Explanation:
Here, \(\lambda = 600\;nm = 6 \times {10^{ - 7}}\;m\)
\(\theta=30^{\circ}\)
condition for minimum is,\(a \sin \theta=m \lambda(m=1,2,3, \ldots)\)
For first minimum, \(\sin \theta=\dfrac{\lambda}{a}\)
or \(\sin 30^{\circ}=\dfrac{6 \times 10^{-7}}{a}\) or \(a=12 \times 10^{-7}\)
or \(a = 1200\;nm = 1.2\,\mu m\)