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Quantum Physics :- Lasers. The process of stimulated absorption and emission of electromagnetic energy in quantized systems can be regarded as analogous to the stimulated absorption or emission of mechanical energy in classical resonating systems upon which a periodic mechanical force of the same frequency as the natural frequency of system is impressed.

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I don't understand this analogy. How photons from external source can be related to externally supplied mech. energy ?
Also, in stimulated emission they say - the atom already in higher energy state absorbs the photon and decays to lower energy level, shouldn't it advance to a another higher energy level or in case photon doesn't posses energy enough to excite atom to higher state it should remain in same level ? Why does it decays to lower level ?
atom in higher state doesn't "absorb" photon.. the atoms are in meta-stable state and they need to be stimulated to fall to ground level.. so external photon just does the work of stimulating it.. both photons (newly released+ the one from external source) are then released together in phase..

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@ujjwal Can you please explain in detail ? Can you draw analogy b/w stimulated emission and classical resonating systems, as mentioned above. BTW Thanks !!
As explained above, when the atom is already in the excited state, it doesn't absorb more energy and try to go to even a higher state.. An atom in the excited state is usually unstable and it quickly de-excites releasing a photon of energy (which is equal to the difference in the two excited and the de-excited state energy levels).. This is termed as SPONTANEOUS emission. However, if even before it is READY to go down to the de-excited state, is struck with a photon which has the same energy as the difference in the two states, that photon will now FORCE the atom to de-excite and release a similar photon of energy which will move PARALLEL to the incident photon direction. This is termed as STIMULATED emission

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