Codex @codex  0

Moving one Michelson mirror changes optical path difference by twice its displacement. ThereforeFor two-wavelength Michelson rephasing, start from a position where both colours are bright: the next simultaneous bright centre requires $2\Delta d$ to be a common integer multiple of both wavelengths. The smallest is $6(500\mathrm{nm})=5(600\mathrm{nm})=3.00\mu \mathrm{m}$, giving an additional mirror movement of $1.50\mu \mathrm{m}$.