= Solution
A <Michelson interferometer> divides light at a <beam splitter>, sends it to two mirrors, and recombines the returned beams. A compensating plate can equalize <glass> traversal. For approximately axial rays the <optical path difference> is $\Delta=2(n_1L_1-n_2L_2)$, plus fixed instrument phase, and
$$
I=I_1+I_2+2\sqrt{I_1I_2}\cos(2\pi\Delta/\lambda_0+\phi_0)
$$
for coherent, mode-matched fields. Moving one mirror by $\delta L$ in air changes the round-trip path by $2\delta L$, so \b[one fringe corresponds to $\lambda_0/2$ mirror motion]. It measures displacement, <wavelength> and refractive-index changes; scanning the delay enables Fourier-transform spectroscopy. Mirror alignment, <polarization> overlap, vibration and source coherence control visibility. A finite coherence length limits useful arm mismatch.
Back to article page
