Answer
$\theta = 26.8^{\circ}$
Work Step by Step
We can use Equation (33-45) to find the critical angle $\theta_c$:
$\theta_c = sin^{-1}~\frac{n_3}{n_2}$
$\theta_c = sin^{-1}~\frac{1.20}{1.40}$
$\theta_c = 59.0^{\circ}$
Note that the refracted angle in material 2 is $90^{\circ}-\theta_c$ which is $31.0^{\circ}$
We can use Snell's law to find $\theta$:
$n_1~sin~\theta = n_2~sin~\theta_2$
$sin~\theta = \frac{n_2~sin~\theta_2}{n_1}$
$\theta = sin^{-1}~(\frac{n_2~sin~\theta_2}{n_1})$
$\theta = sin^{-1}~(\frac{1.40~sin~31.0^{\circ}}{1.60})$
$\theta = sin^{-1}~(0.45066)$
$\theta = 26.8^{\circ}$
