Find the distance between two successive position of the movable mirror of the Michelson interferometer giving best fringes in the case of sodium source with lines of $\lambda=5890\text{A}^\circ$ and $5896\text{A}^\circ$}
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For a sodium doublet with a difference of $6\text{ \AA}$, the distance moved by the mirror for successive distinct fringes is always approximately $0.29\text{ mm}$ (or $289\ \mu\text{m}$).
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