An effective power of a combination of $5$ identical convex lenses which are kept in contact along the principal axis is $25 \ D$. Focal length of each of the convex lens is: (in $cm$)

$A$ convex lens $A$ of focal length $20 \,cm$ and a concave lens $B$ of focal length $56 \,cm$ are kept along the same axis with a distance $d$ between them. If a parallel beam of light falling on $A$ leaves $B$ as a parallel beam, then the magnitude of distance $d$ (in $cm$) is

The dispersive powers of the materials of two lenses forming an achromatic combination are in the ratio of $4:3$. If the effective focal length of the combination is $+60 \ cm$, then the focal lengths of the lenses should be:

$A$ convex lens of focal length $30 \ cm$ forms an image of height $2 \ cm$ for an object situated at infinity. If a concave lens of focal length $20 \ cm$ is placed coaxially at a distance of $26 \ cm$ in front of the convex lens,what will be the size of the final image in $cm$?

Two lenses of power $+10 \, D$ and $-5 \, D$ are placed in contact. Where should an object be held from the lens so as to obtain a virtual image of magnification $2$ (in $, cm$)?

$A$ convex lens of focal length $40 \ cm$ is in contact with a concave lens of focal length $25 \ cm$. The power of the combination is: