the work of certain substances

Number of atoms spontaneous emission light emitted, does not have the phase, polarization, propagation direction of the line is physically called non-coherent light. The parameter of laser pointer  should also be included in the drawing.
Einstein's 1917 theory that: In addition to the spontaneous radiation, high energy level E2 is on the way the particles can also be another transition to a lower level. He pointed out that when the frequency is = (E2-E1) / h of the incident photons, the particles will lead to a certain probability, the rapid transition from level to level E2 E1, while an external photon radiation frequency, phase, polarization the same state and the photon propagation direction, a process called stimulated emission. Can imagine, if a large number of atoms in a high energy level E2, when there is a frequency = (E2-E1) / h of the incident photons, thus stimulating E2 on the stimulated emission of atoms, photons get exactly the same two features, which two-photon excitation E2 level on another atom, but also to produce stimulated emission, obtained the same four characteristics of the photon, which means that the original optical signal to be amplified. This in the stimulated emission was generated in the process is the laser light amplification. 1917 Einstein proposed stimulated emission, the laser has come out in 1960, separated by 43 years, why? The main reason is, ordinary light is produced by stimulated emission of particles in a very small probability. This kind of laser pointer is one of the most welcomed laser pointer .
When the frequency of light launched into the work of certain substances, stimulated emission and stimulated absorption of two processes exist, the number of photons by stimulated emission of radiation to increase the number of stimulated photon absorption while filling decreases. Substances in thermal equilibrium, the particle distribution on each level, follow the equilibrium statistical distribution of particles under the law. By statistical distribution of the lower energy level E1 in the number of particles at the higher level must be greater than the number of particles of E2.