A point object is placed at a distance of 20 cm from a thin plano-convex lens of focal length 15 cm , if the plane surface is silvered. The image will form at

A biconvex lens of focal length $f$ forms a circular image of sun of radius $r$ in focal plane. Then
Graph of position of image versus position of point object from a convex lens is shown. Then, the focal length of the lens is

A massless rod is suspended by two identical strings AB and CD of equal length. A block of mass $m$ is suspended from point $O$ such that BO is equal to $x$. Further, it is observed that the frequency of 1st harmonic (fundamental frequency) in AB is equal to 2 nd harmonic frequency in CD. Then, length of BO is

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