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 = $180 – 2i_1$ and the deviation produced by second mirror = $180 – 2i_2$.

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

So, $180° = 360° – 2(i_1 + i_2)$

$∴ i_1 + i_2 = θ = 90°$