The radii of curvature of a double convex lens are 15 cm and 10 cm and its refractive index is 1.3. Its focal length will be:

+20 cm

The correct answer is Option (3) → +20 cm

Using Len's Maker formula,

$\frac{1}{f}=(μ-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

$=(1.3-1)\left(\frac{1}{15}-\frac{1}{-10}\right)$

$=0.3×\frac{5}{30}=+20cm$