Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with each other. The combination has a focal length equal to

Question

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with each other. The combination has a focal length equal to (A) Infinite (B) 50 cm (C) 60 cm (D) 10 cm

Tardigrade Admin · Accepted Answer

Powers: P1 =(1/0.2) and P2=-(1/0.2) = 5 D and -5 D Net power =0 ∴ Focal length =(1/0)=∞

Q. Two thin lenses of focal lengths $20 c m$ and $- 20 c m$ are placed in contact with each other. The combination has a focal length equal to

Infinite

50 cm

60 cm

10 cm

Powers: $P_{1} = \frac{1}{0.2}$ and $P_{2} = - \frac{1}{0.2}$
$= 5 D$ and $- 5 D$
Net power $= 0$
$∴$ Focal length $= \frac{1}{0} = \infty$

Q. Two thin lenses of focal lengths 20cm and −20cm are placed in contact with each other. The combination has a focal length equal to