Question

A convex mirror of focal length $10cm$ is placed in water. The refractive index of water is $\frac{4}{3}$. What will be the focal length of the mirror in water.

• A. $10cm$
• B. $\frac{40}{3}cm$
• C. $\frac{30}{4}cm$
• D. None of these

Answer 1

Answer: (C)

$\frac{30}{4}cm$

$\begin{array}{cc}\text{Focal length of convex mirror}& =10cm=f\\ \text{refractive index}\mu & =\frac{-4}{3}\\ \text{focal length in water}& =\frac{f}{\mu }\\ & =\frac{10\times 3}{4}\\ & =\frac{30}{4}\end{array}$