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, 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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