material that glows in the dark after being exposed to light contains molecules
that absorb energy for long periods of time, then later release that energy as
light. The phenomenon is "phosphorescence" and the materials are said
to be "phosphorescent."
in a molecule are normally arranged in a state of the least possible energy. However,
when a molecule is exposed to light it may absorb a particle of light, called
a "photon." The photon rearranges the electrons to a highly energetic
state. If the material is phosphorescent, the extra energy from the photon becomes
trapped in the molecule. Eventually, the electrons shift to a lower energy state
and the extra energy that had been trapped is released as a photon of light. As
these higher-energy electrons release light to return to their original states,
fewer and fewer are left to escape. That is why the glow continually diminishes
until it fades into nothing.
types of substances can glow in the dark. The first is the phosphorescent material
that absorbs light and then gradually releases it, like the phosphorescent
material we just described. The second type can emit light without any outside
help. For example, the firefly has the rare chemical luciferin and the enzyme
luciferase in its abdomen. These interact with oxygen to produce the glow. Radioactive
materials are the third type. These emit high-energy rays that react with surrounding
materials to produce light. Radium is a good example of a radioactive material
that gives off light.
1602 Vincenzo Casciarolo, a cobbler by trade and dilettante alchemist, discovered
Phosphorus" (barium sulphide) on Monte Paderno just outside of
was this natural stone, subsequently referred to also as the "Bolognian Stone"
or "Litheophosphorus", that became the first object of scientific study
of luminescent phenomena. This was followed by the discovery of a number of other
substances which become luminous either after exposure to light or on heating,
or by attrition, and to which the general name of "phosphori" was given.
Among these may be mentioned Homberg's phosphorus (calcium chloride), John Canton's
phosphorus (calcium sulphide) and Balduin's phosphorus (calcium nitrate).
term "fluorescence" was coined. While phosphorescence refers to a material
that absorbs light and releases it over a period of time, fluorescence refers
to a material that absorbs light in one spectral color and immediately emits light
in a different spectral color. If you've ever seen a mineral glow under an ultraviolet
light (black light) then you've seen fluorescence.
in minerals is a selective transformation of incoming ultraviolet energy, which
is invisible to the human eye, into emitted visible light. Mineral fluorescence
was discovered near the end of the 19th Century, when mines were electrified.
Sparks from knife switches and the mine "trolleys" emitted ultraviolet
light, revealing the fluorescent quality of willemite, calcite and other minerals.
female fireflies do not have wings and look very much like flat larvae. These
are called "glowworms." Most glowworms give off a greenish light, but
the "railroad worm" also has a red headlight. Some frogs eat so many
fireflies that they also glow. Microscopic single celled (unicellular) algae called
Dinoflagellates also cause phosphorescence. These are present throughout the ocean,
and emit light when they are mechanically disturbed, for example by a boat paddle,
the arm of a swimmer or wave action. This leads to a chemical reaction within
the organism, resulting in light production.
"Do Fish Drink Water?" by Bill McLain