= Spectral doublet from Michelson visibility revivals
For two equal-intensity mutually incoherent narrow spectral lines with <wavenumbers> $k_1,k_2$, a <Michelson interferometer> with <optical path difference> $x$ has normalized intensity $I/I_0=1+\cos(\bar kx)\cos(\Delta kx/2)$, where $\bar k=(k_1+k_2)/2$ and $\Delta k=|k_1-k_2|$. With mirror displacement $d=x/2$, the fast period is $\pi/\bar k\simeq\bar\lambda/2$; successive <interference visibility> revivals are separated by $\pi/\Delta k\simeq\bar\lambda^2/(2\Delta\lambda)$. The first visibility zero is halfway to the first revival. A signed envelope has twice the period of its absolute value. Unequal line strengths prevent perfect zeros.
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