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Question

# A concave lens of focal length f forms an image which is n times the size of the object. The distance of the object from the lens is:

A
(1n)f
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B
(1+n)f
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C
(1+nn)f
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D
(1nn)f
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Solution

## The correct option is D (1−nn)fLet the object distance be u, image distance be v and focal length be f. From lens formula, 1v−1u=1f ⇒uv−1=uf .......(i) Let height of the image be hi and height of the object be ho Given, hi=n×ho or magnification, m=hiho=vu=n ⇒v=n×u From equation (i) we have, ⇒uu×n−1=uf ⇒(1n−1)=uf ∴u=(1−nn)f

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