Question

Radii of curvature of a converging lens are in the ratio $1:2$. Its focal length is $6cm$ and refractive index is $1.5$. Then its radii of curvature are

• A. 9 cm and 18 cm
• B. 6 cm and 12 cm
• C. 3 cm and 6 cm
• D. 4.5 cm and 9 cm

Answer 1

Answer: (D)

Given, $\frac{R_1}{R_2}=\frac{1}{2},f=6cm$ and $\mu =1.5$
Let $R_1=R$ and $R_2=2R$
We know the focal length of converging lens
By using relation $\frac{1}{f}=(\mu -1)\left[\frac{1}{R_1}+\frac{1}{R_2}\right]$
Substituting all the values, we get $R=4.5cm$
$\therefore R_1=4.5cm$ and $R_2=2\times 4.5=9cm$