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is a form of energy. To create light, another form of energy must be
supplied. There are two common ways for this to occur,
incandescence and luminescence.
is light from heat energy. If you heat something to a high enough temperature,
it will begin to glow. When an electric stove's heater or metal in a
flame begin to glow "red hot", that is incandescence. When
the tungsten filament of an ordinary incandescent light bulb is heated still
hotter, it glows brightly "white hot" by the same means.
The sun and stars glow by incandescence.
is "cold light", light from other sources of energy, which can
take place at normal and lower temperatures. In luminescence, some
energy source kicks an electron of an atom out of its "ground"
(lowest-energy) state into an "excited" (higher-energy) state; then
the electron gives back the energy in the form of light so it can fall back to
its "ground" state.
If you lift a rock, your
muscles are supplying energy to raise the rock to a higher-energy
position. If you then drop the rock, the energy you supplied is
released, some of it in the form of sound, as it drops back to its original
low-energy position. It is somewhat the same with luminescence, with
electrical attraction replacing gravity, the atomic nucleus replacing the earth,
an electron replacing the rock, and light replacing the sound.
There are several
varieties of luminescence, each named according to what the source of energy is,
or what the trigger for the luminescence is.
are luminescence where the energy is supplied by electromagnetic
radiation (rays such as light, which will be discussed later); photoluminescence
is generally taken to mean luminesce from any electromagnetic radiation, while
fluorescence is often used only for luminescence caused by ultraviolet, although
it may be used for other photoluminescences also. Fluorescence is seen in
fluorescent lights, amusement park and movie special effects, the redness of
rubies in sunlight, "day-glo" or "neon" colors, and in
emission nebulae seen with telescopes in the night sky. Bleaches
enhance their whitening power with a white fluorescent material.
should not be confused with reflection, refraction, or scattering of light,
which cause most of the colors you see in daylight or bright artificial
lighting. Photoluminescence is distinguished in that the light is
absorbed for a significant time, and generally produces light of a frequency
that is lower than, but otherwise independent of, the frequency of the absorbed
is luminescence where the energy is supplied by chemical reactions.
Those glow-in-the-dark plastic tubes sold in amusement parks are examples of
is luminescence caused by chemical reactions in living things; it is a
form of chemiluminescence. Fireflies glow by bioluminescence.
is luminescence caused by electric current. Cathodoluminescence is
electroluminescence caused by electron beams; this is how television pictures
are formed. Other examples of electroluminescence are neon lights,
the auroras, and lightning flashes. This should not be mistaken for
what occurs with the ordinary incandescent electric lights, in which the
electricity is used to produce heat, and it is the heat that in turn produces
is luminescence caused by nuclear radiation. Older glow-in-the-dark clock
dials often used a paint with a radioactive material (typically a radium
compound) and a radioluminescent material. The term may be used to
refer to luminescence caused by X-rays, also called photoluminescence.
is delayed luminescence or "afterglow". When an
electron is kicked into a high-energy state, it may get trapped there for some
time (as if you lifted that rock, then set it on a table). In some
cases, the electrons escape the trap in time; in other cases they remain trapped
until some trigger gets them unstuck (like the rock will remain on the table
until something bumps it). Many glow-in-the-dark products,
especially toys for children, involve substances that receive energy from light,
and emit the energy again as light later.
is phosphorescence that is triggered by mechanical action or
electroluminescence excited by electricity generated by mechanical action.
Some minerals glow when hit or scratched, as you can see by banging two quartz
pebbles together in the dark.
is phosphorescence triggered by temperatures above a certain point.
This should not be confused with incandescence, which occurs at higher
temperatures; in thermoluminescence, heat is not the primary source of the
energy, only the trigger for the release of energy that originally came from
another source. It may be that all phosphorescences have a minimum
temperature; but many have a minimum triggering temperature below normal
temperatures and are not normally thought of as thermoluminescences.
stimulated luminescence is
phosphorescence triggered by visible light or infrared. In this case
red or infrared light is only a trigger for release of previously stored energy.
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VIRGIL G. RICHARDS