  |
|||||||||||
|
Warren|About us|What's Here|How to Visit|Specimen Photos|About Fluorescence|Educators|Search|Links|Contacts |
|
|||||||||
|
|
|||||||||||
|
|
|
|||||||||
 |
|||||||||||
What is Fluorescence?
"Fluorescence" in common parlance refers to the emission of visible light from a substance being irradiated by ultraviolet light, which the human eye cannot see. Fluorescent art posters viewed under "black light" are perhaps the most familiar examples of this phenomenon in everyday life. In reality, however, this definition is too narrow, for the light given off by many substances falls outside the visible range. Some minerals, for example, fluoresce in the infrared, but we cannot see the emitted light. Moreover, the agent that causes a given substance to fluoresce need not be ultraviolet light-it could just as well be an electron beam, or X-rays, or even visible light of a different wavelength from that emitted. All fluorescence, however, involves the addition of energy by some means to a substance, and the reemission of part of that energy as electromagnetic radiation. Fluorescence is a process distinct from incandescence, which refers to emission of light from an object heated to such high temperatures that it glows. Heating is generally detrimental to the process of fluorescence, and most substances, when fluorescing, produce very little heat. For this reason fluorescence has commonly been referred to as "cold light". What is Phosphorescence?To most people "phosphorescence" refers to the continued emission of light from a substance after being irradiated with ultraviolet light. In this sense it is equivalent to an afterglow, as when a diamond is seen to continue glowing even after the ultraviolet lamp used to excite its fluorescence is turned off. Items that commonly utilize phosphorescent materials include the hands on dial watches and the wide variety of plastic "Glow-in-the-Dark" stars, Halloween ornaments, insects, and the like. This simple view of phosphorescence as a visible afterglow is different from the definition accepted by physicists, wherein the difference between fluorescence and phosphorescence depends on the atomic mechanism whereby the light is emitted. In most of the hobbyist literature, however, fluorescence refers to what one sees when the ultraviolet lamp is on, and phosphorescence to what one sees after it is off. Types of FluorescenceA potentially confusing variety of terms related to fluorescence greets those who delve into the literature on this subject. Most of those terms, however, simply refer to the means by which the fluorescence is produced. Here are a few of them:
Mechanism of FluorescenceIn the standard conceptual model of an atom the electrons are envisioned as revolving in circular orbits ("shells") around a central nucleus. An atom in this sense resembles a miniature solar system, with the nucleus taking the place of the Sun and the electrons the planets at various distances from it. This simple view of an atom is not quite correct, but it does serve to outline the general process of fluorescence. A common result of beaming ultraviolet light on a material that absorbs rather than transmits it is that one or more of the electrons of an atom are "kicked" into a higher energy state, which here can be envisioned as moving to an orbit farther out from the nucleus. All such "excited" electronic states are unstable, and sooner or later the electron will lose its excess energy and fall back to its original orbit. This excess energy can be dissipated in several ways, the most common being simply to increase atomic vibrations in the material, but some materials also emit some of the energy as light. This is what we see as fluorescence. Activators of FluorescenceThe fluorescence of many substances is due not to the substance itself but to some chemical impurity or a defect in its crystal structure. For example, pure zinc silicate (synthetic willemite) prepared in a laboratory does not fluoresce under ultraviolet light, but if a bit of manganese is added the substance will glow bright yellow-green. The manganese in this instance is said to be the "activator" of fluorescence. Numerous activators have been identified in minerals, but for most the suspected activators remain unproven, and for some they remain a genuine mystery. Chemical activators in minerals are diverse and include not only metallic elements such as manganese and lead, but also nonmetallic elements (e.g., nitrogen in diamond), atomic groups such as the uranyl and tungstate groups, and organic compounds. Coactivators of FluorescenceFor any material to fluoresce it must be capable of absorbing ultraviolet light. The presence of an activator may or may not help; manganese, for example, is a potent activator but a poor absorber of ultraviolet light. For such materials an additional chemical impurity, one capable of absorbing ultraviolet light and transmitting the added energy to the activator, is necessary for fluorescence. Impurities that operate in this way in conjunction with an activator are termed "coactivators". Lead is perhaps the most common of these in minerals, both because it is a widespread element in the Earth's crust and because it is an effective coactivator at low concentrations. Poisoners of FluorescenceSome chemical impurities deter fluorescence by promoting "radiationless transitions"-that is, by causing a given substance to lose energy by processes that do not involve the emission of light. Typically the added energy is dissipated as atomic vibrations instead. Among minerals the most notorious poisoners of fluorescence are iron, cobalt, nickel, and copper. For example, a ruby crystal containing 1% chromium will probably fluoresce bright, deep red (chromium is the activator here), but if it also contains 1% iron it probably will not fluoresce at all. Spurious FluorescenceSome materials that appear to fluoresce may not actually be doing so. The bright yellow fluorescence of some fluorite crystals from Illinois, for example, is due not to the fluorite itself, but to microscopic droplets of oil that are included in the mineral. Other minerals appear to fluoresce because of thin coatings of a second mineral whose presence may be difficult to detect in daylight, and still others because they are transmitting the light emitted by a mineral beneath. Such cases of spurious fluorescence are common and are the cause of some confusing reports in the hobbyist literature. Uses of FluorescenceFluorescence is a part of our everyday lives, though many people seem not to realize it. Here are three examples. Computer monitors and TV screens: The light that comes from these devices is emitted by tiny phosphor dots that coat the inside of the screen, and that are excited into fluorescence by a stream of electrons emitted by a cathode-ray tube. Three colors of dots-blue, red, and green-are used in most computer monitors. If you examine any on-screen image with a magnifying glass you should be able to see the dots. Fluorescent office lights: The "fluorescent light tubes" commonly used for office and commercial lighting are well named, for the light they emit is indeed produced by the process of fluorescence. The insides of the tubes are coated with a phosphor that emits a bright white light when exposed to the ultraviolet light produced inside the tube. Clothing: Nearly all laundry detergents contain a fluorescent dye that emits strongly in the blue when exposed to sunlight. The blue light counteracts the yellow tinge of old or incompletely cleaned clothing and thus makes clothes appear cleaner than they really are. The dye is designed to fluoresce in daylight. Further InformationThe Fluorescent Mineral Society maintains a bibliography for those interested in learning more about fluorescence, particularly that of minerals. For a list of publications highly regarded by the Society click here. Those new to this subject are particularly referred to the section on Bibliography - Collecting.
|
|||||||||||
|
Warren|About us|What's Here|How to Visit|Specimen Photos|About Fluorescence|Educators|Search|Links|Contacts [HOME to the Sterlinghill Mining Museum]
Contact Information
|
|||||||||||
