The word “Laser” is an acronym—it stands for “light amplification by stimulated emission of radiation.”  Here’s how a laser works.

A simple ruby laser consists of a ruby rod, a flash bulb similar to that on a camera, and two mirrors—one on each end of the ruby rod.  One mirror is “fully silvered,” meaning it reflects all the light that hits it.  The other is “half silvered,” meaning it lets some light through and reflects the rest.

Atoms consist of a proton-neutron core orbited by electrons.  If you inject some energy into an atom—by exposing it to heat, light, or electricity—you “excite” the electrons, causing them to move to more “excited” orbits farther away from the nucleus.

Electrons can’t stay in an excited state forever.  Eventually they relax and move back to their original orbits, closer to the nucleus.  When they do this, however, they emit a photon—a single particle of light.

The ruby rod in a simple laser contains atoms to excite.  The flash bulb exposes them to light, forcing their electrons into higher orbits.  When the electrons “relax,” they release photons.  Some of the photons escape, but others are released in a horizontal direction, and they bounce back and forth between the mirrors.

When the photons bounce back and forth, they knock into the atoms in the ruby rod, exciting them and forcing them to release more photons.  Some of these photons escape through the half-mirrored side; that’s your laser beam.

Most light is multi-colored, but laser light is completely monochromatic.  The light is “organized”—the photons are released in lock step, moving up and down in a synchronized wave motion.  It’s also extremely focused—it releases in just one direction, as opposed to a flashlight or light bulb, which releases light in diffuse directions.

Lasers are useful in many different technologies—including reading CD’s and DVD’s.  Without them, we would not have recording technology as we know it today.