Science of Everyday Things Vol. 2 - Physics Episode 13 ppt

Luminescence
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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.)
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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
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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
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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). <d-
er.com/enc/32000/luminescence.html> (May 5,
2001).
“Luminescence.” Xrefer (Web site).
< (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
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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).
< />lumin1.html> (May 5, 2001).
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