Question

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

• A. -0.20 m
• B. -0.60 m
• C. 0.3
• D. 0.40 m

Answer 1

The correct option is B -0.60 m

For convex lens,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
$R_1=20cm,R_2=-20cm.$
$=(1.5-1)(\frac{1}{20}+\frac{1}{20})$
$=\frac{1}{2}\times \frac{2}{20}=\frac{1}{2}\times \frac{1}{10}=\frac{1}{20}$
$\Rightarrow f=20cm=0.2m$
For water lens,
$R_1=\infty$ (for plane surface)
$R_2=-20cm$
$\frac{1}{f^{\prime }}=(\frac{4}{3}-1)(\frac{1}{\infty }-\frac{1}{-20})=\frac{1}{3}\times \frac{-1}{20}=\frac{-1}{60}$
$f^{\prime }=-60cm=-0.60m$