If an object is placed 21cm away from a converging lens, the image formed is slightlysmaller than the object. If the object is placed 19cm away from the lens , the image formed is slightly larger than the object.The approximate focal length of the lens is?

When object is placed at the 2F in front of the convex lens then size of the image is same as that of the object.
radius of curvature is approximately equal to 20 cm
then focal length = 10 cm.

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f = 20 cm (approx)
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