Lasers are everywhere. We use them in medicine, telecommunications, manufacturing, and even in our personal computers (to burn a CD, for example). How exactly do they work, though? Typically whenever we turn on a light of some sort, the photons it emits propagate in all directions, yet with a laser, all the photons flow in a neat, straight, steady stream (making them ideal pointers). How does that happen?
Many people do not realize it, probably because the term has become so commonplace, but the word "LASER" was originally an acronym that stood for Light Amplification by Stimulated Emission of Radiation. Lasers earned their technical-sounding from the process scientists use to create them.
Essentially, light behaves in two ways: like a wave and like a particle. The particle aspect of light, known as a photon, is essentially a tiny bundle of energy. In a laser, electricity sends energy into a sea of excitable atoms, and when the atoms become "excited," their electrons "jump" to a new orbit further from the nucleus. This new orbit is called an energy level, because electrons require an energy transfer to jump from one level to another. In this case, electrons first jump up a level when given energy from a stream or burst of electricity. Afterward, they drop back to their normal energy level, but in order to do so, they must discharge the energy they gained that allowed them to jump in the first place. That discharge occurs in the form of a photon, and when billions of electrons in billions of atoms all emit photons of the same frequency in the same direction at relatively the same time, you have that steady flow of photons we call a laser.
Not all photons have the same energy. Max Planck showed that the energy of a photon is directly proportional to its frequency, and the frequency of a photon determines its color as well. Therefore, lasers that emit low-energy photons will appear red, for example, while high-energy photons will appear blue or purple, and mid-energy photons will fall somewhere in the middle, like yellow or green. Thus, the color of the laser emitted from a particular device depends on the energy of the photons being emitted from the electrons as they fall back to their normal energy levels.
It's pretty amazing how much physics goes into something as simple-looking as a laser.