A beam of unpolarized light is incident on the surface of a...

When unpolarized light is incident at the polarizing angle (Brewster's angle), the reflected light is completely plane-polarized.

According to Brewster's law, at the polarizing angle $i_p$, the reflected and refracted rays are mutually perpendicular.

Therefore, $i_p + r = 90^{\circ}$, where $r$ is the angle of refraction.

Given $i_p = 60^{\circ}$, we have:
$60^{\circ} + r = 90^{\circ}$
$\Rightarrow r = 30^{\circ}$

Answer: $30^{\circ}$