If the radii of curvature of the faces of a double convex lens are 10 cm and 15 cm, respectively and its refractive index is 1.5, the focal length of the lens in air is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 12 cm
Use the lens maker's formula for a thin lens in air. For a double convex lens, \( R_1 = +10\,\mathrm{cm} \) and \( R_2 = -15\,\mathrm{cm} \) (the second face curves the other way).
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
\( \displaystyle \frac{1}{f} = (1.5 - 1)\left(\frac{1}{10} + \frac{1}{15}\right) = 0.5\times\frac{5}{30} = \frac{1}{12} \)
\( \displaystyle f = 12\,\mathrm{cm} \)