Answer
$0.6^{\circ}$
Work Step by Step
For $\lambda$= 400nm, $\theta_{b}$= arcsin$(\frac{1.sin(35)}{1.67})$= 20.1$^{\circ}$
For $\lambda$= 700nm, $\theta_{b}$= arcsin$(\frac{1.sin(35)}{1.62})$= 20.7$^{\circ}$
$\Delta\theta$= 20.7$^{\circ}$-20.1$^{\circ}$= 0.6$^{\circ}$
