Tag: incandescent light bulbs

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.