$\begin{array}{ll}\mathrm{I}_{\mathrm{p}}=\mathrm{I}_0=4 \mathrm{I}_0 \cos ^2 \frac{\Delta \phi}{2} & \\ \cos \frac{\Delta \phi}{2}=\frac{1}{2} \text { or }-\frac{1}{2} & \\ \frac{\Delta \phi}{2}=\frac{\pi}{3} \text { or } \frac{2 \pi}{3} & \\ \frac{2 \pi}{\lambda}=\Delta \mathrm{x}=\Delta \phi=\frac{2 \pi}{3} & \text { or } \\ \frac{2 \pi}{\lambda}=\Delta \mathrm{x}=\Delta \phi=\frac{4 \pi}{3} \\ \Delta \mathrm{x}=\frac{\lambda}{3}=2000 \mathrm{~A}^{\circ} & \text { or } \\ \Delta \mathrm{x}=\frac{2 \lambda}{3}=4000 \mathrm{~A}^{\circ}\end{array}$