Now here’s a bright idea—a lamp that saves you money and helps the environment! It lasts 10 times longer than a standard electric lamp and uses 80 percent less energy. If you care about tackling global warming, lamps like this are a great place to start. During its lifetime, a typical energy-saving lamp will stop about one ton of carbon dioxide from entering the atmosphere and pay for itself many times over. So it’s good for your pocket and kind to the Earth as well. But how exactly does it work?
How to make light the hot way (incandescence)
To understand what’s so good about energy-saving lamps, we first need to understand what’s so bad about ordinary ones.
Most lamps are incandescent. This means they give off light because they are hot. A typical electric light bulb is a glass globe with a very thin piece of wire inside it. The thin wire, called a filament, gets extremely hot when electricity flows through it. Now, hot things often give off light. Fires, for example, look red, orange, yellow, or white because they are hot. Put an iron bar in a fire and it will glow red when the temperature reaches about 950°C (1750°F); this is what we mean by “red hot.” If the temperature rises to about 1100°C (2000°F), the bar glow yellows. If it gets hotter still, say about 2500°C (4500°F) it will glow with a bright, white light. The filament in a lightbulb looks white because it is glowing white hot.
Hot iron looks red, yellow, or white because it is giving off light—but why should it give off light at all? When you heat iron, the atoms inside it absorb the heat energy you supply. The electrons inside the atoms push out farther from the nucleus to soak up this extra energy. But this makes them unstable, so they quickly return to their original or “ground” state. When they do so, they have to get rid of some energy and do so by giving off a tiny packet of light called a photon. Depending on how much energy they get rid of, the photon appears as light of a particular color. See our article on light for a fuller explanation of how atoms make light.
Photo: This incandescent lamp makes light when the filament gets white hot.
You might think heating up a bit of wire is a pretty inefficient way to make light—and you’d be right. A fire, a hot iron bar, and the wire filament in a lamp all give off light, but they also give off heat. If making light is our only objective, any heat we make is wasted energy. If you’ve ever put your hand near a typical incandescent lamp, you’ll know it gets incredibly hot—far too hot to touch, so don’t try it! In fact, an incandescent bulb wastes about 90 percent of the electricity it uses by getting hot.
Find out more in our article on incandescent lamps.
How to make light the cool way (fluorescence)
Energy-saving lights save energy by making light without the heat using a completely different process called fluorescence. This is a trick similar to the one used by creatures like fireflies and glow-worms, whose bodies contain chemicals that make “cool light” without any heat. The general name for light made this way is luminescence.
From the outside, a fluorescent lamp seems to have two main sections: a squarish base out of which two or more white, glass tubes emerge. Inside, things are a bit more complex. The base is the bit that plugs into the power socket (1). Inside it, there’s a small electronic circuit (2), containing atransformer, that boosts the voltage of the incoming electricity. (You can see a photo of the circuit below.) This means the lamp can produce more light than it would otherwise do and also helps to reduce flicker. The circuit is connected to a couple of electrical contacts called electrodes (3). When electricity flows into the electrodes, electrons (shown here as red dots) boil” from their surface and shoot off down the thin white tubes, which contain mercury gas (4). As the electrons hurtle down the tubes, they collide with atoms of the mercury (5), shown here as blue dots. The collisions give the mercury atoms energy so their electrons jump to higher energy levels. But this makes the mercury atoms unstable, so the electrons quickly return to their ground states. When they do so, they give off photons of invisible ultraviolet light (slightly higher frequency than the blue light we can see).
Photo: The electronic circuit inside an energy-saving lamp. The transformer is the big orange/gold thing in the center. The black cylinder on the left is a capacitor.
If fluorescent lights make invisible light, how come they glow white? Here’s the clever part. The thin glass tubes of a fluorescent light are covered in white-colored chemicals called phosphors. When the ultraviolet light strikes atoms in the phosphors, it excites their electrons in just the same way that the mercury atoms were excited (6). This makes the phosphor atoms unstable, so they give off their excess energy as photons—which, this time, happen to be visible, white light.
So, in short, fluorescent lights make their energy in a three-step process:
- Electrodes take electrical energy from the power supply and generate moving electrons.
- The moving electrons collide with mercury atoms in the tubes to make ultraviolet light.
- The white phosphor coating of the tubes converts the ultraviolet light into visible light (that we can see).
In case you’re wondering, this is what a compact fluorescent light looks like inside. (Don’t break one apart yourself; there is some health risk from the mercury inside if you do so.) Sorry the photo is a bit blurred. Next time one of my lamps breaks, I’ll take a better photo!
Photo: Inside a compact fluorescent lamp. The numbers on this photo correspond to the numbers in the artwork up above: 1) Connection to power socket; 2) Transformer circuit; 3) Electrodes; 4-6) Glass tubes with white phosphor coating inside.