Energy-saving lamps
Last updated: March 1, 2008.
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?
Hot light (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.
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.
Cool Light (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.
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
transformers, that boost the
voltage of the incoming electricity. 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.)
The numbers on this photo correspond to the numbers in the diagram up above.
