Explaining Single Slit Diffraction

fringe spacing w	=	λD s	slit separation
width of central fringe W	=	2λD a	slit aperture width

Wavelets emitted simultaneously at point sources within the slit contribute to the new wavefront.Waves arriving from two points within the slit superpose. Dark fringes (minima) occur where a sin θ = nλ, where a is the slit width.