A third type of photon-related electron transitions in an atom is stimulated emission. Suppose an electron is in a higher energy level and a photon comes along with an energy equal to the difference between the electron's energy and a lower energy. What will happen is that the photon will stimulate the electron to fall into the lower energy state, thereby emitting a photon. This is pictured below.
The emitted photon will have the same energy as the original photon, and viewed as waves we will then have two waves emerging from the atom in phase with the same frequency. Such waves will constructively interfere, leading to a more intense wave. This is the principle behind the laser, which stands for Light Amplification by Stimulated Emission of Radiation. In a laser atoms are kept in an excited state by ``pumping'' the laser, and some photons are inserted. This causes some atoms to undergo stimulated emission, and the resulting photons cause other atoms to undergo stimulated emission, leading to a chain reaction. The resultant light is very intense and coherent (composed of one frequency), and can be easily focused. Originally viewed as a curiosity, lasers now are found in a wide variety of applications, including surgery, precision cutting tools, CDROM readers, supermarket checkout stand scanners, and holograms.
