(a) As refractive index for $z > 0$ and $z \leq \ 0$ is different $X - Y$ plane should be boundary between two media
Angle of incidence,
$\cos i = \left| \frac{A_{z}}{\sqrt{A_{x}^{2} + A_{y}^{2} + A_{z}^{2}}} \right| = \frac{1}{2}$
$\therefore i = 60{^\circ}$
From Snell’s law
$\frac{\sin i}{\sin r} = \frac{\sqrt{3}}{2}$
$\Rightarrow r = 45{^\circ}$