the consensus on the direction of propagation

This radiation process is called spontaneous emission. Many atomic spontaneous radiation emitted light does not have a phase, polarization state, the consensus on the direction of propagation, is mentioned in the physically non-coherent light. In 1917, Einstein's theory that: In addition to spontaneous emission of particles in high energy level E2 can also be another way to transition to a lower level.  Laser Pointer  is specially manufactured for the school.
He pointed out that when the frequency ν = (E2-E1) / h photon incident will give rise to particles with a certain probability. Quickly from level E2 transitions to the level of E1, while the radiation with external photon frequency, phase, polarization and propagation direction are the same photon, a process called stimulated emission. Can be assumed that if a large number of atoms in a high energy level E2 a frequency ν is = (E2-E1) / h of the photon incident, which inspired the E2 atoms generated by stimulated emission of radiation, the two features in exactly the same photon two photons to motivate the E2 level on the atom, to produce stimulated emission, can get the four characteristics of the photon, which means that the original optical signal is amplified. This process of stimulated emission and amplification of light is a laser.  Laser Pointer  can be easily bought in the market. The package weight of Laser Pointer is a part of the gross weight
Einstein's 1917 proposed by stimulated emission of radiation, lasers come out in 1960, separated by 43 years, why? The main reason is that ordinary light particles generated by a very small probability of stimulated emission of radiation. When the frequency of the light launched into the working substance, stimulated emission and the shock absorption of the two processes at the same time, subject to stimulated emission increase in the number of photons, while filling the number of photons by stimulated absorption decreases. Substances in thermal equilibrium, the distribution of particles on each level, follow the equilibrium statistical distribution of the particles. Laws of statistical distribution of the number of particles in the lower level of E1 must be greater than the number of particles in the higher level of E2.