For the most part, prior to the nineteenth century, scientists had little concept of light without heat. Even what premodern observers called "phosphorescence" was not phosphorescence as the term is used in modern science. Instead, the word was used to describe light given off in a fiery reaction that occurs when the element phosphorus is exposed to air.
There are, however, examples of luminescence in nature that had been observed from ancient times onward—for instance, the phosphorescent glow of the ocean, visible at night under certain conditions. At one time this, too, was mistakenly associated with phosphorus, which was supposedly burning in the water. In fact, the ocean's phosphorescence comes neither from phosphorus nor water, but from living creatures called dinoflagellates. This is an example of a phenomenon known as bioluminescence—fireflies are another example—discussed below.
Modern understanding of luminescence owes much to Polish-French physicist and chemist Marie Curie (1867-1934). Operating in fields that had been dominated by men since the birth of the physical sciences, Curie distinguished herself with a number of achievements, becoming the first scientist in history to receive two Nobel prizes (physics in 1903 and chemistry in 1911). While working on her doctoral thesis,
Curie—who also coined the term "radioactivity"—helped spark a revolution in science and technology. As a result of her work and the discoveries of others who followed, interest in luminescence and luminescent devices grew. Today, luminescence is applied in a number of devices around the household, most notably in television screens and fluorescent lights.
As indicated in the introduction to this essay, the difference between the two principal types of luminescence relates to the timing of their reactions 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 mathematician and physicist George Gabriel Stokes (1819-1903), who coined the term "fluorescence," first discovered this difference in wavelength. However, in a special type of fluorescence known as resonance fluorescence, the wavelengths are the same. Applications of resonance include its use in analyzing the flow of gases in a wind tunnel.
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 fluorescent 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 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," simply by activating or deactivating the ultraviolet lamp.
Fluorescence 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 surface 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 molecule. 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 dispersion—and its intensity could be concentrated on a small, highly energized pinpoint of space.
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 output 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 applications.) The phosphor, which coats the inside surface 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 phosphor, a fluorescent lamp gives off much more light than an incandescent one, and does so without producing heat.
In contrast to the nearly instantaneous "on-off" of fluorescence, phosphorescence involves 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. The hands and numbers of a watch that glows in the dark, as well as any number of other items, are coated with phosphorescent 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.
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 substances luciferase and luciferin, which chemically react with oxygen in the air above the water to produce light with minimal heat levels. Though dinoflagellates are microscopic creatures, in large numbers they produce a visible glow.
Nor are dinoflagellates the only bioluminescent organisms in the ocean. Jellyfish, as well as various species of worms, shrimp, and squid, all produce their own light through phosphorescence. This is particularly useful for creatures living 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 cypridina mixes its luciferin and luciferase with sea water to create a bright bluish glow. When dried to a powder, a dead cypridina can continue toproduce light, if mixed with water. Japanese soldiers in World War II used the powder of cypridina to illuminate maps at night, providing themselves with sufficient reading light withoutexposing themselves to enemy fire.
The phenomenon of bioluminescence actuallygoes beyond the frontiers of physics, into chemistry and biology. In fact, it is a subset of chemiluminescence, or luminescence produced bychemical reactions. Chemiluminescence is, in turn, one of several processes that can create luminescence.
Many of the types of luminescence discussed above are described under the heading of electroluminescence, or luminescence involving electromagnetic energy. Another process is triboluminescence, in which friction creates light. Though this type of friction can produce a fire, it is not to be confused with the heat-causing friction 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.
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