As you might have suspected, it's done with mirrors.

Not just mirrors, of course. But the heart of even the most complex, state-of-the-art laser machine is essentially a tube with mirrors at each end. The tube contains a substance -- usually a gas or crystalline solid -- that is stimulated by an electric current.

The energized material in the tube begins to glow and give off light in all directions, much as a fluorescent light does. The light that hits the side of the tube is absorbed, but the portion of light hitting the ends of the tube is reflected by the mirrors, which bounce it back and forth between them in an increasingly dense, pure "cascade" of amplified light.

One of the mirrors is totally reflective, allowing no light to escape. The other is only partially reflective, allowing the now-intense beam of laser light to leave the tube and be used for a variety of purposes -- from cutting skin to lighting the Statue of Liberty's torch.

"Laser" is an acronym, standing for Light Amplification by Stimulated Emission of Radiation.

Laser light differs from ordinary light in three ways: It's monochromatic, having only one wavelength or color. Ordinary light passed through a prism will separate into the colors of the rainbow; laser light won't. Its color, which depends on the wavelength of the substance energized in the laser tube, stays the same. For example, an argon laser is blue-green, and a carbon dioxide laser is invisible. It's coherent, or in phase. All the light waves are perfectly aligned with one another, instead of overlapping. It's collimated -- the waves are parallel, moving in exactly the same direction instead of flaring out like a flashlight beam. A laser beam directed at the moon -- 238,857 miles away -- spreads out only about a few miles by the time it strikes the lunar surface and bounces back to Earth.

Aside from these crucial differences, which give lasers their power and intensity, lasers behave like ordinary light. They travel at the same speed -- about 186,000 miles per second -- and reflect off mirrors.

Einstein laid the theoretical groundwork for the laser in 1917, anticipating what he called "a splendid light." He theorized that an electrically charged atom struck by a photon -- a tiny packet of light -- would be stimulated to emit an identical photon traveling in the same direction. One photon would go in, and two would come out, creating the potential for a chain reaction and, under the right conditions, a stream of the purest light ever seen.

"A laser is basically an amplifier," said Dr. Douglas E. Gaasterland, director of the glaucoma and laser service in the Center for Sight at the Georgetown University Medical Center. "You put in a little bit of light over a long period of time, and you get out a lot in a short period of time."

The original laser, produced in 1960, used synthetic ruby as the source of its light. Today, lasers come in dozens of sizes and varieties, suited to particular uses.

The main types of lasers are named for the source materials in the laser tube: carbon dioxide, argon or a tongue-twister that combines a rare metal called neodymium and a synthetic diamond crystal called yttrium aluminum garnet. For obvious reasons, this last one is commonly known as the Nd:YAG or YAG laser.