If incident ray MP and reflected ray QN are parallel to each other, find the angle θ between the mirrors.

90°

The deviation produced by first mirror $=180-2 i_1$ and the deviation produced by second mirror $=180-2 i_2$.

The total deviation is 180° as MP and QN are anti parallel

So, $180^{\circ}=360^{\circ}-2\left(i_1+i_2\right)$

∴ $i_1+i_2=\theta=90^{\circ}$