An object, a convex lens of focal length 20cm and a plane miror are arranged as shown in Fig. How far behind the mirror is the final image after reflection is formed?
A
20cm
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B
30cm
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C
40cm
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D
50cm
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Solution
The correct option is C40cm Putting u=−12cm and f=+20cm in the lens formula 1v−1u=1f 1v−1−12=120 1v=120−112 1v=12−2020×12=1−30 v=−30cm,
i.e., the image is formed at a distance of 30cm from the lens on the same side as the object. The distance of this image from the mirror =30+10=40cm. Therefore, the final image is formed behind the mirror at a distance of 40cm from it. Hence the correct answer is (c).