= Full-cycle phase-plate invisibility
{title2=$(n-1)t=m\lambda$}
In a monochromatic two-path interferometer, an ideal <phase plate> of thickness $t$ and <refractive index> $n$ adds <phase> $2\pi(n-1)t/\lambda$ at normal incidence. The <interference> pattern is unchanged when $(n-1)t=m\lambda$ for integer $m$, provided transmitted <amplitudes> and alignment are unchanged. Calling the smallest index requires specifying the admissible range; $n>1$ excludes the optically null $n=1$ solution.
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