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As far as the working of lasers is concerned, I was convinced till the part that the light flashes on a ruby stone and the electrons are excited and then again go back to their ground state and then they emit the photons(stimulated emission). This emitted photons in turn are absorbed by electrons of other atoms and this continues...
The photons emitted are emitted in all the directions. But they are reflected again and again such that they all get in a straight line. But, each reflection would have taken up some energy from photon hence, wavelength would change. And it is not certain that all the photons will undergo same number of reflections and it isn't certain either that all the excited electrons will undergo same transitions. Then how do we get resultant light that is of one wavelength.
Nor, am I convinced that all the emitted photons will be in phase with each other. That is coherence, right?
P.S.: I am not getting this concept well :(