A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point $15 \mathrm{cm}$ from the lens on the opposite side. If the lens is removed the point where the rays meet will move $5 \mathrm{cm}$ closer to the lens. The focal length of the lens is

Graph of position of image vs position of point object from a convex lens is shown. Then, focal length of the lens is

An object is located $4 \mathrm{m}$ from the first of two thin converging lenses of focal lengths $2 \mathrm{m}$ and $1 \mathrm{m}$ respectively. The lenses are separated by $3 \mathrm{m}$. The final image formed by the second lens is located from the source at a distance of

A plano-convex lens fits exactly into plano-concave lens as shown in the figure. Their plane surfaces are parallel to each other. If the lenses are made of different materials of refractive indices $1.6$ and $1.5$ respectively, if $R$ is the radius of curvature of the curved surface of lenses, then the focal length of the combination