For three parallel slits of equal size and with neighbor to neighbor distances being $a$ and $b$ , respectively. The intensity at diffraction minima is zero, if $\dfrac{a}{b} = \dfrac{n}{m}$ , where $n$ and $m$ are positive integers when a plane wavefront of light perpendicular to slits illuminates them.

Which of the following is a possible value of $n-m$ ?

Assume that we ignore single slit diffraction.

2
4
5
3

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for the sum of three vectors of equal magnitude to be zero, they must form an equilateral triangle

and $a/b = n/m$ such that $n-m$ is a multiple of 3