Answer
The upper limit of the Brewster angle is $~~55.8^{\circ}$
Work Step by Step
On the graph in Figure 33-18, we can see that the index of refraction in quartz for light with a wavelength of $400~nm$ is $n = 1.470$
We can find the Brewster angle:
$\theta_B = tan^{-1}~\frac{n_2}{n_1}$
$\theta_B = tan^{-1}~\frac{1.470}{1.00}$
$\theta_B = 55.8^{\circ}$
The upper limit of the Brewster angle is $~~55.8^{\circ}$
