Two plano concave lenses of glass of refractive index 1.5 have radii of curvature 20 cm and 30 cm respectively. They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive index 5/2. The focal length of the combination is (in cm)

see full answer

Talk to JEE/NEET 2025 Toppers - Learn What Actually Works!

Real Strategies. Real People. Real Success Stories - Just 1 call away

An Intiative by Sri Chaitanya

– 92

108

answer is D.

(Unlock A.I Detailed Solution for FREE)

Ready to Test Your Skills?

Check your Performance Today with our Free Mock Test used by Toppers!

Take Free Test

Detailed Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} + \frac{1}{f_{3}}$

$(μ_{1} - 1) (\frac{- 1}{R_{1}}) + (μ_{2} - 1) (\frac{1}{R_{1}} + \frac{1}{R_{2}}) + (μ_{2} - 1) (\frac{- 1}{R_{2}}) \frac{1}{F} = (1.5 - 1) (\frac{- 1}{20}) + (\frac{5}{2} - 1) (\frac{1}{20} + \frac{1}{30}) + (1.5 - 1) (\frac{- 1}{30}) \frac{1}{F} = \frac{1}{2} \times \frac{- 1}{20} + \frac{3}{2} \times \frac{5}{60} + \frac{1}{2} \times \frac{- 1}{30} = \frac{- 3 + 15 - 2}{120} F = \frac{120}{10} = 12 cm$