Light bulbs rely not only on simple materials but on esoteric ions and compounds. And while we take their emissions, visible light, for granted, the inner workings of these deceivingly simple gadgets depend on the complex behavior of electrons.
We’ll discuss four types of light bulbs:incandescent bulbs, halogens, fluorescent lights (including CFLs) and LEDs.A) INCANDESCENT BULBS
The light bulb of the short-lived variety, is the traditional tungsten incandescent bulb. Inside the glass, electricity flows through a thin filament of the element tungsten (chemical symbol, W, for its old name wolfram).
Because the wire is so thin, resistance is high, and it raises the temperature of the tungsten wire, so chosen because of its high melting point of 3410 oC. At the bulb’s temperature, which is about 1000o cooler, excited electrons that return to lower energy states release photons of a frequency that is visible to the human eye. The radiation is intense in the red to yellow regions but compared to daylight, the spectrum of an incandescent light bulb is very weak in the 400 to 500 nm region (blue). This would be nice to verify with a prism, and is the reason that plants don’t do as well if grown under such light. Although the heat is not sufficient to melt the tungsten it would certainly fry the heck out of it in an oxidizing atmosphere. Thus manufacturers replace oxygen with a mixture of the less reactive nitrogen and the noble gas argon. Note that a vacuum would not be a good solution because the tungsten would vaporize even more easily and dramatically shorten the bulb’s lifespan.
Even within an argon-nitrogen atmosphere, however, the heat causes some of the tungsten to sublimate. Some of it returns to the wire as it bounces off the argon gas, but a good deal ends up on the glass. This is one of the reasons incandescent bulbs tend to get darker with increased use. The glass suffers more abuse from plain old electrons which fly off the filament as if it were a cathode ray tube from a conventional television set. Such electrons cause tiny black spots to appear. These first caught Edison’s attention, but he had never time for further investigations; otherwise, as David Bodanis suggests, Edison may have discovered electrons before J.J. Thomson. To create a more diffuse light but perhaps in an attempt to camouflage all the future damage, manufacturers treat light bulb glass with hydrofluoric acid one of the few acids that can attack glass) which creates that familiar frosty look.
Eventually the tungsten wire becomes so thin, that it snaps, breaking the circuit and sending you off to the hardware store. At one point someone got tired of the bulb’s short lifespan and invented the halogen light bulb. This still uses tungsten but along with argon it includes a small amount a halogen gas, namely chlorine. The reactive gas combines with the tungsten vapour and deposits it again on the filament. In other words it recycles the tungsten, rather than letting it wastefully deposit on the glass. Of course, it is very unlikely that the metal will be perfectly and evenly replaced all along the coiled filament. Weak spots eventually develop, and the coil still breaks, but it takes a lot longer, and halogen bulbs outlive their incandescent counterparts. The glass has to be able to withstand higher temperatures, so they use a purer form of silicon dioxide, one that unfortunately gets ruined by oils on our skin. If these bulbs are mishandled as such, the grease should be washed away with alcohol.
To avoid wasting energy in the form of heat, fluorescent lights, ubiquitous in schools and other institutions, operate by a totally different principle. They contain a small amount of
mercury(Hg), which emits ultraviolet light when excited by electrical energy. The story
cannot end there because ultraviolet(UV) is invisible to the human eye. The walls of the
bulbs are coated with a phosphor, usually a halophosphate such as Ca5(PO4)3(F,Cl) with
ions of Sb3+ and Mn2+ that absorb the UV radiation. The excited electrons then release visible
light, compliments of fluorescence. In this form of luminescence, an excited electron
returns from a specific molecular orbital to a lower one without inverting its spin. The
resulting light has the bulk of its intense wavelengths in the yellow and blue regions. But
relative to natural light, fluorescence is weak in the red regions. Plants will again remain
"unhappy", unless you buy more expensive fluorescent lights which try to compensate for
this weakness by substituting antimony and manganese ions in the phosphor with
europium and terbium ions.
Next we come to the smaller version of type 3 bulbs, compact fluorescent bulbs(CFLs), which are overrated for three reasons:
(1) Practically, the CFLs are not as bright as halogens and although they match the light intensity of incandescent bulbs, they take a while to reach their peak intensity.
(2) They were not designed for cold climates, where the traditional bulb’s inefficiency is less of a drawback. The heat generation is actually desirable for about 9 months of the year in the northern states and Canada because it means the main heat source in the home does not have to work as hard.
(3) It is ironic that something marketed as an environmental savior actually contains mercury. According to Environment Canada, the Hg content varies from 1 to 25 mg per bulb. There are about 115 million American households. If each household breaks 3 bulbs per year either by accident or indirectly by sending them to a landfill, then between 300 kg and 9000 kg of mercury (one significant figure) are added to the environment in the United States alone. The annual mercury emissions from all sources in the United States are estimated at 43 700 kg ( over 12000 kg from Alaska).
To gain insight into how an LED (light emitting diode) bulb works we need to be reminded that a diode consists of two adjacent wafers of silicon doped with different impurities. The latter do not have the same valence number as silicon. If the impurity or "doping agent" is short of an electron(for example, boron), its wafer will receive an electron from the wafer with the opposite problem(example arsenic). Since electrons are stepping down from a higher energy level, photons are released. The energy gap is usually small and will only emit in the infrared, but it’s still useful if you want to use the remote control to turn off your daughter’s music channel.
In LEDs silicon is not the primary material. Common combinations include GaAsxP1-x, GaxIn1-xP, AlxIn1-xP, AlxGayInzP, and GaxIn1-xN. To get visible light you need to get away from the classic boron-arsenic combo representing a valence of 3 and 5, respectively. If aluminum and gallium(each with a valence of 3) replace boron, one can create a red LED. If indium replaces aluminum, the transition energy increases, and blue light is released. The third primary color is created by replacing arsenic with another valence 5 element, phosphorus, and combining it with aluminum and gallium. A white color can result from combining all three recipes or by coating the bulb with a phosphor.
Although there are still technical challenges ahead, LED lights will probably replace CFLs. But perhaps not to add too much arsenic to the environment, we should also use incandescent light or simply wait for sunrise to read science.References
University Corporation for Atmospheric Research http://www.ucar.edu/news/releases/2007/nicc-table.shtml
Environment Canada http://www.ec.gc.ca/mercure-mercury/default.asp?lang=En&n=2486B388-1
Efficiency of CFLs http://www.cbc.ca/news/canada/manitoba/story/2009/03/04/mb-light-bulbs.html
US Department of Energy http://www1.eere.energy.gov/buildings/ssl/how.html
Bodanis, David. Electric Universe. Crown. 2005
Britannica. DVD edition. 2000
Haber Schaim and Al. PSSC Physics. Heath. 1971