= Two-wavelength Michelson rephasing
{title2=$2d=m\lambda_1=n\lambda_2$}
Moving one mirror by $d$ changes the round-trip optical path by $2d$. Starting at a common bright centre for two commensurate <wavelengths>, the next common bright centre occurs when $2d=m\lambda_1=n\lambda_2$ for the smallest positive integers. This is a common-multiple condition on optical path, not on the mirror displacement itself. Incommensurate exact <wavelengths> have no finite exact recurrence in the ideal model.
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