Problem 27.8

27.8. a)
From equation 27.11, we see that
$\frac{I}{I_{0}} = \cos^{2}(\frac{\phi}{2})$
Solving for $\phi$ we have
$\phi = 2 \cos^{-1} (\frac{I}{I_{0}})^{1/2}$
$\phi = 2 \cos^{-1} (.64)^{1/2}$
$\phi = 1.29$ radians

b)
The path difference can given by
$\delta = \frac{\lambda \phi}{2\pi}$
$\delta = 9.98 \times 10^{-8} m$