\(\mathrm{I}=\mathrm{I}_{\mathrm{o}} \cos ^{2}\left(\frac{\phi}{2}\right)\)
where \(\mathrm{I}_{0}\) is the maximum intensity.
NOTE : This formula is applicable when \(I_{1}=I_{2} .\) Here \(\phi=\pi / 3\)
\(\therefore \quad \frac{I}{I_{0}}=\cos ^{2} \frac{\pi}{6}=\left(\frac{\sqrt{3}}{2}\right)^{2}=\frac{3}{4}\)