The word laser is actually an acronym, and is derived from the initial letters of a phrase that describes the effect that the instrument is designed to produce, namely Light Amplifica¬tion by Stimulated Emission of Radiation.

An ordinary laser consists of three components. The first is a lasing medium usually a crystal or a gas. In this case it is a synthetic ruby, whose atoms can be energized (‘excited’) in a way that defines the properties of the emitted light. The second is a pump mechanism, such as a flash tube as you can see here, which provides the energy required for this excitation. And the third is an optical resonator, consisting of two mirrors between which the input light is repeatedly reflected.

Atoms are made up of a heavy central nucleus, which is surrounded by much lighter and more agile electrons. In a simplified picture, the electrons can be thought of as following orbital paths at different distances from the nucleus. Each orbital defines one specific energy level. Electrons are generally in the ‘ground state’, the lowest energy state available. In an excited atom, however, one or more electrons have been raised to a higher energy level. The energy required to do so is supplied by a pumping process – for example, atoms can absorb light quanta - so called ‘photons’. After some time, the excited electron will re-emit that energy in the form of light of a defined wavelength. This phenomenon is known as spontaneous emission.

But if such a photon with the matching energy interacts with an already excited atom, it can cause that atom to relax and emit its excitation energy in form of a second photon, by a process called stimulated or induced emission. This second photon is an exact copy of the one that induced its emission. Both photons have the same wavelength and propagate in the same direction and with the same phase.

In a laser, this whole process takes place in a cavity defined by two mirrors between which the photons are bounced back and forth. As a result, more and more identical photons interact with already excited atoms, such that the system self-amplifies until an equilibrium is reached. Since one of the mirrors is less than 100% reflec¬tive, the transmitted portion emerges from the resonator as a narrow beam of intense and coherent laser light.