$A$ lens of power $+2$ diopters is placed in contact with a lens of power $-1$ diopter. The combination will behave like

Two lenses having focal lengths in the ratio $f_1:f_2 = 2:3$ are combined to produce no net dispersion. Find the ratio of the dispersive powers of the glasses used.

As shown in the figure, a combination of a thin plano-concave lens and a thin plano-convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is $30\,cm$ and the refractive index of the material for both the lenses is $1.75$. Both the lenses are placed at a distance of $40\,cm$ from each other. Due to the combination, the image of the object is formed at a distance $x = ..........cm$ from the concave lens.

$A$ convex lens and a concave lens,each having the same focal length of $25\, cm$,are put in contact to form a combination of lenses. The power in diopters of the combination is:

Two thin equiconvex lenses each of focal length $0.2 \,m$ are placed coaxially with their optic centres $0.5 \,m$ apart. Then the focal length of the combination is (in $\,m$)

In which optical instruments is a combination of lenses used?