There are many different types of lasers. Most of them work in a similar way. There is a tube inside that has a material or a gas inside. They put a bit of electricity in there which causes the the atoms inside to become excited. Excited means that they have more energy than they usually do. When the atoms return to normal, they each send out a photon. A photon is a little bit of light.
Here's where it gets a bit tricky. These photons bounce around inside the tube and cause other atoms lose their energy by giving off more photons. What's special about a laser is that the new photon waves follow the same wave pattern as the old ones. Pretty soon, there are a lot of photons inside the tube, all in the same wave pattern.
On one end of the tube, there is a mirror to reflect the photons back. On the other side, there is a special mirror that will let a little bit of light out and reflect the rest back. The light that gets out is special because it is all pointing in almost the same direction. Even very far away, like on the moon, the laser beam still forms a pretty small dot. The light from a flashlight spreads out much more.
Another important thing to know is that you can't see light unless it bounces off of something. You can see the dot that a laser makes but not the beam of light that is moving towards that dot. The reason you can sometimes see the beam of light is that if there is something in the air, like fog or smoke, the light can bounce off it. Then we can see the light well.
(republished on 07/28/06)