Question

An object is placed at a distance of $10cm$ from the concave mirror and the image is formed $20cm$ in front of the mirror. The focal length of the mirror is:

• A. $-\frac{20}{3}cm$
• B. $\frac{3}{20}cm$
• C. $15cm$
• D. $-20cm$

Answer 1

The correct option is A $-\frac{20}{3}cm$

Here, $u=-10cm$ $v=-20cm$

Now, $\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

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

$f=-\frac{20}{3}cm$