Photons and atoms can interact with each other by means of absorption, spontaneous emission and induced emission. Induced emission is most relevant when producing laser light. In fact, it is the basis for the coherence and monochromatic characterics of laser light.

Light can only be amplified when the process of induced emission has a higher probability than absorption. This is achieved by circumventing the thermodynamic equilibrium which forbids this probability. The method called pumping makes population inversion possible.

A resonator is used in bringing the emitted waves (photons) to interfere constructively with one another. A laser can work continuously when a part of this resonance is extracted.

The HeNe laser used in this tutorial produces polarised light owing to the Brewster windows in the resonator.