Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact. The focal length of this combination is

As shown in the figure,a concave lens of focal length $50 \, cm$ and a convex lens of focal length $25 \, cm$ are placed in contact with each other. If a parallel beam of light is incident on the system,how will it emerge?

$A$ combination of two thin lenses in contact has a power of $+10 D$. The power reduces to $+6 D$ when the lenses are $0.25 m$ apart. The power of each individual lens is:

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 thin lenses have a combined power of $+9 D$. When they are separated by a distance of $20 \,cm$, their equivalent power becomes $+\frac{27}{5} D$. Their individual powers (in diopters) are respectively:

$A$ thin convex lens of focal length $30 \, cm$ forms an image $2 \, cm$ high,of an object at infinity. $A$ thin concave lens of focal length $20 \, cm$ is placed $26 \, cm$ from the convex lens on the side of the image. The height of the final image is.....$cm$.