Usually, the wavelength of the re-emitted radiation is longer than the wavelength of the radiation the substance absorbed.. Under ordinary visible light, a black-light poster does not lo
Trang 1Curie 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 10-5seconds 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
B L A C K L I G H T S A N D F L U O
-R E S C E N C E 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
L IKE MANY MARINE CREATURES , JELLYFISH PRODUCE THEIR OWN LIGHT THROUGH PHOSPHORESCENCE (Photograph by Mark A Johnson The Stock Market Reproduced by permission.)
Trang 2of 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
R U B I E S A N D L A S E R S 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
F L U O R E S C E N T L I G H T S 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
P H O S P H O R E S C E N C E 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
P H O S P H O R E S C E N C E I N S E A
C R E A T U R E S 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
Trang 3dinoflagellates 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 illumcyprid-inate 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
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
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
lumines-cence whereby a substance absorbs radia-tion and begins to re-emit the radiaradia-tion
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 10-5seconds after the energy source
is removed
passing through a given point during the interval of one second The higher the fre-quency, the shorter the wavelength
K E Y T E R M S
Trang 4turn, one of several processes that can create
luminescence
Many of the types of luminescence discussed above are described under the heading of
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
W H E R E T O L E A R N M O R E
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)
light without heat There are two principal varieties of luminescence, fluorescence and phosphorescence
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
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
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
K E Y T E R M S C O N T I N U E D
Trang 5Luminescence 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).