Answer
$\theta_{critical}=34^{\circ}$
Work Step by Step
The critical angle between two substances is defined as $$\theta_{critical}=sin^{-1}(\frac{n_2}{n_1})$$ Substituting values of $n_1=1.8$ for benzene and $n_2=1.0$ for air yields a critical angle of $$\theta_{critical}=sin^{-1}(\frac{1.0}{1.8})=34^{\circ}$$
