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:
Answer & explanation
Correct answer: option 2
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,
$ρ=\frac{100}{f}=\frac{-3×100}{200}=-1.5D$