The diffraction pattern of a single slit is described by the square of a sinc function.

      
w sinθ = mλ
single slit diffraction minima (w = slit width)

E_slit(θ) = E₀cos(kr₀ - ωt) sin(β)/β, with β = πw(sinθ)/λ.
<I(θ)> = <I₀>sin²(β)/β².
Angular half-width of the central maximum:   θ ~ λ/w.

The diffraction pattern of a circular aperture is described by an Airy Disk.

Angular half-width of the central maximum (D = diameter):  θ ~ 1.22 λ/D

The minimum angular separation of two objects which can just be resolved by an optical instrument is given by θ_min=1.22λ/D.