Revered Members, An electron in ground state makes its way to an excited state upon absorption of photon of energy, equivalent to energy difference between ground state and excited state, and after some time, it decays by emitting the photon and returns to the ground state. This is called spontaneous decay. For Laser action, population inversion and stimulated emission should occur. Now let me explain the scenario 1) An atom(electron) in the energy level E1 absorbs a photon and goes to a state of higher energy say E3 2) While decaying from E3 to E1, it reaches a metastable state E2. Now, due to longevity of the stay in E2 than in E3, we can achieve population inversion in E2 My question is , 1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission? 2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?