Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / B7 / b

A slit of width $w$ has its first zeros at $\sin \theta =\pm \lambda /w$. Thus the full central-envelope widths areThe narrower envelope determines the overlap, so it spansThis is the usual approximate “50 fringes” count. If counting nominal bright fringe centres strictly inside both first zeros, they have indices $-24,\dots ,24$, giving 49 centres. The endpoints $\pm 25$ lie at the wider slit's first zeros, where two-slit visibility vanishes; they should not be counted as fully interfering bright centres.