= Evacuated-cell interferometric refractometry
{title2=$n-1=N\lambda_0/l$}
Evacuating a transparent cell of optical length $l$ in one coherent beam changes its optical path by $l(n-1)$ relative to an unchanged cell in the other beam. With vacuum wavelength $\lambda_0$, the number of completed fringe periods is $N=l(n-1)/\lambda_0$. The initial maximum is not an additional completed period. Counting only integer fringes leaves a phase uncertainty of order one fringe unless the final fractional phase is measured; the corresponding index scale is $\lambda_0/l$. Fixed equal window contributions cancel in the path difference.
Back to article page
