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A concave lens of focal length f forms an image which is 1/3 times the size

A concave lens of focal length f forms an image which is 1/3 times the size of the object. Then, the distance of object from the lens is

Options

(a) 2f
(b) f
(c) 2/3f
(d) 3/2f

Correct Answer:

2f

Explanation:

Given, f = -f ⇒ v = (1/3)u

According to the lens formula, 1/f = (1/v) – (1/u) – (1/f)

= [1/(-1/2)u] + (1/u) – (1/f) 1/f

= [(-3+1)/u] – (1/f)

= -2/u ⇒ u = 2f

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