==> \({I_0} = 4{I_0}{\cos ^2}(\phi /2)\)
==> \(\cos (\phi /2) = \frac{1}{2}\) or \(\frac{\phi }{2} = \frac{\pi }{3}\)
==> \(\phi = \frac{{2\pi }}{3}\) \(\Delta = \frac{{xd}}{D} = \frac{\lambda }{{2\pi }} \times \phi \)
==> \(x \times \,\left( {\frac{d}{D}} \right) = \frac{{6000 \times {{10}^{ - 10}}}}{{2\pi }} \times \frac{{2\pi }}{3}\)
==> \(x = 2 \times {10^{ - 3}}m = 2\,mm.\)