How Everything Works
How Everything Works How Everything Works

Incandescent Lightbulbs
Page 4 of 5 (42 Questions and Answers)

1061. What makes a three-way touch lamp work? What makes a three-way light bulb work? - CY
A three-way touch lamp is much like a simple touch lamp—it detects your touch by applying a high frequency alternating charge to the lamp's surfaces and uses this fluctuating charge to measure the lamp's electric capacitance—the ease with which charge can moved on or off the lamp's surfaces. When you touch the lamp, the lamp's capacitance changes and the lamp's electronics detect this change.

In a three-way touch lamp, the lamp's electronics control 4 different light levels alternately: dim, medium, bright, and off. How these light levels are obtained depends on the lamp. If the lamp uses a three-way light bulb, which contains two separate filaments, then it can obtain the 3 brightness levels by turning on one or both of the filaments. It uses just the small filament for dim, just the large filament for medium, and both filaments for bright. That's exactly what a normal three-way lamp does.

But if the lamp uses a normal bulb and obtains three light levels from it, then it uses the same technique as a dimmer switch. In this technique, an electronic switching device called a triac is used to limit the times during which electric current can flow through the bulb and deliver power to it. In the bright setting, the triac permits current to flow through the bulb at all times and the bulb appears as bright as possible. But in the dim or medium settings, the triac prevents current from flowing at certain times. The triac takes advantage of the fact that the power flowing through a household lamp is alternating current—current that reverses directions 120 times a second (in the United States) for a total of 60 full cycles of reversal, over and back, each second (60 Hz). At the beginning of each current reversal, the electronic devices that control the triac start a timer. This timer allows those devices to wait a certain amount of time before they trigger the triac and allow it to begin carrying current to the light bulb. Once triggered, the triac will allow current to flow through the bulb until the next reversal of current in the power line. Thus the amount of energy that reaches the bulb during each half-cycle of the power line depends on how long the electronic devices wait before triggering the triac. The longer they wait, the less energy will reach the bulb and the dimmer it will glow. In the bright setting, the triac is triggered immediately after each current reversal so that power always flows to the bulb and it glows brightly. But in the medium and dim settings, the triac is triggered well into the half-cycle that follows the reversal. A normal dimmer gives you complete control over this delay, but a three-way touch switch only provides three preset delays. The medium setting has a medium delay while the dim setting has a long delay.

1067. How is infrared light produced?
There are many ways of producing infrared light. First, any warm surface emits infrared light. For example, a heat lamp or an electric space heater emits enormous amounts of it. That's because the thermal radiation of a warm object lies mostly in the invisible infrared portion of the electromagnetic spectrum.

Second, many light-emitting electronic devices emit infrared light. For example, the light emitting diodes in a television remote control unit emit infrared light. In this case, the infrared light is emitted by electrons that are shifting from one group of quantum levels in a semiconductor to another group—from conduction levels to valence levels. This emission isn't thermal radiation; it doesn't involve heat.

Lastly, some infrared light is produced by lasers. In this case, excited atoms or atomic-like systems amplify passing infrared light to produce enormous numbers of identical light particles—identical photons. Infrared industrial lasers are commonly used to machine everything from greeting cards to steel plates.

1090. How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? — SF, Lake Almanor, CA
The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle's yellow glow is about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be hotter still, except it's usually colored artificially by the presence of excited atoms. Atomic emissions are colored because atoms can't emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they preferentially emit only specific colors. That's why when you burn copper, you see blue-green light, even when the copper isn't very hot. The copper atoms just can't emit red or yellow light, even though those would be the more appropriate colors at the temperature of the burning copper.

1152. I was told by an electrician to use 130-volt bulbs, which he said were outlawed by the electric bulb makers because they last so long. He said that electricians can buy them and not the public. I found them and have used them for 5 years and he is right! They last forever. Why is that? How do they compare to more energy efficient lights? — J
When you use a bulb designed for 130 volts in a fixture that operates at 120 volts, the bulb's filament runs at less than its rated temperature. This temperature change has two consequences—one good and one bad. The good news is that operating the filament at less than its normal temperature slows the evaporation of tungsten atoms and prolongs the filament's life. That's why your bulbs are lasting so long. The bad news is that incandescent bulbs become much less energy efficient as you lower their filament temperatures. The light emitted by the filament is thermal radiation and its color spectrum and brightness depend almost exclusively on its temperature. These 130-volt bulbs emit redder and dimmer light than a normal bulb and they are significantly less energy efficient as a result. Incandescent bulbs already emit far more invisible infrared light than visible light and operating them at reduced temperatures only makes this problem worse. I recently read the statement "this bulb burns cooler than a normal bulb" on a package of super-long-life bulbs—as though burning cooler was a good thing rather than a serious shortcoming.

As energy becomes more and more precious, making the most of it becomes more and more important. I would suggest saving these 130-volt bulbs for fixtures that are so difficult to reach that you want to avoid changing bulbs at all costs. In more easily accessible fixtures, replacing bulbs is only a minor inconvenience associated with improved energy efficiency. Better still, switch to fluorescent lamps—which are much more energy efficient than even the best incandescent lamps.

1164. How does an ear thermometer work so quickly? — SN, West Covina, California
An ear thermometer examines the spectrum of thermal radiation emitted by the inner surfaces of a person's ear. All objects emit thermal electromagnetic radiation and that radiation is characteristic of their temperatures—the hotter an object is, the brighter its thermal radiation and the more that radiation shifts toward shorter wavelengths. The thermal radiation from a person's ear is in the invisible infrared portion of the light spectrum, which is why you can't see people glowing. But the ear thermometer can see this infrared light and it uses the light to determine the ear's temperature. The thermometer's thermal radiation sensor is very fast, which accounts for the speed of the measurement.

1165. I understand that an ear thermometer measures a person's temperature by studying the thermal radiation emitted by their ear. What is the farthest range that a person can emit thermal radiation that can still be received? Does this range depend on how hot the inner person is? — M
The thermal radiation that a person emits is mostly infrared light and, like all light, it can travel forever if nothing gets in its way. In principle, if you can observe something through a telescope, you can also measure its temperature. For example, astronomers can measure the temperature of a distant star by studying the star's spectrum of thermal radiation.

However, there are several complications when using this technique to measure a person's temperature. First, anything that lies between the person and you, and that absorbs or emit thermal radiation, will affect your measurement. That's because some of the thermal radiation that appears to be coming from the person may be coming from those in between things. Fortunately, air is moderately transparent to thermal radiation but many other things aren't. In fact, to get an accurate reading of person's temperature, you'd have to cool the telescope and the light detector so that they don't add their own thermal radiation to what you observe. You'd also have to use a mirror telescope because glass optics absorb infrared light.

Second, the temperature that you observe will be that of the person's skin and not their inner core temperature. That's because the person's skin absorbs any infrared light from inside the person and it emits its own infrared light to the world around the person. You can't observe infrared light from inside the person because the person's skin blocks your view. All you see is their skin temperature.

1176. How are incandescent light bulbs made? — SU
The glass enclosures are made from a ribbon of hot glass that's first thickened and then blown into molds to form the bulb shapes. These enclosures are then cooled, cut from the ribbon, and their insides are coated with the diffusing material that gives the finished bulb its soft white appearance.

The filament is formed by drawing tungsten metal into a very fine wire. This wire, typically only 42 microns (0.0017 inches) in diameter is first wound into a coil and then this coil is itself wound into a coil. The mandrels used in these two coiling processes are trapped in the coils and must be dissolved away with acids after the filament has been annealed.

The finished filament is clamped or welded to the power leads, which have already been embedded in a glass supporting structure. This glass support is inserted into a bulb and the two glass parts are fused together. A tube in the glass support allows the manufacturer to pump the air out of the bulb and then reintroduce various inert gases. When virtually all of the oxygen has been eliminated from the bulb, the tube is cut off and the opening is sealed. Once the base of the bulb has been attached, the bulb is ready for use.

1253. How does an acetylene miner's lamp work? How does a propane gas lamp work? Why do gas lamps need a mantle and what is the mantle made of? — DK, Washington, DC
An acetylene miner's lamp produces acetylene gas through the reaction of solid calcium carbide with water. An ingenious system allows the production of gas to self-regulate—the gas pressure normally keeps the water away from the calcium carbide so that gas is only generated when the lamp runs short on gas. In contrast, a propane lamp obtains its gas from pressurized liquid propane. Whenever the propane lamp runs short on gas, the falling gas pressure allows more liquid propane to evaporate.

Only the propane lamp needs a mantle to produce bright light. That's because the hot gas molecules that are produced by propane combustion aren't very good at radiating their thermal energy as visible light. The mantle extracts thermal energy from the passing gas molecules and becomes incandescent—it converts much of its thermal energy into thermal radiation, including visible light. Mantles are actually delicate ceramic structures consisting of metal oxides, including thorium oxide. Thorium is a naturally occurring radioactive element, similar to uranium, and lamp mantles are one of the few unregulated uses of thorium.

The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.

In contrast, the acetylene miner's lamp works pretty well without a mantle. I think that's because the flame contains lots of tiny carbon particles that act as the mantle and emit an adequate spectrum of yellow thermal radiation. Many of these particles then go on to become soot. A candle flame emits yellow light in the same manner.

One last feature of a properly constructed miner's lamp, a safety lamp, is that it can't ignite gases around it even if those gases are present in explosive concentrations. That's because the lamp's flame is surrounded by a fine metal mesh. This mesh draws heat out of any gas within its holes and thus prevents the flame inside the mesh from igniting any gas outside the mesh.

1301. How does a halogen bulb work and is it really better than a regular bulb?
A halogen bulb uses a chemical trick to prolong the life of its filament. In a regular bulb, the filament slowly thins as tungsten atoms evaporate from the white-hot surface. These lost atoms are carried upward by the inert gases inside the bulb and gradually darken the bulb's upper surface. In a halogen bulb, the gases surrounding the filament are chemically active and don't just deposit the lost atoms at the top of the bulb. Instead, they react with those tungsten atoms to form volatile compounds. These compounds float around inside the bulb until they collide with the filament again. The extreme heat of the filament then breaks the compounds apart and the tungsten atoms stick to the filament.

This tungsten recycling process dramatically slows the filament's decay. Although the filament gradually develops thin spots that eventually cause it to fail, the filament can operate at a higher temperature and still last two or three times as long as the filament of a regular bulb. The hotter filament of a halogen bulb emits relatively more blue light and relatively less infrared light than a regular bulb, giving it a whiter appearance and making it more energy efficient.

1316. Is there any equipment that can track people in a large, dense forest? — BRAR, India
To track someone in a forest, he must be emitting or reflecting something toward you and doing it in a way that is different from his surroundings. For example, if he is talking in a quiet forest, you can track him by his sound emissions. Or if he is exposed to sunlight in green surroundings, you can track him by his reflections of light.

But while both of these techniques work fine at short distances, they aren't so good at large distances in a dense forest. A better scheme is to look for his thermal radiation. All objects emit thermal radiation to some extent and the spectral character of this thermal radiation depends principally on the temperatures of the objects. If the person is hotter than his surroundings, as is almost always the case, he will emit a different spectrum of thermal radiation than his surrounds. Light sensors that operate in the deep infrared can detect a person's thermal radiation and distinguish it from that of his cooler surroundings. Still, viewing that thermal radiation requires a direct line-of-sight from the person to the infrared sensor, so if the forest is too dense, the person is untrackable.
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