\(I_{\max }\) for \(\phi=0 \& I_{\min }\) for \(\phi=\pi\)
\(I_{\max }=\left(\sqrt{I_{1}}+\sqrt{I_{2}}\right)^{2}=(\sqrt{9 I}+\sqrt{4 I})^{2}=25\,I\)
\(I_{\min }=\left(\sqrt{I_{1}}-\sqrt{I_{2}}\right)^{2}=(\sqrt{9 I}-\sqrt{4 I})^{2}=I\)
\(I_{\max }-I_{\min }=25 I - I =24\,I\)