= Interferometer
An <interferometer> splits or combines coherent <wave> amplitudes so that an observable depends on their relative <wave phase>. A <Michelson interferometer> sends <light> through two reflected paths before recombination; moving one <mirror> changes the round-trip <optical path difference>.
For two detected contributions, $I=I_1+I_2+2\sqrt{I_1I_2}\cos\Delta\phi$ under ideal mutual coherence and matched <polarization>. The varying cross term turns tiny path changes into measurable <intensity> changes. Stable interpretation requires control of vibrations, beam overlap and <optical coherence>. A lack of visible fringes can result from those limitations rather than from a lack of light in one path.
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