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:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → +20 cm
Using Len's Maker formula,
$\frac{1}{f}=(μ-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
$=(1.3-1)\left(\frac{1}{15}-\frac{1}{-10}\right)$
$=0.3×\frac{5}{30}=+20cm$