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Explainer: What is a laser

A lesson on the brightest light source on Earth

Digital video discs (DVDs), laser movie discs and compact discs used to store music and data all rely on a laser, like the one shown here, to read the zeros and ones that encode the material stored on these discs. You can’t actually “see” those zeros and ones, so an artist has graphically superimposed some onto the top of the disc to suggest what the laser is “reading.” 

kstimage/iStockphoto

Lasers are the brightest source of light on Earth. They also produce the purest form of color possible. And they can be focused down to the tiniest spot possible. These qualities make them useful for a seemingly endless list of applications.

Theodore Maiman at the Hughes Research Laboratories in Malibu, Calif., created the first laser in 1960. He used a powerful flash bulb wrapped around a short ruby rod about as long as your finger. When the flash bulb fired, it excited atoms in the ruby. Mirrors on the ends of the rod reflected light through the ruby crystal. When some of the light leaked through one of the mirrors, it left as an intense burst of red light. Thus the first laser was born.

The word “laser” is an acronym. Its letters stand for light amplification by stimulated emission of radiation. Lasers therefore produce an intense or “amplified” pulse of light. This pulse results when atoms are stimulated, or excited, by light but then fall down into a lower energy level and emit energy. Atoms can remain excited for only about one-millionth of a second. When atoms return to their normal, non-excited states, they produce photons. Photons are the basic units of light.

Visible light, such as sunlight, consists of many different colors. In contrast, laser light consists of just one pure color.

Light travels in waves. These have peaks and valleys, just like the waves of the ocean. Light waves from sunlight or a flashlight scatter in different directions. The wavelengths emitted by a laser don’t. They flow in perfect formation, like the rows in an amazingly precise marching band. Because waves of laser light move together so precisely, beams of this light can be focused into a remarkably tiny area — one much smaller than a pinhead.

Digital video discs, or DVDs, contain digital messages written by lasers. Those messages are decoded by lasers inside of DVD players. A laser at the grocery store checkout line reads the bar code on your box of cereal. Lasers can weld and shape metal. Lasers play a role in manufacturing most goods, including every major automobile part — from air bags to cloth seats, brakes, clutch and engine. Doctors use lasers in delicate eye surgeries to improve vision. Bouncing laser light off of the moon can give physicists a precise gauge of the moon’s distance from Earth, to within a few centimeters (couple of inches). Today, half of the total income of the United States (which is known as the gross domestic product, or GDP) depends on the lasers used in manufacturing and in delivering digital information.

In 2010, scientists at the SLAC National Accelerator Lab began some of the first experiments using the world’s first X-ray laser (a device that had been unveiled in September 2009). Since the wavelength of X-rays is similar to the distance between atoms, this laser can take snapshots of very small stuff, such as the bonds between atoms in proteins. (Proteins are strings of molecules that fold into complex structures and perform lots of services, such as breaking down the food we eat and using it to build muscles.) A big initial goal: using X-ray lasers to study how proteins change shape as one chemical bond breaks and another forms.

Further out, scientists envision using lasers to help harness the power of the sun for carbon-free energy. Scientists expect that the next generation of lasers will be 10 to 100 times more powerful than those in use today.

Power Words

DVD  (Digital Video Disc) A thin, circular plate of plastic used to store digital information, including text, music and photos.

laser  Any of several devices that emit highly amplified and coherent radiation of one or more discrete frequencies.

physicist  A scientist who studies physics, or the interactions between matter and energy.

wavelength  The distance between one peak, or crest, of a wave of light, heat or other energy and the next corresponding peak or crest.

X-rays  A type of high-energy radiation that moves in waves. The wavelengths of this type of radiation are exceedingly small — similar to the distance between atoms bound into a molecule of some chemical, such as a protein.

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