Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. What will be the focal length of the combination in $cm$?

$A$ 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 $3\,D$ and $-5\,D$ are combined to form a compound lens. An object is placed at a distance of $50\,cm$ from this combination. At what distance (in $cm$) will the image be formed?

In which optical instruments is a combination of lenses used?

$A$ convex lens has $9 \ cm$ focal length and a concave lens has $-18 \ cm$ focal length. The focal length of the combination in contact will be.....$cm$

Find the position of the final image with respect to the rightmost lens formed by the lens combination shown in the figure.