Find the distance of the final image from the object $O$,formed by the combination of lenses shown in the figure (in $cm$):

$A$ convex lens of focal length $A$ and a concave lens of focal length $B$ are placed in contact with each other. The focal length of this combination is ......

Two similar plano-convex lenses are combined together in three different ways as shown in the adjoining figure. The ratio of the focal lengths in three cases will be

$A$ biconvex lens is formed by using two thin planoconvex lenses, as shown in the figure. The refractive index and radius of curved surfaces are also mentioned in the figure. When an object is placed on the left side of the lens at a distance of $30 \ cm$ from the biconvex lens, the magnification of the image will be:

Two thin lenses in contact form an achromatic doublet of power $+2 \text{ D}$. The power of the convex lens is $+5 \text{ D}$. What is the ratio of the dispersive powers of the converging and diverging lenses?

$A$ plano-convex lens of material of refractive index $\mu_1$ exactly fits into a plano-concave lens of material of refractive index $\mu_2$. If $R$ is the radius of curvature of the curved surfaces of the lenses and the plane surfaces of the lenses are parallel,the focal length of the combination is: