L.A.S.E.R.

U.S. Army Research Laboratory, 2013

For those who don't know, "laser" is an abbreviation for light amplification by stimulated emission of radiation. That really does sum things up nicely, but a little more explanation might be in order.

First of all, whenever an atom drops from a high energy state to a lower, the atom emits a photon of energy equal to the difference in energy states. This type of thing occurs spontaneously in nature all the time, but a laser works by forcing this kind of reaction.

To stimulate this photon emission, the gas inside the laser cavity is energized from an external source, such as an electrical current from a battery. For example, a mixture of helium and neon gets zapped, which causes the atoms to move around rapidly. The idea is that the twitchy helium atoms would bump into the neon atoms and cause them to jump to a high-energy state. Within moments, one of the neon atoms will naturally drop to a lower state and emit a photon.

When one of these photons bumps into another energized atom, it somewhat counter-intuitively causes the atom to drop to a lower energy level (the book uses the term "sympathetic vibration", but I'm not too clear on how that works right now) and release a photon with the exact same properties as the one that struck it; one photon becomes two. These reactions snowball into more photon emissions as the new photons react with other energized atoms. In a very short time, a lot of photons are bouncing around with the exact same properties as all the others.

All of this is occurring within a cylinder with carefully aligned mirrors on each end. The photons bounce back and forth between these mirrors until they gradually line up and fit through the hole at one of the ends, all at the same wavelength (colour), moving in parallel, and in the same direction. Voilà! A laser beam.

"Physics for the Rest of Us" - Roger S. Jones