File:Michelson interferometer white light.svg

English: Michelson interferometer using white light. White light has only a very limited coherence length. When using a white light source, the two optical paths must be equal for all wavelengths. To meet this requirement, both the longitudinal and transverse light paths must cross an equal thickness of glass of the same dispersion. In this figure, the longitudinal beam crosses the beam splitter three times, while the transverse beam crosses the beam splitter once. To equalize the path lengths, a compensating plate identical to the beam splitter, but without the semi-reflective coat, is inserted into the path of the transverse beam. If one uses a half-silvered mirror as the beam splitter, the horizontally traveling beam will undergo a front-surface reflection at the mirror, and a rear-surface reflection at the beam splitter. The vertically traveling beam will undergo two front surface reflections at the beam splitter and the mirror. At each front-surface reflection, the light will undergo a phase inversion. Since light traveling the two paths will undergo a different number of phase inversions, when the two paths differ by a whole number (including 0) of wavelengths, there will be destructive interference and a weak signal at the detector. If they differ by a whole number and a half wavelengths (e.g., 0.5, 1.5, 2.5 ...) there will be constructive interference and a strong signal. Results will differ if a cube beam-splitter is employed, since a cube beam-splitter makes no distinction between a front- and rear-surface reflection.