n of light. input output Amplifying Medium Figure 4.8 All lasers consist of an energized substance that increases or amplifies the intensity of light passing through it. That substance can be a solid, a liquid, or a gas. Light Sources and Detectors SPONTANEOUS EMISSION The top portion of Figure 4.9 is a simplified view of the change in the state of an electron. The process in which an electron falls to the lower energy level is referred to as absorption. As the electron is absorbed, it spontaneously emits a photon of light. Thus, the movement of electrons is also referred to as absorption with spontaneous emission. The actual absorption process results in the energy of the photon being equal to the difference between the two energy levels, which in our simplified example are levels 0 and 1. Because photons have different energy levels specified in electronvolts, which correspond to their wave- length, this also means that only photons of a particular frequency (or wavelength) will be absorbed. Because the best result that can be achieved under spontaneous emission is for one photon to be generated for each photon that is absorbed, this process does not result in the amplification of light. STIMULATED EMISSION Instead of spontaneous emission, a laser must perform stimulated emission. To do SO requires a photon of light to interact with an atom in an excited state, stimulating the return of the electron to the lower state and causing two photons to be emitted. A a. Absorption and spontaneous emission. Figure 4.9 Spontaneous versus = electron stimulated emission. Energy State photon of light A photon absorbed by an atom is initially in a low energy state (0). As the energy of the electron is raised (1) it remains in an excited state for a short period of time and spontaneously returns to the lower state (0) where it emits a photon of light. b. Stimulated emission. two photons in phase If an electron is excited and interacts with a previously excited atom, two photons, each in phase with the other, result in the gene