverley. Marie Curie: Pioneer in the Study of Radiation. Milwaukee, WI: Gareth Stevens Children’s Books, 1990. Evans, Neville. The Science of a Light Bulb. Austin, TX: Raintree Steck-Vaughn Publishers, 2000. “Luminescence.” Slider.com (Web site). <http://www.slid- er.com/enc/32000/luminescence.html> (May 5, 2001). “Luminescence.” Xrefer (Web site). <http://www.xrefer.com/entry/642646> (May 5, 2001). HERTZ: A unit for measuring frequen- cy, named after ninetenth-century German physicist Heinrich Rudolf Hertz (1857- 1894). LUMINESCENCE: The generation of light without heat. There are two principal varieties of luminescence, fluorescence and phosphorescence. PHOSPHORESCENCE: A type of luminescence involving a delayed emission of radiation following absorption. The delay may take as much as several minutes, but phosphorescence continues to appear after the energy source has been removed. PROPAGATION: The act or state of travelling from one place to another. RADIATION: In a general sense, radia- tion can refer to anything that travels in a stream, whether that stream be composed of subatomic particles or electromagnetic waves. RADIOACTIVE: A term describing materials which are subject to a form of decay brought about by the emission of high-energy particles or radiation, includ- ing alpha particles, beta particles, or gamma rays. SPECTRUM: The continuous distribu- tion of properties in an ordered arrange- ment across an unbroken range. Examples of spectra (the plural of “spectrum”) include the colors of visible light, the elec- tromagnetic spectrum of which visible light is a part, as well as emission and absorption spectra. TRANSVERSE WAVE: A wave in which the vibration or motion is perpendi- cular to the direction in which the wave is moving. VACUUM: An area of space devoid of matter, including air. WAVELENGTH: The distance between a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The shorter the wavelength, the higher the fre- quency. KEY TERMS CONTINUED set_vol2_sec9 9/13/01 1:16 PM Page 371 Luminescence Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Pettigrew, Mark. Radiation. New York: Gloucester Press, 1986. Simon, Hilda. Living Lanterns: Luminescence in Animals. Illustrated by the author. New York: Viking Press, 1971. Skurzynski, Gloria. Waves: The Electromagnetic Universe. Washington, D.C.: National Geographic Society, 1996. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996. “UV-Vis Luminescence Spectroscopy” (Web site). <http://www.shu.ac.uk/virtual_campus/courses/241/ lumin1.html> (May 5, 2001). 372 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:16 PM Page 372 . Though 369 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec9 9 /13/ 01 1:16 PM Page 369 Luminescence 370 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS dinoflagellates. TERMS set _vol2 _sec9 9 /13/ 01 1:16 PM Page 370 Luminescence 371 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS turn, one of several processes that can create luminescence. Many of the types of. site). <http://www.shu.ac.uk/virtual_campus/courses /24 1/ lumin1.html> (May 5, 20 01). 3 72 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec9 9 /13/ 01 1:16 PM Page 3 72