Incandescent lamps

Last updated: October 13, 2008.

No more candles, no more gas lamps—just imagine how amazing people found the very first practical electric lamps toward the end of the 19th century. Incandescent lamps (ones that make light by making heat) are getting something of a bad press these days because they waste so much energy, but they've longed been considered among the greatest inventions of all time and a burning-bright light bulb is still widely used as the symbol of a great idea. Let's take a look at these marvels of technology and find out how they work.

Photo: An incandescent lamp makes light by passing electricity through a very thin wire filament. The filament gets red or white hot and gives off light (as well as lots of heat). You can see the filament clearly as a bright white stripe in this short exposure photograph.

Why hot things give off light

Set fire to a big bunch of logs and you'll get a nice red glow as well as a warm feeling. People have known that hot things give off light ever since the discovery of fire, somewhere between one and two million years ago. But just why do hot things give off light? When things burn, what's actually happening is a chemical reaction called combustion in which a fuel (such as the wood in the logs) reacts with oxygen in the air to produce carbon dioxide, water (in the form of steam), and a great deal of energy. Some of that energy is heat, some is light... and there's even a bit of sound energy produced too (in the crackling and hissing of the logs). Hot things give off light when the atoms they're made of gain energy and become excited. That makes them unstable and, to become stable again, they give off the energy they gained as particles of light called photons. (Read more and see a diagram of this in our main article on light.)

Candles used to be our main way of making light from heat. A candle is a mini-chemical factory that produces a continuous flame by slowly converting the energy stored in its oily wax into heat and light. A basic law of physics called the conservation of energy tells us exactly why candles always burn out eventually: all the energy we need to make continuous candlelight has to come from the wax, which must slowly burn away. Now just imagine if you could make a candle that never burned out—one that never needed replacing. You'd need a flame that never died and an endless supply of energy. And that's pretty much what you have in an incandescent electric light.

How incandescent lights work

Why do incandescent lamps glow when electricity flow through them? Electricity flows better through some materials than others. Metals that let electricity flow easily are good conductors that have low electrical resistance; plastics, wood, and other insulators have a high resistance. Some metals are better conductors than others: silver is better than gold, gold is better than copper, and copper is better than aluminum. Not all conductors are metals, however. Carbon is a good conductor and it has little in common with most metals.

Take a piece of a conducting material and you can make electricity flow through it a little bit better by doing two things: first, by making it shorter (the longer your conductor, the more work electricity has to do to get through it); second, by making it thicker (the fatter the conductor, the easier it is for the electric current to flow). Now suppose you could make a conductor that's both short and thin and pass electricity through it. Fashion it just right and it'll have a really high resistance. Switch on the electric current and your conductor (which is usually called a filament) will heat up. Use enough electricity and the filament will heat up so much that it'll glow red or white hot and give off light. That's the basic idea behind the incandescent electric light.

Photo: A modern, electric incandescent lamp (left) and a close-up of the filament (right). The filament is a length of tightly coiled tungsten metal stretched between two terminals that let the current flow through it.

The only trouble is that an incandescent lamp has to produce an incredible amount of heat to make a decent amount of light. Roughly 90-95 percent of the electricity you feed into a lamp like this is wasted as heat. That's why people are now so keen on switching away from incandescent technology to energy-saving fluorescent lamps (sometimes known as compact fluorescent lamps or CFLs), which last several times longer and save roughly 80 percent of the energy.