BEHINDLIGHTthe

Before the invention of the light bulb, illuminating the world after the sun went down was a messy, arduous, hazardous task. It took a bunch of candles or torches to fully light up a good-sized room, and oil lamps, while fairly effective,

tended to leave a residue of soot on anything in their general vicinity. When the science of electricity really got going in the mid 1800s, inventors everywhere were clamoring to devise a practical, affordable electrical home lighting device. Englishman Sir Joseph Swan and American Thomas Edison both got it right around the same time (in 1878 and 1879, respectively), and within 25 years, millions of people around the world had installed electrical lighting in their homes. The easy-to-use technology was such an improvement over the old ways that the world never looked back. The amazing thing about this historical turn of events is that the light bulb itself could hardly be simpler. The modern light bulb, which hasn’t changed drastically since Edison’s model, is made up of only a handful of parts. In this article, we’ll see how these parts come together to produce bright light for hours on end.

THE LIGHT BULB

Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called light photons, are the most ba-sic units of light. Atoms release light photons when their elec-trons become excited. If you’ve read How Atoms Work, then

you know that electrons are the negatively charged

particles that move around an atom’s nucleus (which has a net posi-tive charge). An atom’s electrons have different lev-

els of energy, de-

pending on several factors, including their speed and distance from the nucleus. Electrons of different energy levels occupy different orbitals. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus. When an atom gains or los-es energy, the change is expressed by the movement of electrons. When something passes energy on to an atom, an electron may be temporarily boosted to a higher orbital (farther away from the nucleus). The electron only holds this position for a tiny fraction of a second; almost immediately, it is drawn back toward the nucleus, to its original orbital. As it returns to its original orbital, the electron releases the extra energy in the form of a photon, in some cases a light photon. The wavelength of the emitted light (which deter-mines its color) depends on how much energy is released, which depends on the particular position of the electron. Consequently, different sorts of atoms will release different sorts of light photons. In other words, the color of the light is determined by what kind of atom is excited. This is the basic mechanism at work in nearly all light sources. The main difference between these sources is the process of exciting the atoms.In the next section we’ll look at the different parts of a light bulb.

HOW DOES IT WORK?There are two types of lightbulbs: incandescent and fluorescent. An incandescent bulb uses heat caused by an electrical current. When electrical current passes through a wire, it causes the wire to heat. The wire, or filament, gets so hot that it glows and gives off light. Everyday incandescent light bulbs have a filament made of tungsten. Since the hot tungsten would quickly burn away if it were exposed to oxygen, it must be placed in a sealed glass bulb which is either evacuated or filled with a gas that won’t let it burn. Another common type of light is the fluorescent lamp. A fluorescent lamp is a glass tube filled with argon gas and a tough of mercury. When electrical current is passed through the gas the atoms of the gas pick up energy and radiate it in the form of ultra-violet light (and

THIS IS WHAT THE AVERAGE PERSON KNOWS

some heat). The UV light then strikes the inside of the tube, which is coated with a phosphor. The phosphor glows, giving off the light we see. Fluorescent lamps don’t require high temperatures to produce light, like incandescent bulbs do. Energy must be used in heating the incandescent bulb, and a large part of that energy is lost as heat, not light. In the fluorescent lamp, a larger portion of the energy is radiated as light Experiments were made with different materials to use as the filament, including natural fibres, pure metals and alloys of different metals, to find the material which had the longest life whilst glowing brightly enough to give out visible light. The metal Tungsten was found to be the best. Also experiments were made trying a vacuum or different kinds of gas inside the glass bulb to find out which was the best. For many years Nitrogen gas was found to be the best but other gases or mixtures of gases may now be used. Fluorescent light bulbs electrically charge a gas (sometimes one of the inert gases like neon). Whilst it is normal every-day talk to say that a light bulb or a lamp is “burning”, that is not strictly accurate because, speaking strictly scientifically, the word “burning” has a very precise meaning. When something is said to be “burning” it means the material is combining with the element Oxygen to form a compound called an Oxide.

Modern light bulbs don’t hold a vacuum. Instead they are filled with an inert (electrically non-conducting) gas such as Nitrogen. An inert gas is used to fill the bulb (instead of just pumping out almost all the air to leave a near-vacuum) because the action of filling the bulb with an inert gas can be used to flush away ALL of the air. In addi-tion the inert gas has the very useful physical property of helping to conduct heat from the glowing filament to the glass bulb. This allows the whole surface area of the glass bulb to radiate heat into the surrounding air. It is important to understand that the inert gas does not allow the filament to “burn away”, it just allows it to glow brightly. If some air were still present in the bulb - as sometimes happens if a light bulb gets knocked and gets even a tiny crack in its glass bulb - then the oxygen present in ordinary air will quickly make the filament burn away.If the light bulb just held a vacuum (as was the case in the early days of electric lighting) the main way the heat from the hot glowing filament could get out was along the wires feeding current to the filament and also along the insulators which support the filament. (Relatively little heat passes through a vacuum compared with what can pass through an inert gas.) So the feed wires and insulators got very much hotter compared to the temperature they reach in modern light bulbs. This caused the old vacuum light bulbs, which glowed at a much higher temperature than radio tubes, to have a much shorter useful life compared to vacuum radio tubes. An earlier answer was given saying the filament has to be made from a material that has a negative temperature coefficient (as temperature increases, resistance decreases) but, if that were correct, then the decreasing resistance would cause more and more current to be taken as the lamp heated up - and the temperature would get higher and higher in a runaway manner - until either the power supply’s breaker would trip or (more likely) the light bulb’s filament would simply explode!

Imagine that you’re holding a garden hose -- one with no nozzle attached. With nothing to obstruct the water, it pours out of the hose’s end freely. But if you place your thumb over the end of the hose, the water’s going to squirt out. The reason it does is because of the resistance created by your thumb.It works much the same way for a light bulb. Electrons move relatively freely through the wire, then they come to the bulb’s filament, which resists the flow of electrons. The electrons can get through, but not as easily as they can through the wire. The work done overcoming the resistance causes the filament to heat up and to give off light.

WHATS INSIDE THE BULB

In an incandescent light bulb, a positive and negative (or neutral wire) are connected by a tung-sten filament in a vacuum. An electrical current passes through the thin filament, heating it very hot and causing it to glow. Even-tually, after repeated use, the tungsten filament gets quite thin and eventually breaks, which is what happens when the light bulb burns out! Also, if the filament is exposed to oxygen while the cur-rent is flowing, the filament will break melts.

LED light bulbs are much cool-er. When you’re running fans or an air conditioner this summer, having burning-hot incandes-cent bulbs just makes it harder to manage the heat. LEDs run much cooler than incandescent bulbs and significantly cooler than CFLs. LED bulbs do get hot but the heat is dissipated by metal heat sinks that wick away the heat from the light source itself. Keeping them cool with heat sinks or even liquid cool-ing is important to ensuring they last as long as advertised. You get instant full light. You get the full brightness of an LED lamp when you turn it on, which is an advantage over CFLs in a cou-

WHY LED?

ple of ways. For starters, you don’t need to wait for full light if you’re running in and out of a room. But frequent cycling also degrades the life of CFLs. LED lights don’t attract bugs. The reason that bugs don’t fly to-ward LEDs is because bugs are attracted to ultraviolet light and at least some LEDs don’t give off this type of light. But that’s not universally true for all types of LEDs, according to people who have commented online. In one discussion, white LEDs do not create ultraviolet radiation like a fluorescent light bulb, and most residential LED bulbs give off almost no UV light.