The combination of a convex lens $(f = 18 \ cm)$ and a thin concave lens $(f = 9 \ cm)$ is:

Two convex lenses of focal lengths $20 \,cm$ and $30 \,cm$ are placed in contact with each other co-axially. The focal length of the combination is (in $\,cm$)

The given lens is broken into four parts and rearranged as shown. If the initial focal length is $f$,then after rearrangement,the equivalent focal length is

An eye specialist prescribes spectacles having a combination of a convex lens of focal length $40\, cm$ in contact with a concave lens of focal length $25\, cm$. The power of this lens combination in diopters is

Two similar thin equi-convex lenses,each of focal length $f$,are kept coaxially in contact with each other such that the focal length of the combination is $F_{1}$. When the space between the two lenses is filled with glycerin (which has the same refractive index $\mu = 1.5$ as that of glass),the equivalent focal length is $F_{2}$. The ratio $F_{1} : F_{2}$ will be

$A$ convex lens of focal length $30 \ cm$ is placed in contact with a concave lens of focal length $20 \ cm$. An object is placed at $20 \ cm$ to the left of this lens system. The distance of the image from the lens in $cm$ is . . . . . .