Category: incandescent light bulbs

How can I differentiate between daylight and incandescent light?

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.

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?

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.

At what point is it more efficient to leave a light on when leaving and the returning to a room?

Since turning an incandescent bulb on and off doesn’t shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn’t true of a fluorescent tube—turning it on ages its filaments significantly (due to sputtering processes) so you shouldn’t turn a fluorescent lamp off if you plan to restart it in less than about 1 minute.

Can I produce light without using electric power?

Since light carries energy with it, something must provide that energy. However, the energy doesn’t have to come from electric power. Since objects emit visible thermal radiation when they have temperatures above about 500 C, anything that heats an object to high temperatures will make light. But light can also be made without heat. There are many ways to convert electric energy into light without making anything hot (for example, a neon sign or a light-emitting diode). But you ask about making light with electricity. The next best choice is light-emitting chemical reactions, such as those used in light sticks (liquid-filled plastic sticks that you bend to activate and which then glow bright green for about 12 hours). However, such reactions don’t produce all that much light and they consume the chemicals fairly quickly. If you are trying to produce large amounts of light without electric power, I’m afraid that you’ll have to burn sometime. That’s what people did before 1879 and the electric lamp.

How does a heat-seeking missile and a radar-homing missile work?

A heat-seeking missile studies the infrared light coming toward it from the sky in front of it. It uses a lens to form a real image of that light on an array of infrared sensors. If there is a hot object in front of the missile, that object will emit more infrared light than its surroundings and the missile’s lens will form a bright image of the hot object on one of the infrared sensors. If the bright image falls on the central sensor, the missile will do nothing—it will flight straight ahead. But if the bright image falls on one of the side sensors, the missile will turn. It will turn by deflecting its rocket exhaust so that the missile begins to rotate in flight. As the missile rotates, the image of the hot object will move from one sensor to the next and it will eventually fall on the central sensor. At that point, the missile will stop turning and will flight straight ahead. Since the missile automatically turns to head toward the hot object, it will eventually fly right into the hot object and explode. A radar-seeking missile will do that same things, except that it will look for an object that is emitting lots of microwaves (radar), rather than lots of infrared light. A radar-guided missile is much more complicated, since it must first emit a burst of microwaves and then analyze the reflected microwaves to look for something to fly toward. Many laser-guided missiles are just like heat-seeking missiles except that they look for an object that is reflecting a laser beam. The people who fire the missile simply illuminate the target with a bright laser beam and the missile flies directly toward the laser spot on the target.

How does a regular lamp (light bulb) work?

A normal incandescent lamp contains a double-wound tungsten filament inside a gas-filled glass bulb. By “double-wound”, I mean that a very fine wire is first wound into a long, thin spiral and then this spiral is again wound into a wider spiral. While the final filament looks about 1 or 2 centimeters long, it actually contains about 1 meter of fine tungsten wire. When the bulb is on, an electric current flows through the filament from one end to the other. The electrons making up this current carry energy, both in their motion and in the forces that they exert on one another. As they flow through the fine tungsten wire, these electrons collide with the tungsten atoms and transfer some of their energy to those tungsten atoms. The tungsten atoms and the filament become extremely hot, typically about 2500° Celsius. Tungsten wire is used because it tolerates these enormous temperatures without melting and because it resists sublimation longer than any other material. Sublimation is when atoms “evaporate” from the surface of a solid. The gas inside the bulb is there to slow sublimation and extend the life of the filament.

Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation. For any object that is hotter than about 500° Celsius, some of this thermal radiation is visible light and for an object that is about 2500° Celsius, about 10% is visible light. The light that you see from the bulb is the visible portion of its thermal radiation. However, most of the filament’s thermal radiation is invisible infrared light. While you can feel this infrared light warming your hand, you can’t see it. Because only about 80% of the electric power delivered to the bulb becomes thermal radiation and only about 12% of that thermal radiation is visible, an incandescent light bulb is only about 10% energy efficient. Other types of lamps, including fluorescent and gas discharge lamps, are much more energy efficient.

Is there a better way to construct a light bulb? For instance, is there a way to prevent the surface of the bulb from heating so quickly and generating so much heat? Is glass the best cover?

Unfortunately, there is not much that can be done to increase the efficiency of an incandescent bulb. It emits light by creating a very hot filament. Some of the filament’s heat is emitted as visible light but most ends up as hot air or infrared light (which you cannot see). There are tricks used to increase the bulb’s visible light output slightly (e.g. heating the filament hotter as in a halogen bulb or reducing the heat transport in the bulb gas as in a krypton bulb), but mostly there is nothing that can be done. Glass is about the best material for a bulb: it’s clear and a relatively poor conductor of heat.

On a three-way lamp, what are the switch settings for? Does it pump in more energy?

The lamp has four switch positions: off, filament 1 on, filament 2 on, and both filaments on. The bulb has three electrical connections to its filaments. One contact delivers electrical power to filament 1, another contact delivers electrical power to filament 2, and the third contact returns electricity from both filaments to the power plant. The switch carefully controls the flow of electricity to the two filaments so that at the low light setting, only the small filament is on, at the medium setting, only the large filament is on, and at the high setting, both filaments are on.

Which electric light bulb is best for the money, i.e. uses least electricity and has greatest light. I remember my high school physics teacher saying something like 50 watts -> 100 watts doesn’t double the light, just eats electricity.

For a given type of light bulb, the higher wattage bulbs are more energy efficient. Each light bulb has some “overhead” of wasted power that goes into heating the supporting structure and glass envelope. The higher wattage bulbs produce a little more light per watt of power. But not all types of bulbs are equally efficient. Long life bulbs are the least energy efficient because they run cooler than normal bulbs. The filament lasts a long time, but wastes more power producing infrared light. Some “energy miser” bulbs aren’t as good as normal bulbs. They may have lower wattages (typically 55 W instead of 60 W or 90 W instead of 100 W), but they actually produce significantly less light and thus consume more watts of power for each unit of light they produce. The most efficient incandescent bulbs are halogen lamps. These lamps, with their chemical recycling process, run substantially hotter than normal bulbs and produce more light per watt. They also last longer than normal light bulbs. They also produce whiter light (less red) and are just plain better bulbs than normal light bulbs. They cost more money up front, but it’s worth it in most cases.

Why aren’t you supposed to touch halogen bulbs with your bare hands?

When they’re operating, halogen bulbs become extremely hot, so you certainly wouldn’t want to touch them then. But even when a bulb is cool, touching it would deposit greases and salts from your skin onto its surface. The aluminosilicate glass used in the lamp’s envelope would be weakened when these salts are baked into the glass during the lamp’s operation and the greases would scorch and darken the bulb’s surface.