Question

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A convex lens produces an image of an object on screen with magnification of 1/3 when lens is moved 20cm from object the magnification of an image on screen becomes -3 find focal length​

Answer 1

for convex lens

$m \: = \frac{f}{u + f} \\ = > \frac{1}{3} = \frac{f}{u + f} \\ = > u +f = 3 f \\ = > u = 2f \\ = > \frac{f}{u - 20 + f} = - 3 \\ = > \frac{f}{2f - 20 + f} = - 3 \\ = > f = - 9f + 60 \\ = > 10f = 60 \\ = > f = 6$

focal lenght of lens is +6cm

hope it helps you