Question

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

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

Answer 1

Answer: (B)

$\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
$\frac{1}{10}=(1.5-1)\left(\frac{1}{7.5}-\frac{1}{ R _{2}}\right)$
$\frac{1}{10}=\frac{1}{2}\left(\frac{1}{7.5}-\frac{1}{R_{2}}\right)$
$R _{2}=-15 cm$