Two thin lenses, one of focal length + 60 cm and the other of focal length - 20 cm are put in contact, the combined focal length is

Question

Two thin lenses, one of focal length + 60 cm and the other of focal length - 20 cm are put in contact, the combined focal length is (A) 15 cm (B) -15 cm (C) - 30 cm (D) 30 cm

Tardigrade Admin · Accepted Answer

The focal length (f) of the combination of concave and convex lens is (1/f)=(1/f1)+(1/f2) where f1=+60 cm, f2=-20 cm ∴ (1/f)=(1/60)-(1/20) ⇒ (1/f)=(-2/60) ⇒ f=-30 cm

Q. Two thin lenses, one of focal length $+ 60 c m$ and the other of focal length $- 20 c m$ are put in contact, the combined focal length is

15 cm

-15 cm

- 30 cm

30 cm

The focal length $(f)$ of the combination of concave and convex lens is
$\frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}}$
where $f_{1} = + 60 c m$ ,
$f_{2} = - 20 c m$
$∴ \frac{1}{f} = \frac{1}{60} - \frac{1}{20}$
$\Rightarrow \frac{1}{f} = \frac{- 2}{60}$
$\Rightarrow f = - 30 c m$

Q. Two thin lenses, one of focal length +60cm and the other of focal length −20cm are put in contact, the combined focal length is