LED By the Nose

Bad Astronomy
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June 21 2005 6:12 PM

LED By the Nose

There's a road not far from my house that cuts across a lot of farmland. I drive on it to get to my boss's house, or when I go to visit the wine country in Sonoma or Napa valley. It's a beautiful drive, with lots of rolling hills, vineyards, and cows.

At the crest of a hill is a crossroad that leads into a residential neighborhood. To give the folks who live there a chance to get out, the cars on the main road have to stop at the hill top. A stop sign isn't very useful there, because the speed limit is 60 mph on the big road, and you'd never see it in time. A stop light might help, but they're expensive, and the small amount of traffic from the neighborhood doesn't warrant it.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


So the engineers put in a flashing red light, which can be seen for hundreds of meters away. On the drive, I can always spot it from quite a distance, blinking on and off about once every second. As I mentioned, the drive is quite nice in that area, which is conducive to long, unbroken chains of thought. A few months ago I was watching the light blinking as I approached it, and I suddenly realized that if the light blinks once per second, it must flash 31 million times per year!

I was amazed. What kind of light bulb could take that kind of stress? A normal bulb would explode after a short time. Ever notice how bulbs always seem to burn out when you flip the switch to turn them on? That's because the current flowing into them is like a flood, and it stresses the filament. A nice constant flow of electricity doesn't put much stress on the filament, but a sudden torrent of it does.

So how does the red light at that hilltop last for so long? One day, I happened to mention this story to my officemate, and he looked at me like I was an idiot. "It's an LED," he said.

Aha! Light Emitting Diodes answered my question. You've seen LEDs: they're behind the Hulkishly green glowing numerals used in alarm clocks, for example. LEDs are fundamentally different than incandescent bulbs. A regular bulb has a piece of wire which gets hot when you run a current through it. So in a sense it gives off light for the same reason stars do: hot things emit light.

LEDs, though, give off light for a very different reason. Indulge me in an analogy: When you jump down off the last step in a staircase, your feet hit the floor and make a noise. You have just converted gravity to sound! The energy of your descent (which comes from gravity) is used to move air in waves (which makes sound). The same thing happens to electrons, too. When they move in an electric potential (in a sense, the electromagnetic equivalent of gravity) they also convert their energy gained into waves, but in this case those waves are light.

In an LED, electrons move when a current is applied. They respond by giving off light. There are a number of advantages here: for one thing, there is no filament. Electrons are happy to move back and forth zillions of times without worry. So LEDs last a long, long time. It's not hard to make a lot of light using LEDs, either. I have a pen with an LED in one end, and it's so bright it's hard to look at, yet it only uses two little batteries. The color can be controlled as well, so you can have red, green, and blue LEDs.

That's why the stop light at that hilltop uses LEDs. They're bright, so I can see the light from a long way off. They can be made red, which is rather useful for a stop light. It can blink on and off literally millions of times a year without malfunction. And they can last for many years, so you don't have to replace them very often.

And it gets better. You can combine red, green, and blue LEDs to make white light. The light generated this way is bright, cheap (LEDs are tremendously more efficient than not only incandescent bulbs but also compact fluorescents), and last virtually forever. I think they may soon replace light bulbs. Other people do too. This could easily save billions of dollars in energy costs, which is particularly useful these days.

I love stuff like this. It's so cool! And it's another reminder to people that understanding science can lead to amazing and unexpected ways of making our lives better.

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