Two thin lenses are of same focal lengths $(f)$,but one is convex and the other one is concave. When they are placed in contact with each other,the equivalent focal length of the combination will be:

$A$ convex lens is in contact with a concave lens. The magnitude of the ratio of their focal lengths is $2/3$. Their equivalent focal length is $30 \ cm$. What are their individual focal lengths?

$A$ 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

Concave and convex lenses are placed touching each other. The ratio of magnitudes of their power is $2:3$. The focal length of the system is $30 \ cm$. The focal lengths of individual lenses are

Two lenses have focal lengths $f_1$ and $f_2$ and their dispersive powers are $\omega_1$ and $\omega_2$ respectively. They will together form an achromatic combination if

$A$ collimated beam of light of diameter $2 \ mm$ is propagating along the $x$-axis. The beam is required to be expanded into a collimated beam of diameter $14 \ mm$ using a system of two convex lenses. If the first lens has a focal length of $40 \ mm$, then the focal length of the second lens is . . . . . . $mm$.