$A$ 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

$A$ 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:

$A$ lens of power $3.5 \, D$ is placed in contact with a lens of power $-2.5 \, D$. The combination behaves as:

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

The two lenses of an achromatic doublet should have: