Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 108 m/s, the focal length of the lens is :

Engineering

Physics

Curved Surface Refraction

Question

Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 10⁸ m/s, the focal length of the lens is :

15 cm

10 cm

20 cm

30 cm

LiveFree JEE Main Previous Year Online Papers Live Free JEE Advance Previous Year Online Papers

College PredictorLive

Know your College Admission Chances Based on your Rank/Percentile, Category and Home State.

Get your JEE Main Personalised Report with Top Predicted Colleges in JoSA

3² + (R – 0.3)² = R²

32 + R² – 0.6R + 0.09 = R²

$\Rightarrow R = \frac{3^{2}}{0 .6} = 15 cm$

$\frac{1}{f} = (µ - 1) (\frac{1}{R} - \frac{1}{\infty}) = \frac{1}{30}$