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Fluorescent Lights
and Neon Signs
The physics behind these ubiquitous devices
by Nick Guilbert
Two of the most common plasma devices on the planet are
the fluorescent light bulb, and its cousin, the neon sign. Since
their development in the 1940's, fluorescent bulbs have become the
lighting fixture of choice in offices, factories, and schools, and
they are beginning to be found more widely in homes as well. Neon
signs operate on similar principles, and are nearly as common.
This page will outline some of the physics behind these ubiquitous
devices, focusing on the fluorescent light. We will begin with the
light we can see from the outside of the bulb and work our way
inward to see what makes them work.
The Light
The light from fluorescent light bulbs looks white in most cases,
and that white color is a combination (as it is with sunlight) of
all of the colors of the visible spectrum. In the case of the
fluorescent bulb, the material that is actually doing the glowing
is a white powder applied to the inner wall of the bulb's long
glass tube.
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This powder (commonly called a 'phosphor', although it
may not have any phosphorus in it) is giving off the white light
we see through a process called fluorescence, which is the basis
of the name 'fluorescent' light bulb. Fluorescence occurs when an
atom (or molecule) absorbs energy from some source (like a photon
of light, or a collision with another atom) and then releases that
energy in the form of light in two or more consecutive steps. In
the fluorescent bulb, high-energy ultraviolet light from within
the tube is absorbed by the phosphor, which then re-radiates the
energy by emitting two or three lower-energy light waves. Since
the visible spectrum to which our eye is sensitive is at a lower
energy than is ultraviolet (uv) radiation, we can use the
fluorescing phosphor as a light source.
Where Did The Ultraviolet Come From?
In order to glow with its familiar white light, the phosphor
needed to be bombarded with uv light from within the bulb. This uv
was emitted by mercury atoms present in the partially-evacuated
fluorescent tube. When the mercury absorbs energy inside the bulb
(which it does usually as a result of impacts by very swift free
electrons also present in the tube), it emits very efficiently in
the ultraviolet region of the spectrum, mostly at a wavelength of
253.7 nm (i.e., 253.7 billionths of a meter). Only a small
fraction of the gas within the bulb is mercury; argon gas atoms
outnumber the mercury atoms by about 300 to 1. Both kinds of atoms
combined are only at a total of about 1/100 of atmospheric
pressure within the bulb.
Where Do The Free Electrons Get Their Energy?
The free electrons that collide with the mercury atoms and excite
them had initially been stripped off the mercury atoms themselves.
Not many mercury atoms are 'ionized' like this: only a few percent
of them have lost an electron or two. But once a free electron is
liberated from an atom, it rushes toward the end of the bulb that
is the more positive one (remember, fluorescent bulbs are
electrical devices, so one end of the tube is always more
'positive' relative to the other end).
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