Download Process_Description_Fluorescence

Today fluorescent lighting is a common method of commercial lighting because of
their relative efficiency compared to that of their incandescent counterparts. You
probably have encountered their icon glow at the store or in the workplace. They
are even set to overshadow incandescent bulbs in the home and make them a thing
of the past. This is largely due to the fact that they require less energy to kick start
the physical processes that enable the emission of light.
The construction of these fluorescent tubes relies on some interesting physics that
has only been understood with theory developed within the last century. We shall
see that each step in a fluorescent lamps design is motivated directly from these
ideas.
A florescent lamp is an electric powered light source that uses florescence to
produce visible light. In order to accomplish this many pieces come together each
taking advantage of some different physical principle. In Figure 1 the basic
anatomy of a florescent lamp is outlined along with a brief description of what
happens in each part.
(1) Figure 1:
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Glass tube: This is the outside housing of the lamp
Contact pins: This is what plugs into the wall and provides power to the electrode which
connects to a filament and heats it up.
Electode/Filament: Heats up to emit electron when a current is ran through it.
Physical Principle - Thermionic Emission
Mercury: Turns gaseous upon heating. In this state in releases ultraviolet light after
encountering electrons.
Physical Principle – Atomic Photon Emission
Phosphor coating: Contains florescent material that absorbs ultraviolet light to emit
visible light.
Physical Principal- Fluorescence
The entire process can be broken down into 3 main steps each corresponding to a
physical principle Step 1: The filament heats up and emits electrons
Step 2: The electrons excites the mercury and it emits ultraviolet light
Step 3: The ultraviolet light excites the phosphor and it emits visible light
Through a series of steps the florescent lamp produces visible light by the end of
the process. We will go into further detail on what is going on in these steps and
how they work.
The first step is the heating the filament. When a voltage is applied across the
filament from the contact pins it heats up and glows. This is similar to an
incandescent bulb only instead of only emitting
light the filament also emits electrons through
thermionic emission. Figure 2 illustrates thermionic
emission when a voltage is applied to a filament.
This happens when electrons escape the atoms they
are bounded too due to the presence of thermal
energy. When this happens electrons can occupy
open space. In our case with florescent lighting
electrons will serve a purpose in this open space
during Step 2.
(2) Figure 2: Thermionic emission illistrated by
the detection of current between heated
filament and a collection plate. This current
represents the emission of electrons.
After the eletrons are emited from heating the filament they are released into the
glass tube. This tube is filled with murcury vapor. Some of the electrons interact
with the mercury atoms. When an electron
scatters off a mercury atom inelastically the
electron losses energy and leaves the mercury
atom in an excited state. This excited state is
unstable and the mercury quickly relaxes back
down to the state it was before. When the
atom completely relaxes it releases all the
energy it received for the electron in the form
of a light emission as illustrated in Figure 3.
By then end of this step there is a light
emission; however, it is not in the visible
spectrum yet. That is the focus of the next
and final step.
Figure 3: Energy is absorbed from the electron in an inelastic
collision. The atom later releases this energy by the emission
of a photon. This happens quickly and after a mercury atom
emits it is ready to be excited again.
Before this set in the process there is already a photon produced. The problem is
that the energy of that photon is currently too high and out of the visible spectrum.
This will be corrected by the use of a flourescing material. Flueresence is the
emission of radiation as a result of incident radiation of a larger energy. This
allows the ultraviolet radiation to be reduced in energy down to the visible
spectrum. In this case the fluorescing material is the phosphor coating on the inside
of the glass. This allows for any escaping light to first be conveted into visible light
before escaping the lamp. The way the phosphor does this is very similar to how
the mercury excites and relaxes during atomic photon emission in step 2. First the
phospur molecules absorb the untraviolet radiation. This cause them to go into an
excited state as it was with the mercury. Recall with mercury it restabalizes from
this higher energy state and releases the same amount of energy. The difference
with the phosphor molecule is that some of that energy is first lost through
molecular collisions. This effect is illustrated in Figure 4 with a slight shift in
energy state before the
final emission. This loss
requires the final emission
to have a lower energy
than the exciting radiation.
It is this enegry difference
that effectively converts
incoming photons to lower
(3) Figure 4: The phosphor absorbs the incoming radiation. Then it
dissipates energy and falls to a slightly less energetic state. Finally it
energy photons. In the case
fluoresces the visible light at a smaller energy value.
of the phosphur coating
this difference is just
enoguh to convert the ultraviolet light from the mercury into visible as it passes
through the glass.
Conclusion
Fluorescent lighting is becoming the standard of today. This is a direct response to
the call for smarter energy solutions. With a couple of physical statements we can
inspire technology and we see the results here. First the filament is heated by
applying a voltage across it causing it to emit electrons. Then the electrons are
inelastically scattered with mercury. This excites the mercury causing it to
temporarily absorb some of this energy. The mercury then releases this energy in
the form of ultraviolet light. This light interacts with the phosphor coating where it
is converted into lower energy radiation within the visible spectrum. This passes
through the glass as the very same light that illuminates a room that used to
produced by the less efficient predecessor. This is a tangible outcome and we see a
couple of abstract statements on energy and radiation come to life and change our
everyday lives. As our understanding of the world evolves so does our ability to
change it. With this in mind more efficient energy technology always remains just
around the corner.
Works Cited
1)
Harris, T. (n.d.). How Fluorescent Lamps Work. Retrieved from
http://home.howstuffworks.com/fluorescent-lamp2.htm
2)
Sharma, V. (n.d.). Thermionic Emission and Radioactivity. Retrieved from
http://www.slideshare.net/vipulsharma936/thermionic-emission-radioactivity
3)
Babou, B. (n.d.). Fluorescent Probes. Retrieved from https://www.thermofisher.com/us/en/home/lifescience/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-proteinmethods/fluorescent-probes.html