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How Do Lasers Work?

Lasers are used everywhere from the dentist's office to science fiction films. But how do they work? Why are laser pointers so different from a regular flashlight? Everyday Einstein beams up.

By
Sabrina Stierwalt, PhD,
Episode #141

Hi I’m Dr. Sabrina Stierwalt and I’m Everyday Einstein bringing you Quick and Dirty Tips to help you make sense of science.

There are so many everyday uses for lasers, from dentistry and tattoo removal to printing and CD players (remember them?). They also pop up frequently in science fiction movies like in the famous light sabers of Star Wars.

However, lasers are no ordinary form of light. They are both monochromatic and coherent, which are special properties that allow us to use them in unique ways that would be impossible with a regular flashlight or a light bulb.

So what are lasers exactly? How do they work? Let’s look at how lasers can help us correct our vision and why I would rather fight someone with a light saber than a flashlight.

The Structure of an Atom

To understand how lasers work, we must first take a look at the atom. Everything we interact with on a daily basis – the chair you sit on, the air you breathe, even our bodies – are all made up of small particles called atoms. As shown in the periodic table of the elements, only about 100 different kinds of atoms exist. Different materials consist of different combinations of these elements.

See also: Atomic Bonds - The Ties That Bind

 

Structure of an atomEach atom contains a nucleus (made up of protons and neutrons) and electrons that are constantly in motion to orbit the nucleus. Atoms have a ground level energy state, one that does not require any additional energy to maintain, where the electrons are orbiting closest to the nucleus. Those electrons can also be bumped or stimulated into higher energy level orbits so that the atom is considered to be in an “excited” energy state.

Thanks to quantum mechanics, we now know that this view of an atom is a bit simplified and that electrons aren’t likely to actually travel in discrete, well-defined orbits. However, such a picture is still a helpful description for relating the physics of small particles like electrons to the laws of physics we experience every day.

To excite an electron into a higher energy state, all that is needed is energy, usually in the form of light or heat.

When all those electrons decide to relax again, they can rejoin their neighbors in the lower energy levels when the atom releases energy in the form of photons or packets of light. The energy difference between the electron’s starting and ending orbits determines the energy of the photon that is released, which in turn sets the wavelength or color of the emitted light.

What Is a Laser and How Does it Work?

You may remember from Grammar Girl that the word “laser” is actually an acronym. It stands for Light Amplification by Stimulated Emission of Radiation. Doesn’t roll quite as easily off the tongue, does it?

The key word in all of that is “stimulated.” This is what sets lasers apart from more ordinary forms of light. When you turn on a normal flashlight, for example, light leaves in all directions and does so at random times. The resulting light is diffuse (or spread out) and relatively weak.

A laser, on the other hand, is “coherent” meaning that all of the photons leave in unison and in one direction. This kind of ordered exit results in a narrower, more intense beam of light.

So how do you get photons to follow such strict marching orders?

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