Question

A thin biconvex lens of refractive index $\frac{3}{2}$ and radius of curvature $30 \, cm$ is put in water (refractive index = $\frac{4}{3}$ ). Its focal length is

• A. $0.15 \, m$
• B. $0.30m$
• C. $0.45m$
• D. $1.20m$

Answer 1

Answer: (D)

Using lens-makers formula
$\frac{1}{\mathrm{f}}=\left(\frac{\mu_1}{\mu_2}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)$
$\frac{1}{\mathrm{f}}=\left(\frac{3 / 2}{4 / 3}-1\right)\left(\frac{1}{0.3}+\frac{1}{0.3}\right)$
Or $\frac{1}{f}=\left(\frac{9}{8}-1\right)\left(\frac{2}{0.3}\right)$
Or $\frac{1}{f}=\frac{1}{8}\times \frac{2}{0 .3} \, \, $ or $f=1.20 \, m$