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Incandescent Lightbulbs
Page 2 of 5 (42 Questions and Answers)

477. Does everything (all matter) emit radiation? What about if something is at absolute zero? What about if it's inside a black hole? Does a black hole emit radiation? Are Hawking particles emitted by the black hole or are they spontaneously created? If a black hole causes particles to be created, is that the same as the black hole emitting them?
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To begin with, matter always emits radiation. That's because, at any temperature above absolute zero, the electrically charge particles in matter are in thermal motion and they accelerate frequently. Any time an electrically charged particle accelerates, it emits electromagnetic radiation. If you could cool matter to absolute zero, the thermal motion would vanish and the matter wouldn't emit radiation. However, absolute zero is an unreachable destination—it can't be achieved—so everything experiences thermal motion and emits radiation.

The issue of radiation emitted by a black hole is another story. For decades, people thought of a black hole as perfectly black—it absorbed radiation perfectly but emitted none itself. However, Stephen Hawking showed that a black hole does emit radiation and that it behaves like a normal blackbody: an object that emits thermal radiation characteristic of its temperature. The temperature of a black hole is inversely proportional to its mass. For black holes of any reasonable size, this temperature is so extraordinarily low that the black hole emits very little Hawking radiation.

This radiation originates in the vicinity of the event horizon, the surface inside which the black hole's gravity finally becomes strong enough to prevent even light from escaping. At that surface, quantum fluctuations in which particles are temporarily created and destroyed can occasionally lead to the creation of a particle that escapes the black hole forever. In effect, two particles are created simultaneously, one of which falls into the black hole and is lost and the other of which escapes forever. The particle that falls into the black hole actually decreases the mass of the black hole, and the missing mass escapes with the other particle. As for whether the black hole causes this emission or is actually doing the emission, there is no difference. The only feature that the black hole has (other than electric charge and angular momentum) is its event horizon (actually a characteristic of its mass). If the event horizon is causing the particles to be created, then the black hole itself is at work creating those particles.


478. How can I differentiate between daylight and incandescent light?
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Actually daylight is a form of incandescent light. Incandescent light is the thermal radiation emitted by a hot object such as the filament of a light bulb or the surface of the sun. But the spectrum of incandescent light emitted by an object depends on its temperature. Since the filament of an incandescent light bulb has a temperature of only about 2500° C, its light is much redder than the light emitted by the 6000° C sun. That's why photographs taken indoors with incandescent lighting turn out so orange—the light just isn't white, it's orange-red. So you can differentiate between sunlight and the light from an incandescent bulb by comparing the spectrums. Look for the relative intensities of red, green, and blue lights. Sunlight will have much more blue in it than light from an incandescent bulb.

496. What is the efficiency of a 60-watt bulb to convert electricity to light?
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Since only about 80% of the heat a 60-watt bulb releases is thermal radiation and only about 12% of that thermal radiation is visible light, the bulb emits about 6 watts of visible light. A halogen bulb is a little more efficient than this and a long-life bulb is a little less efficient than this.

511. How does the wattage of a candle compare to the wattage of a light bulb?
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A 60 watt light bulb emits about 6 watts of visible light while wasting the remaining 54 watts of electric power as other forms of thermal energy. A candle probably also consumes about 60 watts of chemical energy (the paraffin wax) but emits much less than 3 watts of visible light. The light bulb is clearly not very efficient at converting electric power into visible light but the candle is even less efficient. That's because the candle flame operates at a lower temperature (about 1700° C) than the filament of the light bulb (about 2500° C) and the spectrum of light emitted by a hot object depends strongly on its temperature. The cooler flame emits relatively more infrared light and less visible light (particularly blue light) than the hotter filament.

529. How does a three-way light bulb work? - AER
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A three-way light bulb has two filaments inside it. One filament is smaller than the other, consuming less electricity and emitting less light. At the low light setting, only the smaller filament has current running through it and the bulb emits a dim light. At the medium light setting, only the larger filament has current running through it and the bulb emits a medium light. At the high light setting, both filaments have currents running through them and the bulb emits a bright light. To control the two filaments, the bulb has three electrical connections. The two filaments share one of the connection and each has one additional connection of its own. A complicated switch in the lamp determines whether to deliver current to one filament or the other or both. In each case, current flows toward the filament through one connection and returns from the filament through the other connection.

550. How does a light bulb work? — DH, Casselberry, FL (and also KH)
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In a common incandescent light bulb, an electric current flows through a double-spiral coil of very thin tungsten wire. As the electric charges in the current flow through this tungsten filament, they collide periodically with the tungsten atoms and transfer energy to those tungsten atoms. The current gives up its energy to the tungsten filament and the filament's temperature rises to about 2500° C. While all objects emit thermal radiation, very hot objects emit some of the thermal radiation as visible light. A 2500° C object emits about 12% of its heat as visible light and this is the light that you see coming from the bulb. Most of the remaining heat emerges from the bulb as invisible infrared light or "heat" light. The glass enclosure shields the filament from oxygen because tungsten burns in air.

566. Years ago I heard or read that some incandescent bulbs in Thomas Edison's house are still burning after being turned on back early in the 20th century. Is this true? What are they made of?
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From comments that I've received over the web, this story is apparently true. However those bulbs must be operating at reduced power levels and are glowing dimly as a result. There is no magic filament material that can operate indefinitely at yellow-white heat. The life of a filament is determined by how quickly its atoms evaporate (actually sublime) from its surface. Modern tungsten filaments operate at about 2500° C. At that temperature, the filament loses atoms slowly enough that it lives for about 1000 hours. If you were to operate the filament several hundred degrees colder, it would live much, much longer but it wouldn't emit nearly as much light and what light it did emit would be relatively reddish. The design of incandescent bulbs is a trade-off of energy efficiency and operating life. Long-life bulbs are substantially less energy efficient than normal bulbs—you don't have to replace them as often but they cost more to operate. Getting back to Edison's bulbs: they can only live long lives by operating at less than normal temperatures. In that case, they may live a hundred years but have very poor energy efficiencies.

577. What are the different types of light bulbs and how do they work? - BS
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An incandescent light bulb works by heating a solid filament so hot that the filament's thermal radiation spectrum includes large amounts of visible light. A fluorescent tube uses an electric discharge in mercury vapor to produce ultraviolet light, which is then transformed into visible light by fluorescent phosphors on the inner surface of the tube. A gas discharge lamp uses an electric discharge in a gas inside that lamp (often high pressure mercury, or sodium vapor, or even neon) to produce visible light directly.

662. Why does a gas lantern use a silk mantle? How does it produce such intense light — BW, Santa Clara, CA
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The mantle of a lantern is actually a ceramic ash. The silk itself burns away completely and leaves behind only of the oxides of materials that were incorporated in the silk mantle when it was manufactured. The principal oxide formed when the standard Welsbach mantle is burned is thorium oxide, with a few percent of cerium oxide and other oxides. This use of thorium oxide or thoria, is a rare example of a radioactive element (thorium is radioactive) permitted in common household use. Thoria glows brightly when heated because it can tolerate extremely high temperatures without melting and because it is a very effective emitter of thermal radiation at temperatures of roughly 2200° C.

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.


663. Why does an object like metal give off light when it is heated? — ER, Fresno, CA
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All objects emit thermal radiation—electromagnetic waves that are associated with the transfer of heat. That's because all objects contain electrically charged particles and whenever electrically charged particles accelerate, they emit electromagnetic waves. Since all objects have thermal energy in them, their electrically charged particles are always undergoing thermal motion and their thermally induced accelerations cause them to emit electromagnetic waves.

At normal temperatures, the electromagnetic waves of thermal radiation are too low in frequency and too long in wavelength for us to see. But when an object's temperature exceeds about 500° C, the object emits a dim glow. By 1800° C, the object emits the yellowish glow of a candle. By 2700° C, the object emits the yellowish-white light of an incandescent bulb. By 5800° C, the object emits the white light of the sun.


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