Example
Suppose ?=405 nm , the slit separation(center to center) is d=19.44 ?m and the slit width a=4.050 ?m
a) how many bright fringes are within the central peak of the diffraction envelope?
Solution: the diffraction minima are given by a sin?=m? m= ? 1, ? 2, ? 3,…
limit of central diffraction peak is the first minima at m=?1 => a sin?= ? [1]
the bright fringes of the interference pattern are given by d sin? =m? m=0,1,2,3,…
substitute for sin? from [1] ==> m=d/a = 4.8