Question

A convex lens of $10\text{cm}$ focal length is joined with a concave lens of $6\text{cm}$ focal length. Find the focal length of the combination.

• A. $15\text{cm}$
• B. $-15\text{cm}$
• C. $30\text{cm}$
• D. $-30\text{cm}$

Answer 1

The correct option is B $-15\text{cm}$

The focal length of their combination is given by,

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

$\Rightarrow \frac{1}{f}=\frac{1}{10}+\frac{1}{(-6)}$

$\Rightarrow f=-15\text{cm}$

​​​​​​​Hence, option $\left(B\right)$ is the correct answer.