A 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."
Electrons
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.
Three
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.
In
1602 Vincenzo Casciarolo, a cobbler by trade and dilettante alchemist,
discovered "Bolognian
Phosphorus" (barium sulphide) on Monte Paderno
just outside of Bologna.
It 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).
Around
1852 the
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.
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.
Some
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.
