An unpolarized light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster’s angle. Then

To determine the behavior and angle of the light beam, we can use the physics of polarization by reflection, specifically Brewster's law.

Brewster's law states that when an unpolarized light beam is incident on a boundary between two media at a specific angle called Brewster's angle (${\theta }_p$), the reflected light is completely polarized (polarized perpendicular to the plane of incidence).

Brewster's angle is mathematically related to the refractive index of the medium ($n$) by the formula:
$\tan \left({\theta }_p\right)=n$

Given that the medium has a refractive index of:
$n=1.73\approx \sqrt{3}$
We can substitute this value into the equation:
$\tan \left({\theta }_p\right)=\sqrt{3}$

Since $\tan (60°)=\sqrt{3}$, the Brewster's angle (angle of incidence) is:
${\theta }_p=60°$

According to the law of reflection, the angle of reflection is equal to the angle of incidence. Therefore, the angle of reflection is also:
${\theta }_r=60°$

Thus, the reflected light is completely polarized and the angle of reflection is close to 60°.