From the ray diagram shown in Fig. calculte the focal length of concave lens.
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Answer

`-20 cm`

Explanation

For the convex lens : `f = 20 cm, u = -60 cm, v = ?`
`(1)/(v)=(1)/(f)+(1)/(u)=(1)/(20)-(1)/(60) = (3 - 1)/(60) = (1)/(30), v = 30 cm`
Image `I'` would have been formed where `CI' = 30 cm`. On refraction through concave lens, rays converging at `I'` become parallel.
Therefore, for concave lens,
`u = (30 - 10) = 20 cm, v = oo`
`(1)/(f) = (1)/(v)-(1)/(u) = (1)/(oo)-(1)/(20) = -(1)/(20)`
`f = -20 cm`.