Two blocks A and B of masses $2 m$ and $m$, respectively, are connected by a massless and inextensible string. The whole system is suspended by a massless spring as shown in the figure. The magnitudes of acceleration of A and B, immediately after the string is cut, are respectively,

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