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Stimulated emission :-
because there was only enough energy from the incident photons in the first place to raise the electrons to that new energy level and not higher. The electrons fall back down immediately, and emit a photon of the corresponding energy difference.
Do you mean E3 - E2 > h.f and hence atom can't reach E3 ?
So, why doesn't it just comes back to E2 again ?
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I mean it rises above E2 because it has more energy than E2 i.e ( E2+ hf ),
atom can't reach E3 because energy is still less than E3 i.e ( E2 + hf < E3 ) and hence it will come back to E2.
In theory it could transition E2 -> E3 with more photon energy, but my understanding is that the electron falls back down immediately to the ground state (E1) and there are so many available ground state electrons that they are nearly always those that are excited.
Now it's an interesting question why there couldn't be electrons that do get raised to the higher levels. It seems to me incorrect that we should expect that never to happen. But at least intuitively it makes sense that this isn't the predominant behavior.
If someone has a very sharp quantum mechanical answer, I'd be fascinated to hear it.
E3-E2 > h.nu.. so electron from E2 doesn't make transition at all to higher energy level.. its not like the electron makes a transition but then can't reach E3 (due to insufficient energy)..that is not the case.. the electron doesn't make transition to higher orbit unless there is sufficient energy greater than the energy gap (or atleast exactly equal to it)..
so when a photon strikes an electron, it makes it fall down since it can't take it to higher energy level.. as simple as that..
I think I'm relating it to macro-scopic world, but why can't we ?
E.g. Ball-earth system, more the K.E the ball possess higher the ball rises. I can't get this line - "the electron doesn't make transition to higher orbit unless there is sufficient energy greater than the energy gap (or atleast exactly equal to it).. "
I know that quantum physics deals with discrete energy systems, but this is so non-intuitive.
@shubham buddy, there is difference between quantum and classical mechanics... you can't think in classical way in this case.. and everything hasn't got an analogy.. u can't think it to be analogous to 'earth ball system'.. they are totally different.. we don't have energy levels around earth for the balls..