Question

Calculate its focal length if the radii of curvature of a double convex lens are 10 cm, and 20 cm and its refractive index is 1.75.

(a) 50 cm

(b) 13 cm

(c) 8.9 cm

(d) 8 cm

I had been asked this question by my college director while I was bunking the class.

This intriguing question comes from Reflection of Light by Spherical Mirrors in chapter Ray Optics and Optical Instruments of Physics – Class 12

Answer 1

The correct answer is (c) 8.9 cm

To explain: \(\frac {1}{f}\) = (μ-1)\( [ \frac {1}{R_1} – \frac {1}{R_2} ] \)

\(\frac {1}{f}\) = (1.75 – 1)\( [ \frac {1}{10} – \frac {1}{(-20)} ] \)

\(\frac {1}{f} = \frac {9}{80}\)

f = +8.89 cm ≈ 8.9 cm.