Question

A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with one another. (a) What is the power of this combination ? (b) What is the focal length of this combination ? (c) Is this combination converging or diverging ?

Answer 1

Power = $\frac{1}{f}$ where f is focal length in meter
now p1 = $\frac{100}{25}$ = 4 (for convex lens)
p2 =$\frac{-100}{10}$ = -10(power is negative because focla length of concave lens is negative)
now resultant power = p1+p2 = 4+(-10) =-6

$F=\frac{1}{P}=\frac{1}{-6}=-0.16m$

and the combination is diverging.