Question

Double convex lens has radii of curvature 4 cm and 8 cm. Refractive index = 1.5. Find focal length.

• A. 16 cm
• B. 12.11 cm
• C. 7.33 cm
• D. 5.33 cm

Hint:

Use sign convention: R is positive for convex surface facing incident light.

Answer 1

The Correct Option is D

Use Lens Maker’s Formula: \[ \frac{1}{f} = (n - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \] Here, \( n = 1.5, R_1 = 4\ \text{cm}, R_2 = -8\ \text{cm} \), \[ \frac{1}{f} = (0.5) \left( \frac{1}{4} + \frac{1}{8} \right) = 0.5 \cdot \frac{3}{8} = \frac{3}{16} \Rightarrow f = \frac{16}{3} = 5.33\ \text{cm} \]