Question

The focal length of a convex lens is $10cm$ and its refractive index is $1.5$. If radius of one surface is $7.5cm$, the radius of curvature of the second surface will be

• A. $7.5cm$
• B. $15cm$
• C. $75cm$
• D. $5cm$

Answer 1

Answer: (B)

According to lens maker formula,

$\frac{1}{f}=({\mu }_g-1)\frac{1}{R_1}-\frac{1}{R_2}$

$\frac{1}{10}=0.5\frac{1}{R_1}-\frac{1}{R_2}$

$\frac{1}{5}=\frac{1}{R_1}-\frac{1}{R_2}$
$\frac{1}{5}=\frac{1}{75}-\frac{1}{R_2}$

$\frac{1}{R_2}=-\frac{1}{5}+\frac{1}{75}$

$\frac{1}{R_2}=\frac{-3+2}{15}$

$R_2=15cm$