Fluorescent Lights and Neon Signs
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. 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). When it does, it will almost certainly collide with an atom along the way, and if its energy is high enough, it can strip an electron off the target atom and create an additional free electron. If its energy is not quite high enough when it collides with a mercury atom, it can excite the mercury in such a way that the mercury will emit uv when it gives up its energy. This collection of free electrons and residual mercury ions classifies the argon-mercury combination as a plasma, and that is why it is of interest to us in this hypertext chart effort.
Page Written by Nick Guilbert.
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