For diffraction at a single slit of width a, the angular position of the first minimum is given by

1. Single-slit diffraction gives dark fringes at a sin θ = n λ, n = ±1, ±2, … 2. The first minimum (n = 1) is at a sin θ = λ. 3. Note: bright fringes are between minima; the central bright is broader. 4. Half-angular width of central maximum is θ ≈ λ/a for small angles. _Source: syllabus Physics, Ch 10 "Wave Optics", §10.6_