How Do Lasers Work? | Who Invented The Laser? - Explain That Stuff

Lasers

by Chris Woodford. Last updated: November 28, 2024.

Lasers are amazing light beams powerful enough to zoom miles into the sky or cut through lumps of metal. Although they seem pretty recent inventions, they've actually been with us over half a century: the theory was figured out in 1958; the first practical laser was built in 1960. At that time, lasers were thrilling examples of cutting-edge science: secret agent 007, James Bond, was almost chopped in half by a laser beam in the 1964 film Goldfinger. But apart from Bond villains, no-one else had any idea what to do with lasers; famously, they were described as "a solution looking for a problem." [1] Today, we all have lasers in our homes (in CD and DVD players), in our offices (in laser printers), and in the stores where we shop (in barcode scanners). Our clothes are cut with lasers, we fix our eyesight with them, and we send and receive emails over the Internet with signals that lasers fire down fiber-optic cables. Whether we realize it or not, all of us use lasers all day long, but how many of us really understand what they are or how they work?

The basic idea of a laser is simple. It's a tube that concentrates light over and over again until it emerges in a really powerful beam. But how does this happen, exactly? What's going on inside a laser? Let's take a closer look!

Photo: A scientific experiment to check the alignment of optical equipment using laser beams, carried out at the US Navy's Naval Surface Warfare Center (NSWC). Photo by Greg Vojtko courtesy of US Navy and Wikimedia Commons.

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Contents

1. What is a laser?
2. How lasers work
3. How do lasers make light?
   • Spontaneous emission
   • Stimulated emission
   • What makes laser light so different?
4. Types of lasers
5. What are lasers used for?
   • Tools
   • Communications
   • Defense
6. Who invented lasers?
7. Find out more

What is a laser?

Lasers are more than just powerful flashlights.

The difference between ordinary light and laser light is like the difference between ripples in your bathtub and huge waves on the sea. You've probably noticed that if you move your hands back and forth in the bathtub you can make quite strong waves. If you keep moving your hands in step with the waves you make, the waves get bigger and bigger. Imagine doing this a few million times in the open ocean. Before long, you'd have mountainous waves towering over your head! A laser does something similar with light waves. It starts off with weak light and keeps adding more and more energy so the light waves become ever more concentrated.

Photo: It's much easier to make laser beams follow precise paths than ordinary light beams, as in this experiment to develop better solar cells. Picture by Warren Gretz courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

If you've even seen a laser in a science lab, you'll have noticed two very important differences straightaway:

• Where a flashlight produces "white" light (a mixture of all different colors, made by light waves of all different frequencies), a laser makes what's called monochromatic light (of a single, very precise frequency and color—often bright red or green or an invisible "color" such as infrared or ultraviolet).
• Where a flashlight beam spreads out through a lens into a short and fairly fuzzy cone, a laser shoots a much tighter, narrower beam over a much longer distance (we say it's highly collimated).

There's a third important difference you won't have noticed:

• Where the light waves in a flashlight beam are all jumbled up (with the crests of some beams mixed with the troughs of others), the waves in laser light are exactly in step: the crest of every wave is lined up with the crest of every other wave. We say laser light is coherent. Think of a flashlight beam as a crowd of commuters, pushing and shoving, jostling their way down the platform of a railroad station; by comparison, a laser beam is like a parade of soldiers all marching precisely in step.

These three things make lasers precise, powerful, and amazingly useful beams of energy.