Incidence-independent adjacent-fringe wavelength ($\lambda =2a|\sin {\theta }_{\max }-\sin {\theta }_{\min }|$)

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.