摘要： Lasers are used in many types of surgery and the examinations will test both your understanding of their underlying principles and aspects related to laser safety. LASER is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. The definition of the word laser also gives a very succinct description of its mode of action. A lasing medium describes the type of laser, for example a ‘CO2’ laser contains CO2 as the lasing medium. The medium is pulsed with energy from an external source, typically an intense light or electrical discharge. The application of external energy raises the electrons in the lasing medium to a higher orbit or energy state around their host atom. When the pulsed external energy is off, the excited electrons return to a lower energy orbit and, in doing so, release photons (light). The wavelength of this light is dependent upon the nature of the lasing medium and the energy difference between the high energy and low energy orbits. Photons travel in random directions although some will travel parallel to the walls of the container that holds the lasing medium. This container has mirrors at both ends that reflect the photons back and forth through the medium. As each photon hits an electron in the excited state it stimulates it to release its own photon and, in doing so, to return to a lower energy orbit. The phenomenon of this stimulated release of photons is that they share the same wavelength and phase as the photon causing the release. The mirror at one end of the container is partially mirrored in that it reflects a proportion of the photons that hit it but also allows a proportion of them out of the container as laser light. Three important properties of laser light produced by this process are that it is monochromatic, collimated and coherent.