Two plane mirrors are inclined to each other such that a ray of light incident on the first mirror and parallel to the second is reflected from the second mirror parallel to the first mirror. Determine the angle between the two mirrors.

60°

Let θ be the angle between the two mirrors $OM_1$ and $OM_2$. The incident ray AB is parallel to mirror $OM_2$ and strikes the mirror $OM_1$ at an angle of incidence equal to θ. From figure we have

$∠M_1BA = ∠OBC = ∠M_1OM_2 = θ$

Similarly for reflection at mirror $OM_2$, we have

$∠M_2CD = ∠BCO = ∠M_2OM_1 = θ$

Now in triangle OBC, $3θ = 180°$, therefore

$θ = 60°$