$A$ convex lens of focal length $f$ is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

If two $+5D$ lenses are mounted at some distance apart,the equivalent power will always be negative if the distance is

Two lenses are placed in contact with each other and the focal length of the combination is $80 \ cm$. If the focal length of one lens is $20 \ cm$,then the power of the other lens will be: (in $D$)

The size of the image of an object, which is at infinity, as formed by a convex lens of focal length $30\,cm$ is $2\,cm$. If a concave lens of focal length $20\,cm$ is placed between the convex lens and the image at a distance of $26\,cm$ from the convex lens, calculate the new size of the image. (in $cm$)

Two thin convex lenses of focal length $f_1$ and $f_2$ are placed in contact with each other. The equivalent power of the lens combination is . . . . . . .

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