The graph in shows the variation of image distance (v) with object distance (u) in case of a lens. Find focal length of the lens. What is the nature of the lens, if image formed is real ?
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Explanation

(i) The lens forms a real image, so it must be a convex lens.
(ii) When object distance, `u=20cm,` image distance, `v=20cm` (from graph)
For a convex lens forming a real image, u is negative and v and f are positive.
So, `u=-20 cm and v=+20cm`
From lens formula,
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(20)-(-(1)/(20))=(1)/(10)`
`rArr" "f=+10cm`.