Laser uses mirrors to reflect photons in order to stimulate atoms to emit photons, but why this is so?. I mean, why does a photon stimulate atoms to produce more photons? If a photon made an atom to produce a photon, is not the first photon absorbed by the atom and therefore there is not total gain in the process?
Answer
Lasers are pumped, so that many electrons already have excited states. In such a situation, when a photon passes through the active material, it's unlikely to be absorbed because there's not enough electrons in non-excited states. Much more likely result is that, by mechanism of stimulated emission, excited electrons can lose their energy, producing a duplicate of the photon which passed by. This happens when passing photon has energy very close to energy of radiative transition from excited state to non-excited.
Mirrors make up the optical cavity, which makes photon travel multiple times across the active material, thus making more and more photons duplicate, resulting in exponential growth of electromagnetic energy, which partially exits the cavity because one (or both) of mirrors are not 100% reflective. And the light which exits is the usable light from laser.
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