\(I _{1}= I _{2}= K / 4\)
\(\Delta x =\lambda / 6 \Rightarrow \Delta \phi=\pi / 3\)
\(I = I _{1}+ I _{2}+2 \sqrt{ I _{1} I _{2}} \cos \phi\)
\(I =\frac{ K }{4}+\frac{ K }{4}+2 \times \frac{ K }{4} \frac{1}{2}\)
\(=\frac{ K }{2}+\frac{ K }{4}=\frac{3 K }{4}=\frac{9 K }{12}\)
\(n =9\)