How
does glow-in-the-dark stuff work?
You see glow-in-the-dark stuff in all kinds of
places, but it is most common in toys. If you have ever
seen any of these products, you know that they all have to
be "charged". You hold them up to a light, and then take
them to a dark place. In the dark they will glow for 10
minutes. Some of the newer glow-in-the-dark stuff will glow
for several hours. Usually it is a soft green light, and it
is not very bright. You need to be in nearly complete
darkness to notice it.
All glow-in-the-dark products contain phosphors. A phosphor
is a substance that radiates visible light after being
energized. The two places where we most commonly see
phosphors are in a TV screen or computer monitor and in
fluorescent lights. In a TV screen, an electron beam
strikes the phosphor to energize it (see How Television
Works for details). In a fluorescent light, ultraviolet
light energizes the phosphor. In both cases, what we see is
visible light. A color TV screen actually contains
thousands of tiny phosphor picture elements that emit three
different colors (red, green and blue). In the case of a
fluorescent light, there is normally a mixture of phosphors
that together create light that looks white to us.
Chemists have created thousands of chemical substances that
behave like a phosphor. Phosphors have three
characteristics:
• The type of energy they require to be energized
• The color of the visible light that they produce
• The length of time that they glow after being energized
(known as the persistence of the phosphor)
To make a glow-in-the-dark toy, what you want is a phosphor
that is energized by normal light and that has a very long
persistence. Two phosphors that have these properties are
Zinc Sulfide and Strontium Aluminate. Strontium Aluminate
is newer -- it's what you see in the "super"
glow-in-the-dark toys. It has a much longer persistence
than Zinc Sulfide does. The phosphor is mixed into a
plastic and molded to make most glow-in-the-dark stuff.
Occasionally you will see something glowing but it does not
need charging. The most common place is on the hands of
expensive watches. In these products, the phosphor is mixed
with a radioactive element, and the radioactive emissions
energize the phosphor continuously. In the past, the
radioactive element was radium, which has a half-life of
1600 years. Today, most glowing watches use a radioactive
isotope of hydrogen called tritium (which has a half-life
of 12 years) or promethium, a man-made radioactive element
with a half-life of around three years.
How
Do LEDs Work?
Light emitting diodes, commonly called LEDs,
are real unsung heroes in the electronics world. They do
dozens of different jobs and are found in all kinds of
devices. Among other things, they form the numbers on
digital clocks, transmit information from remote controls,
light up watches and tell you when your appliances are
turned on. Collected together, they can form images on a
jumbo television screen or illuminate a traffic light.
Basically, LEDs are just tiny light bulbs that fit easily
into an electrical circuit. But unlike ordinary
incandescent bulbs, they don't have a filament that will
burn out, and they don't get especially hot. They are
illuminated solely by the movement of electrons in a
semiconductor material, and they last just as long as a
standard transistor.
A diode is the simplest sort of semiconductor device.
Broadly speaking, a semiconductor is a material with a
varying ability to conduct electrical current. Most
semiconductors are made of a poor conductor that has had
impurities (atoms of another material) added to it. The
process of adding impurities is called doping.
In the case of LEDs, the conductor material is typically
aluminum-gallium-arsenide (AlGaAs). In pure
aluminum-gallium-arsenide, all of the atoms bond perfectly
to their neighbors, leaving no free electrons
(negatively-charged particles) to conduct electric current.
In doped material, additional atoms change the balance,
either adding free electrons or creating holes where
electrons can go. Either of these additions make the
material more conductive.
A semiconductor with extra electrons is called N-type
material, since it has extra negatively-charged particles.
In N-type material, free electrons move from a
negatively-charged area to a positively charged area.
A semiconductor with extra holes is called P-type material,
since it effectively has extra positively-charged
particles. Electrons can jump from hole to hole, moving
from a negatively-charged area to a positively-charged
area. As a result, the holes themselves appear to move from
a positively-charged area to a negatively-charged area.
A diode comprises a section of N-type material bonded to a
section of P-type material, with electrodes on each end.
This arrangement conducts electricity in only one
direction. When no voltage is applied to the diode,
electrons from the N-type material fill holes from the
P-type material along the junction between the layers,
forming a depletion zone. In a depletion zone, the
semiconductor material is returned to its original
insulating state -- all of the holes are filled, so there
are no free electrons or empty spaces for electrons, and
charge can't flow.