What is Laser and how do it work ?

Every time the tube flashes, it "pumps" energy into the ruby crystal. The flashes it makes inject energy into the crystal in the form of photons.

Atoms in the ruby crystal (large green blobs) soak up this energy in a process called absorption.

Atoms absorb energy when their electrons jump to a higher energy level. After a few milliseconds, the electrons return to their original energy level (ground state) by giving off a photon of light (small blue blobs). This is called spontaneous emission.

The photons that atoms give off zoom up and down inside the ruby crystal, traveling at the speed of light.

Every so often, one of these photons stimulates an already excited atom. When this happens, the excited atom gives off a photon and we get our original photon back as well.

This is called stimulated emission. Now one photon of light has produced two, so the light has been amplified (increased in strength).

In other words, "light amplification"(an increase in the amount of light) has been caused by "stimulated emission of radiation" (hence the name "laser", because that's exactly how a laser works!)

A mirror at one end of the laser tube keeps the photons bouncing back and forth inside the crystal.

A partial mirror at the other end of the tube bounces some photons back into the crystal but lets some escape.

The escaping photons form a very concentrated beam of powerful laser light.

If that's as much detail as you want to know about lasers, you can stop reading now or skip further down the page to types of lasers. This section goes over the same points from the box above in a bit more detail, and a little bit more "theoretically."

You'll often read in books that "laser" stands for Light Amplification by Stimulated Emission of Radiation.

That's a complex and confusing mouthful but, if you pull it apart slowly, it's actually a very clear explanation of how lasers make their super-powerful beams of light.

Let's start with the "R" of laser: radiation

The radiation lasers make has nothing to do with dangerous radioactivity, the stuff that makes Geiger counters click, which atoms spew out when they smash together or fall apart.

Lasers make electromagnetic radiation, just like ordinary light, radio waves, X rays, and infrared. Although it's still produced by atoms, they make ("emit") it in a totally different way, when electrons jump up and down inside them.

We can think of electrons in atoms sitting on energy levels, which are a bit like rungs on a ladder.

Normally, electrons sit at the lowest possible level, which is called the atom's ground state.

If you fire in just the right amount of energy, you can shift an electron up a level, onto the next rung of the "ladder." That's called absorption and, in its new state, we say the atom's excited—but it's also unstable.

It very quickly returns to the ground state by giving off the energy it absorbed as a photon (a particle of light).

We call this process spontaneous emission of radiation: the atom is giving off light (emitting radiation) all by itself (spontaneously).

Normally, a typical bunch of atoms would have more electrons in their ground states than their excited states, which is one reason why atoms don't spontaneously give off light.

what if we excited those atoms—pumped them full of energy—so their electrons were in excited states.

In that case, the "population" of excited electrons would be bigger than the "population" in their ground states, so there would be plenty of electrons ready and willing to make photons of light.

We call this situation a population inversion, because the usual state of affairs in the atoms is swapped around (inverted).