\(\frac{I}{I_0}=\frac{sin^2 \beta}{\beta ^2}\)

\(\beta=\frac{1}{2}k b sin \theta\)

\(\Delta=b sin\theta=\frac{3}{4}\lambda\)

\(\beta=\frac{1}{2}(k)(bsin\theta)=\frac{1}{2}(\frac{2\pi}{\lambda})(\frac{3}{4}\lambda)=\frac{3\pi}{4}\)

\(\frac{I}{I_0}=\frac{sin^2\frac{3\pi}{4}}{(\frac{3\pi}{4})^2}=0.09\)