Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. The combined focal length of the combination will be ....... $cm$.

$A$ beam of parallel rays is brought to a focus by a plano-convex lens. $A$ thin concave lens of the same focal length is joined to the first lens. The effect of this is:

The focal length of a combination of lenses formed with lenses having powers of $+2.50 \ D$ and $-3.75 \ D$ will be .... $cm$.

Obtain the equation of power and magnification for a combination of lenses.

$A$ convergent lens is placed $40 \,cm$ to the right of a diverging lens of focal length $15 \,cm$. $A$ parallel beam of light enters the diverging lens from the left, and the beam is again parallel when it emerges from the convergent lens. The focal length of the convergent lens is (in $\,cm$)

What is the power of the combination of a convex lens and a concave lens of equal focal length $25 \text{ cm}$?