1. The problem statement, all variables and given/known data

Is it necessary that a single slit diffraction occur with the two source having the distance of $\frac{a}{2}$?Are there any diffractions occur when the point source having the distance of $\frac{a}{3}$,$\frac{a}{4}$ and so on?
And I would like to know whether are there only destructive interference occur?

2. Relevant equations

3. The attempt at a solution

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