A ray falls from air on to a flat surface of flint glass (n = 1.6).

Step 1. The condition. The two rays are at $90^\circ$ when $i + r = 90^\circ$. Then $r = 90^\circ - i$, so $\sin r = \cos i$.

Step 2. Snell's law. $n = \dfrac{\sin i}{\sin r} = \dfrac{\sin i}{\cos i} = \tan i$, so $\tan i = 1.6$.

Step 3. The angles. $i = 58.0^\circ$, and $r = 90^\circ - 58.0^\circ = 32.0^\circ$.