Match each wave optics phenomenon to its key equation.

• Double-slit bright fringe → $d\sin\theta = m\lambda$
• Single-slit dark fringe → $a\sin\theta = m\lambda,\ m \ne 0$
• Thin-film constructive (in air) → $2nt = (m+\tfrac{1}{2})\lambda$
• Rayleigh resolution limit → $\theta \approx 1.22\,\lambda/D$

Slits that are close together (separation d) give interference maxima; a single opening of width a gives diffraction minima; thin films add a half-wave flip on reflection; and Rayleigh’s criterion sets the smallest angle two sources can be apart and still be distinguished.