Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror.

Let MM' and M'M'' be the two plane mirrors at right angles.

The reflected ray from the second mirror is shown in the figure below:

Given that the ray is incident on mirror MM' at 30°,

∴ ∠AOX = 30°

According to the law of reflection,

∠i = ∠r

∴ ∠XOO' = ∠AOX = 30°

As OX ⊥ MM' and O'X ⊥ M'M''

∴ OX ⊥ O'X

∴ ∠OXO' = 90°

In ΔOXO', by angle sum property.

∠XOO' + ∠OXO' + ∠XO'O = 180°

⇒ 30° + 90° + ∠XO'O = 180°

⇒ ∠XO'O = 180° - 90° - 30°

⇒ ∠XO'O = 60°

∴ Angle of incidence of the ray on M'M'' = 60°

∴ Angle of reflection of the ray from M'M'' = 60° [∵ ∠i = ∠r]