|MadSci Network: Physics|
Dear Mr. Turner,
After consulting with my physicist colleague next door, I can give you the following answer: First of all, the Michelson Interferometer and the laser form a coupled system together. They cannot be considered in isolation from each other. So, if the interferometer is adjusted for destructive interference, i.e. that no light issues from the beam splitter in the direction of the observer, this means that all the light is coupled back (1) into the cavity of the laser. The radiation field inside the laser cavity is shifted to a new equilibrium, as though the mirror at its end, which is designed to let through a small part of the radiation, were a perfect reflector.
(1) because of the phase jump on reflection, destructive interference in the "observed" beam direction, automatically means constructive interference in the direction back to the light source.
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