= Two-line coherence beating
{title2=$|g^{(1)}(\tau)|=|\cos(\Delta\omega\tau/2)|$}
Two equally intense monochromatic spectral components have normalized <first-order optical coherence>
$$
g^{(1)}(\tau)=\tfrac12(e^{-i\omega_1\tau}+e^{-i\omega_2\tau})
=e^{-i\bar\omega\tau}\cos(\Delta\omega\tau/2).
$$
The visibility zeros occur at $\tau=(2j+1)\pi/|\Delta\omega|$, while its revivals repeat every $2\pi/|\Delta\omega|=1/|\Delta\nu|$. This envelope describes self-interference of each spectral component after a delay, not a time-resolved measurement of inter-frequency beating at one fixed point. Unequal line intensities prevent complete cancellation at the minima.
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