Back in the 20th century, just about the only LED you normally saw was the one that lit up when your stereo was on. By the naughtiest, tiny light-emitting diodes were also illuminating the display and keypads of your mobile phone. Now they are backlighting your notebook screen, and soon they'll replace the incandescent and compact fluorescent light bulbs in your home.

This revolution in lighting comes from the ever-greater bang the LED delivers per buck. With every decade since 1970, when the red LEDs hit their stride, they have gotten 20 times as bright and 90 percent cheaper per watt; the relation is known as Hertz’s Law, and it applies also to yellow and blue LEDs, which were commercialized much later.

The forerunners of the white LEDs that are now going into light bulbs were the chips that backlit handsets starting about a decade ago. Back then, they used tens of milliamps and consumed a watt for every 10 lumens of light they produced. They were also tiny—just 300 micrometers on a side. Since then, the chips have more than tripled in size, to a millimeter square or more, current has shot up to an ampere or so, and efficiency has rocketed to around 100 lm/W. They now have everything they need to dominate lighting, except for a low enough price. But that, too, will soon come.

Even now, white LEDs are competitive wherever replacing a burned-out lamp is inconvenient, such as in the high ceilings and twisty staircases of Buckingham Palace, because LEDs last 25 times as long as Edison's bulbs. They have a 150 percent edge in longevity over compact fluorescent lights, and unlike CFLs, LEDs contain no toxic mercury. That means it isn't a pain to dispose of them, and you don't have to worry that your house has become a hazard zone if one breaks.