Question

The focal length of a double convex lens is $20cm$ . The radii of curvature of the surface of this lens are $20 \, cm$ and $40 \, cm$ , respectively. Then, the refractive index of the material of the double convex lens is

• A. $\frac{5}{2}$
• B. $\frac{4}{3}$
• C. $\frac{5}{3}$
• D. $\frac{4}{5}$

Answer 1

Answer: (C)

Here, $R_{1}=20 \, cm \, R_{2}==-40 \, cm, \, f=20 \, cm$
$\frac{1}{20}=\left(\mu - 1\right)\left(\frac{1}{20} + \frac{1}{40}\right);\frac{1}{20}=\left(\right.\mu -1\left.\right)\frac{3}{40}\Rightarrow \mu =\frac{5}{3}$