Question

Two thin lenses one of focal length 20 cm and another of focal length - 40 cm are in the contact. What is the focal length of the combination?

• A. zero
• B. - 20 cm
• C. 20 cm
• D. 40 cm

Answer 1

The correct option is D 40 cm

Given,

Focal length of lenses, $f_1=20cmand{f}_2=-40cm$

Lens combination formula.

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{20}+\frac{1}{-40}$

$f=40cm$

Hence, focal length is $40cm$.