Dear Dr. Universe: How do lasers work? – Manna, 12, India
Humans use lasers for everything from scanning barcodes and putting on light shows to performing delicate eye surgery and measuring the distances between objects in space.
Cats also like to chase lasers, but I wasn’t sure how they worked. I asked my friend Chris Keane, a physics professor at Washington State University. Keane came to WSU from the National Ignition Facility at Lawrence Livermore National Laboratory where he helped work on a laser as big as a football stadium.
First, we have to know a bit about light. Whether it’s light from our sun or your flashlight, light travels in tiny bundles called photons. It normally radiates out in all directions from its source, like the Sun, for example.
It turns out we can also find light energy stored in the atoms, or building blocks, that make up materials inside a laser pointer. There are different materials we can use in lasers, but some popular ones are gases like neon and helium. You may have seen neon atoms at work in a bright, glowing sign. You may also have filled up a balloon with helium atoms to make it float.
Atoms like these are sometimes really excited and other times they are at rest, or at their ground state. One way we can make some of these atoms really excited is to give them a source of energy, something like really strong flash of light or a jolt of electricity from the battery in a laser pointer.
Keane explained that under just the right conditions, you can get more excited atoms than resting atoms inside the tube of your laser pointer. When scientists were experimenting with different kinds of laser materials, they made excited helium atoms collide with resting neon atoms.
Atoms will normally emit photons when they transition from a particular excited state to a resting state. When there are more excited atoms than resting atoms, the first atom to emit light will trigger a kind of chain reaction and a lot of light will build up inside the pointer.
There are also two mirrors in a laser pointer that help keep our chain reaction going. It’s a different process, but in a way it reminds me of how we plug a guitar into an amplifier to increase its volume. But with lasers, instead of amplifying sound, we amplify light. LASER actually stands for Light Amplification by Stimulated Emission of Radiation.
The opening on one end of the laser is the light’s way out. It doesn’t radiate in all directions, but builds up in one very straight, focused point that we usually see as a bright red dot.
We don’t find lasers in nature. We have to make them in factories or labs. But there are naturally occurring “light amplifiers” in our universe. These are similar to our lasers on earth, except they don’t have any mirrors. We usually find them out in big clouds of gas where there are more excited atoms than resting atoms, which results in some brilliant light.
“Ask Dr. Universe” is a science-education project based out of Washington State University. Send questions to Dr.Universe@wsu.edu.