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Question

An object, a convex lens of focal length 20 cm and a plane miror are arranged as shown in Fig. How far behind the mirror is the final image after reflection is formed?

A
20 cm
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B
30 cm
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C
40 cm
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D
50 cm
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Solution

The correct option is C 40 cm
Putting u=12 cm and f=+20 cm in the lens formula
1v1u=1f
1v112=120
1v=120112
1v=122020×12=130
v=30 cm,

i.e., the image is formed at a distance of 30 cm from the lens on the same side as the object. The distance of this image from the mirror =30+10=40 cm. Therefore, the final image is formed behind the mirror at a distance of 40 cm from it. Hence the correct answer is (c).

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