$A$ convex lens of focal length $40 \ cm$ is in contact with a concave lens of focal length $25 \ cm$. The power of the combination is:

Two lenses of power $-15 \ D$ and $+5 \ D$ are in contact with each other. The focal length of the combination is ....... $cm$.

$A$ plano-convex lens of refractive index ${\mu _1}$ and focal length $f_1$ is kept in contact with another plano-concave lens of refractive index ${\mu _2}$ and focal length $f_2$. If the radius of curvature of their spherical faces is $R$ each and $f_1 = 2f_2$,then ${\mu _1}$ and ${\mu _2}$ are related as:

Two lenses have focal lengths of $+10 \ cm$ and $-15 \ cm$. When they are placed in contact,the combination acts as a convex lens. If the chromatic aberration is zero,what is the ratio of their dispersive powers?

Two lenses of powers $-1.75 D$ and $+2.25 D$ are placed in contact. The focal length of the combination is (in $\,cm$)

$A$ magnifying glass is made of a combination of lenses with power $+20\,D$ and $-4\,D$. If the least distance of distinct vision is $25\,cm$,calculate the size of an image of an object $2\,cm$ high seen through the magnifying glass. (in $,cm$)