Question
By what angle should $M_2$ be rotated, so that the light ray after reflection from both mirrors become horizontal (incident angle at $M_1$ mirror is $40^o$)
✓
Answer
$180-2(90-\theta)=40^{\circ}$
$2 \theta=40^{\circ} \Rightarrow \theta=20^{\circ}$
$5$ clock wise rotation.
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