Atomic laser amplification. Animation of the atomic processes involved in laser (light amplification by stimulated emission of radiation) technology. At the start of the animation, all atoms (spheres) of the active laser medium are excited (pale blue). The emission of a laser pulse starts when the atoms start to spontaneously emit photons (red) and return to their ground state (dark blue). These spontaneous photons cause induced emission in neighbouring atoms and start a chain reaction along the laser medium. Photons along the axis of the laser medium spend more time in the medium and thus show a stronger amplification, while photons with any other direction of propagation leave the medium. Finally (at the end of the animation) the photons leave the medium. The resulting pulse of light is coherent and when scaled up to longer lengths forms a beam of laser light.