Under what condition in an arrangement of two plane mirrors,will the incident ray and the final reflected ray always be parallel to each other,regardless of the angle of incidence? Show the same with the help of a diagram.

(N/A) When two plane mirrors are placed at an angle of $90^{\circ}$ to each other,the incident ray and the final reflected ray will always be parallel to each other,regardless of the angle of incidence.
Explanation:
$1$. Let the two mirrors be $M_1$ and $M_2$ placed at $90^{\circ}$.
$2$. The incident ray strikes $M_1$ at an angle of incidence $i$. According to the law of reflection,the angle of reflection $r = i$.
$3$. The reflected ray from $M_1$ acts as the incident ray for $M_2$. By geometry,the angle of incidence at $M_2$ will be $(90^{\circ} - i)$.
$4$. The final angle of reflection at $M_2$ will also be $(90^{\circ} - i)$.
$5$. By calculating the total deviation or the angles with the normal,it can be shown that the final reflected ray is parallel to the initial incident ray.