A laser works as long as it manages to excite a minimum number of electrons in a given material to a higher energy level, in order to obtain a population inversion (when there are more excited electrons than electrons in the ground state). When this occurs, some electrons are stimulated to emit their photons, which will initiate a cascade effect so that the photon emitted by an electron stimulates the next electron to emit another photon of the same wavelength and phase, which will amplifying the emission of light beams of defined and coherent wavelength.

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For all this to work, however, a feedback is necessary, that is, for a certain time to keep emitted photons, in a stimulated way, interacting with other electrons. This is achieved with an optical cavity, a region of space where light is confined for some time by using highly reflective and conveniently aligned mirrors that reflect photons several times. In one of the mirrors there is a small hole through which some photons, after being reflected many times, manage to leave, emitting the collimated beam of light.