Practice question

**Double-slit vs single-slit** double-slit interference pattern has equally spaced bright fringes with envelope due to single-slit diffraction, single-slit central maximum width 2λ D/a, intensity of secondary maxima decreases with order, condition for coherence constant frequency and phase, path difference for bright n λ, dark (n+½)λ. For dark fringes, x_n = ((n + (1/2)) λ D/d) . Second dark fringe, n = 1 . λ = 7.0 × 10⁻⁷ m , d = 3.5 × 10⁻⁴ m , D = 1.5 m . x₁ = ((1 + (1/2)) × 7.0 × 10⁻⁷ × 1.5/3.5 × 10⁻⁴) = (1.5 × 1.05 × 10⁻⁶/3.5 × 10⁻⁴) = 4.5