Question

The plano-convex lens of focal length $20$cm and $30$cm are placed together to form a double convex lens, the final focal length will be :

• A. $12cm$
• B. $60cm$
• C. $20cm$
• D. $30cm$

Answer 1

The correct option is A $12cm$

Equivalent focal length,

$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}$

$=\frac{1}{20}+\frac{1}{30}$

$F=\frac{20\times 30}{20+30}=\frac{600}{50}=12$cm

$F=12$cm.