Light Amplification by Stimulated Emission of Radiation.

Laser is one of the applications where quantum mechanics plays a fundamental role. Assume there is an atom with one electron which has two possible orbits. How can this atom interact with light? According to quantum mechanics, the atom can "see" light of only one particular frequency (determined by the Planck's principle E= hf). Light of any frequency other than this particular one shall be discarded and ignored by the atom. Three kinds of interaction can take place between the atom and this particular monochrome light:

1. If the atom is in its lower energy state it can absorb the light and be excited to its upper state.
2. If the atom is in its upper energy state, it can fall spontaneously to its lower state, and emit light in the process.
3. The third possibility is when the atom is stimulated buy the presence of light to jump down to its lower state, emitting additional light while doing so (Laser). In stimulated emissions, the additional light emitted has the same frequency and directional characteristics as the light it stimulates. This is the crucial feature on which the properties of the laser are based. In order for the laser to work effectively, stimulated emission must predominate over both absorption and spontaneous emission.