The power of a combination of convex lens of focal length 40 cm and a concave lens of focal length 25 cm when placed in contact is:

-1.5 D

The correct answer is Option (2) → -1.5 D

focus of a combination of lenses ($f_1,f_2,....$)

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}+.....$

and,

$f_1=+40cm$ (Convex lens)

$f_2=-25cm$ (Concave lens)

$\frac{1}{f}=\frac{1}{40}-\frac{1}{25}$

$=\frac{5-8}{200}=\frac{-3}{200}$

and,

$ρ=\frac{100}{f}=\frac{-3×100}{200}=-1.5D$