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.” 


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

Theodore Maiman created the first laser in 1960. At the time, this physicist worked at the Hughes Research Laboratories in Malibu, Calif. Maiman wrapped a powerful flashbulb around a short rod that was about as long as your finger. That rod was made of ruby. When the flashbulb fired, it excited atoms in the gem. Mirrors on the ends of the rod reflected light through the 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. Later, when electrons in those atoms fall down into a lower energy level, they’ll emit the excess energy. Atoms can remain excited for only about a millionth of a second. When atoms return to their normal, non-excited states, they produce

. 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. 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. Lasers inside of DVD players later decode those messages. 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 products. 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 in some way on the lasers, especially for 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, such a 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 low–cost energy. Scientists expect that the next generation of lasers will be 10 to 100 times more powerful than those in use today.

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