Question

At what angle must two plane mirrors be placed so that incident and resulting reflected rays are always parallel to each other ?

Answer 1

The incident ray and reflected ray will always be parallel to each other when the two plane mirrors will be placed at right angle i.e. 90° to each other.

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Answer 2

Answer:

The two plane mirrors should be placed at an angle of 90°.

Explanation:

• For a smooth surface like a plane mirror the angle of incidence(i) is equal to the angle of reflection(r).
• Now, it is said that the incident ray and reflected ray is parallel which means δ = 180°.

                                    ∴     δ = 360° - 2θ

                                    ∴    360° - 2θ = 180°

                                           2θ = 360° - 180°

                                     ∴   $\theta = \frac{180}{2} =90$

Therefore, when the incident ray is parallel with the reflected ray then the two plane mirrors are arranged in a right-angled situation, θ =90°.