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Question
An object is placed at a certain distance from a convex lens of focal length 20 cm. Find the distance of the object if the image obtained is magnified 4 times.
Solution
Given:
f = 20 cm
m = - 4 = `"v"/"u"`
v = - 4u
`1/"f" = 1/"v" - 1/"u"`
`1/20 = 1/(- 4"u") - 1/"u"`
`1/20 = (-3)/(4"u")`
u = `(- 60)/4`
u = - 15 cm
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