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The electron in an atom revolve in specific orbits and are associated with specific quanta of energy. An atomic electron is described by its energy state or level which ordinarily prefers lower energy and said to be in the ground state. If external energy is supplied, the electrons are excited to higher energy levels which are discrete. The electrons spend a very short amount of time typically of the order of 10 ns in the excited state. Their return to the ground state is accompanied by the emission of energy in the form of electromagnetic radiation, sometimes in the visible region. Ordinarily the process of emission or abosorption is random and emitted energy will be composed of many frequencies.
In lasers, the de-excitation is allowed from only one energy level that have the ability to retain electrons for more than 10 ns and known as metastable state. The electrons are excited into the metastable state by using an external energy source through a process known as pumping. When the number of electrons in the metastable state is greater than those in ground state, the condition is known as population inversion. An external stimulus will make all the electrons from metastable state to come to the ground state. The outcomming radiation is trapped in an optical cavity and amplified and allowed to emergy only after the required amplification is achieved. The whole process is known as light amplification by stimulated emission of radiation (LASER).
The resulting radiation is monochromatic and coherent.