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Light Bulb Types
Appliance Bulbs
Halogen Bulbs
MR Halogen
Specialty Bulbs
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Before the invention of the light bulb, illuminating the world after the sun went down was a messy, arduous, hazardous task. It took a bunch of candles or torches to fully light up a good-sized room, and oil lamps, while fairly effective, tended to leave a residue of soot on anything in their general vicinity. Light Basics Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called light photons, are the most basic units of light. Atoms release light photons when their electrons become excited. The wavelength of the emitted light (which determines its color) depends on how much energy is released, which depends on the particular position of the electron. Consequently, different sorts of atoms will release different sorts of light photons. In other words, the color of the light is determined by what kind of atom is excited. This is the basic mechanism at work in nearly all light sources. The main difference between these sources is the process of exciting the atoms. Light Bulb Light Bulb Structure Light bulbs have a very simple structure. At the base, they have two metal contacts, which connect to the ends of an electrical circuit. The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. The filament sits in the middle of the bulb, held up by a glass mount. The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon. When the bulb is hooked up to a power supply, an electric current flows from one contact to the other, through the wires and the filament. Electric current in a solid conductor is the mass movement of free electrons (electrons that are not tightly bound to an atom) from a negatively charged area to a positively charged area. As the electrons zip along through the filament, they are constantly bumping into the atoms that make up the filament. The energy of each impact vibrates an atom -- in other words, the current heats the atoms up. A thinner conductor heats up more easily than a thicker conductor because it is more resistant to the movement of electrons. Bound electrons in the vibrating atoms may be boosted temporarily to a higher energy level. When they fall back to their normal levels, the electrons release the extra energy in the form of photons. Metal atoms release mostly infrared light photons, which are invisible to the human eye. But if they are heated to a high enough level -- around 4,000 degrees Fahrenheit (2,200 degrees C) in the case of a light bulb -- they will emit a good deal of visible light. The filament in a light bulb is made of a long, incredibly thin length of tungsten metal. In a typical 60-watt bulb, the tungsten filament is about 6.5 feet (2 meters) long but only one-hundredth of an inch thick. The tungsten is arranged in a double coil in order to fit it all in a small space. That is, the filament is wound up to make one coil, and then this coil is wound to make a larger coil. In a 60-watt bulb, the coil is less than an inch long. Tungsten is used in nearly all incandescent light bulbs because it is an ideal filament material. In the next section, we'll find out why this is, and we'll examine the role of the glass bulb and inert gas. The Filament As we saw in the last section, a metal must be heated to extreme temperatures before it will emit a useful amount of visible light. Most metals will actually melt before reaching such extreme temperatures -- the vibration will break apart the rigid structural bonds between the atoms so that the material becomes a liquid. Light bulbs are manufactured with tungsten filaments because tungsten has an abnormally high melting temperature. But tungsten will catch on fire at such high temperatures, if the conditions are right. Combustion is caused by a reaction between two chemicals, which is set off when one of the chemicals has reached its ignition temperature. On Earth, combustion is usually a reaction between oxygen in the atmosphere and some heated material, but other combinations of chemicals will combust as well. The filament in a light bulb is housed in a sealed, oxygen-free chamber to prevent combustion. In the first light bulbs, all the air was sucked out of the bulb to create a near vacuum -- an area with no matter in it. Since there wasn't any gaseous matter present (or hardly any), the material could not combust. The problem with this approach was the evaporation of the tungsten atoms. At such extreme temperatures, the occasional tungsten atom vibrates enough to detach from the atoms around it and flies into the air. In a vacuum bulb, free tungsten atoms shoot out in a straight line and collect on the inside of the glass. As more and more atoms evaporate, the filament starts to disintegrate, and the glass starts to get darker. This reduces the life of the bulb considerably. In a modern light bulb, inert gases, typically argon, greatly reduce this loss of tungsten. When a tungsten atom evaporates, chances are it will collide with an argon atom and bounce right back toward the filament, where it will rejoin the solid structure. Since inert gases normally don't react with other elements, there is no chance of the elements combining in a combustion reaction. Incandescent light bulbs give off most of their energy in the form of heat-carrying infrared light photons -- only about 10 percent of the light produced is in the visible spectrum. This wastes a lot of electricity. Cool light sources, such as fluorescent lamps and LEDs, don't waste a lot of energy generating heat -- they give off mostly visible light. For this reason, they are slowly edging out the old reliable light bulb. Here are further guidelines. |