A convex lens forms a real image of an object with magnification 0.5. If the object displaced by 20 cm along the principal axis, a real image equal to the size of the object is formed -
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(a, c)
Magnification of lens m = 
.
If initially magnification in magnitude is 0.5 then object must be beyond 2f because size of image is smaller. After shifting through 20 cm, the final image is of same size as that of object. So finally object is at focus
2f = 40 cm ⇒ f = 20 cm
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