Question

A convex lens of focal length $10cm$ is placed in contact with a concave lens of focal length $20cm$. The focal length of this combination of lenses will be :

• A. $+10cm$
• B. $+20cm$
• C. $-10cm$
• D. $-20cm$

Answer 1

The correct option is B $+20cm$

Given,

$f_1=+10cm$ for convex lens

$f_2=-20cm$ for concave lens

The focal length of the combination is given by

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

$\frac{1}{f}=\frac{1}{+10cm}+\frac{1}{-20cm}=\frac{1}{20cm}$

$f=+20cm$

The correct option is B.