Question

An object 20 cm from a convex mirror give rise to a virtual image 15 cm behind the mirror. The focal length of the mirror is

• A. 8.5 cm
• B. 30 cm
• C. 90 cm
• D. 60 cm

Answer 1

The correct option is D 60 cm

Taking new Cartesian sign conventions $\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\Rightarrow -\frac{1}{20}+\frac{1}{15}=\frac{1}{f}$
$\Rightarrow \frac{-3+4}{60}=\frac{1}{f}\Rightarrow f=60cm$