In single-slit diffraction, the intensity at angle θ from centre follows I = I₀ (sinα/α)² where α = πa sinθ/λ. At what value of α is the first secondary maximum approximately?

Two coherent slits S₁ and S₂ act as secondary sources; at each point on the screen, the path difference between light from S₁ and S₂ determines whether the waves arrive in phase (bright fringe) or out of phase (dark fringe), producing the characteristic alternating fringe pattern.