OK I'm going to try to put this question the best way I can. But as far as I understand it, an atom will emit a photon when a photon of just the right energy is absorbed, thereby raising an electron from a lower energy state to a higher energy state. More specifically, the photon is emitted when the electron drops down an energy level from it's new excited state, to a lower energy level. From say n=4 to n=2 for example. But my question is what causes the electron to drop an energy level and thereby emit the photon? An example of this might be hydrogen whereby the electron dropping from the n=4 to the n=2 level results is a blue spectral line. However if just the right frequency of blue light is being shone on a sample to allow for an emission of exactly that frequency of light from the hydrogen sample, then why doesn't the electron stay in a continually 'excited' state?
What I mean is if work is continually being done on the electron as photons strike it, then why doesn't it just stay continuously in the n=4 energy state? Why would it drop down to a lower energy state an emit a photon of the same frequency as the source of light being directed at it?
The only (probably crude) explanations I can think of is that clearly no system is capable of continuously absorbing energy, without at some point releasing some of that energy. Therefore work done on a system, must equate to work produced by that system at some point or other. The only analogy I can think of is perhaps a sealed copper boiler containing water. If this boiler is continuously heated, this will equate to work done, but as the boiler is sealed there is no way for the energy supplied to escape. Therefore the energy of the boiler increases, until inevitably eventually there is an explosion and the energy of the system is transferred by means of work done in a variety of ways.
Therefore the electron cannot continue indefinitely to absorb photons, because to do so would result in it becoming increasingly unstable and would presumably cause it to break free of the atom at some point or other.
I'm not sure this explains everything though. Just because it would become unstable and would eventually break free of the atom, still doesn't explain (to me) why it may drop from one energy level to another.
Therefore the second (again probably erroneous) assumption I can make from this is that in some sense light isn't continuous, but instead comes in packets, rather like a series of tennis balls being thrown at a target, thereby giving the electron sufficient 'time' to absorb the photon, drop from one energy level to another and thereby emit a photon, before being struck by another photon (which is then absorbed), thereby raising it to a higher energy level again - and so on. If this was true (which it probably isn't), it would seem like a very simple mechanical process. So how close, or far off am I?
No doubt someone will 'hand me my ass' for trying to work out what's going on here though lol. I just don't feel I have a clear enough grasp of why electrons change energy levels and emit photons to be confident about this.
All input would be welcomed.