= Incidence-independent adjacent-fringe wavelength
{title2=$\lambda=2a|\sin\theta_{\max}-\sin\theta_{\min}|$}
In oblique-incidence two-slit interference, the incident wave contributes a constant phase offset while changing observation angle changes the outgoing <optical path difference> by $a\Delta\sin\theta$. Subtracting the phase conditions of an adjacent minimum and maximum cancels the unknown incidence angle and order and gives the displayed wavelength. Nonadjacent extrema need their intervening phase-order difference; angular spacing should not be replaced by a small-angle formula unless justified.
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