\(\phi \,' = \frac{{2\pi }}{\lambda }(\Delta )\)
\(\Delta = d\sin \theta \)
==> \(\phi \,' = \frac{{2\pi }}{\lambda }(d\sin \theta ) = \frac{{2\pi }}{\lambda } \times \frac{\lambda }{4}(\sin {30^o}) = \frac{\pi }{4}\)
\(\phi = {\phi _0} + \phi \,' = \frac{\phi }{4}\).
\(I = 4{I_0}{\cos ^2}\,(\phi /2) = 4{I_0}{\cos ^2}\left( {\frac{{\pi /2}}{2}} \right) = 2{I_0}\).