KVPY SA Exam 2018 Question Paper

Question : 20 of 80

Marks: +1, -0

Two plane mirrors are kept on a horizontal table making an angle θ with each other as shown schematically in the figure.
The angle θ is such that any ray of light reflected after striking both the mirrors returns paralled to its incident path.
For this to happen, the value of q should be

30º
45º
60º
90º

Validate

Solution:

(d) As emergent ray is parallel to incident ray, deviation angle

δ

180^{∘} .

But

\;\; δ=360^{∘}-2 θ

where,

θ=

angle between inclined mirrors.

So,

\;\; 360^{∘}-2 θ=180^{∘}

2 θ=180^{∘} ⇒ θ=90^{∘}

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