Category: incandescent light bulbs

How does a heat lamp work and could it be harmful to the eyes of pets from extended exposure? — DM, Osceola, IA

A heat lamp is much like a normal incandescent lamp, except that the heat lamp’s large filament operates at a much lower temperature. Because of this lower temperature, the filament emits relatively little visible light. Instead, it emits mostly invisible infrared light. While you can’t see infrared light, you can feel it as heat. Looking at a heat lamp is no more dangerous than looking at the glowing coals in a fireplace. Their thermal radiation heats your skin and the surfaces of your eyes, and is likely to make you uncomfortable enough to turn away before it causes real damage. In contrast, ultraviolet light from a sunlamp can injure your skin and eyes without causing any immediate pain—it’s only much later that you feel the sunburn on your skin and corneas. That’s why a heat lamp is relatively safe while a sunlamp is not.

How do flashing lights, chasing lights, and any type of Christmas lights work? – N

Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series, electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would repeat endlessly.

But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully timed switching can make patterns of light that appear to move or “chase.” As for the problem with one failed bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a bulb’s filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a little more brightly than normal and their lives are shortened slightly as a result.

Why does a gas lantern use a silk mantle? How does it produce such intense light — BW, Santa Clara, CA

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

Why does an object like metal give off light when it is heated? — ER, Fresno, CA

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.

What are the different types of light bulbs and how do they work? – BS

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.

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?

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.

How does a light bulb work? — DH, Casselberry, FL (and also KH)

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.

How does a three-way light bulb work? – AER

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.

How does the wattage of a candle compare to the wattage of a light bulb?

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.

What is the efficiency of a 60-watt bulb to convert electricity to light?

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.