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

Two lenses of power $6D$ and $-2D$ are combined to form a single lens. The focal length of this lens will be (in $m$)

List-$I$ contains four combinations of two lenses ($1$ and $2$) whose focal lengths (in $cm$) are indicated in the figures. In all cases,the object is placed $20 \ cm$ from the first lens on the left,and the distance between the two lenses is $5 \ cm$. List-$II$ contains the positions of the final images.
Which one of the following options is correct?

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

The ray diagram for two lenses kept at some distance is given in the figure. Which of the following options is correct? ($f_1, f_2 =$ focal lengths,$d =$ distance between lenses)

$A$ 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: