Practice question

**Wavefront types** point source spherical, distant point source plane, after convex lens plane wave focuses to point because lens adds phase delay proportional to thickness, converging spherical wavefront, after concave mirror plane wave becomes spherical converging to focus, illustrating Huygens construction. Minima occur at sin θ = (nλ/a) . For the third minimum, n = 3 . λ = 4.0 × 10⁻⁷ m , a = 4.0 × 10⁻⁶ m . sin θ = (3 × 4.0 × 10⁻⁷/4.0 × 10⁻⁶) = 0.3 , θ = sin⁻¹(0.3) ≈ 17.5° . Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ,