Question

A drop of water is placed on a glass plate. $A$ double convex lens having radius of curvature of each surface $20cm$ is placed on it. The focal length of water lens $\left(\mu =\frac{4}{3}\right),$ will be

• A. 0.20 m
• B. -0.60 m
• C. 0.30 m
• D. 0.40 m

Answer 1

Answer: (B)

From lens formula
$\frac{1}{f}=(\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
where $R_1,R_2$ are radii of curvatures and $\mu$ the refractive index.
Given,
$R_1=-20cm,$
$R_2=\infty$ (plano-convex lens),
$\mu =\frac{4}{3}$
$\frac{1}{f}=\left(\frac{4}{3}-1\right)\left(\frac{1}{-20}-\frac{1}{\infty }\right)$
$\Rightarrow \frac{1}{f}=\frac{1}{3}\times \left(-\frac{1}{20}\right)$
$\Rightarrow f=-60cm=-0.6m$