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

As the image formed is real, the lens must be convex.
From the graph,
when `|u| = 20 cm, |v| = 20 cm`
`:. u = -20 cm, v = + 20 cm`
`(1)/(f) =(1)/(v)-(1)/(u)=(1)/(20)+(1)/(20) = (1)/(10)`
`f = 10 cm`.