Question

Two plano-concave lenses (1 and 2) of glass of refractive index 1.5 have radii of curvature 25 cm and 20 cm. They are placed in contact with their curved surfaces towards each other and the space between them is filled with liquid of refractive index 4/3. The combination is 

• A. convex lens of focal length 70 cm
• B. concave lens of focal length 70 cm
• C. concave lens of focal length 66.6 cm
• D. convex lens of focal length 66.6 cm

Hint:

For lenses in contact with a medium: Pₙet = sum Pₛᵤᵣfₐcₑₛ Always consider refractive index of the filling medium.

Answer 1

The Correct Option is C

Step 1: Power of each curved surface using refraction formula Net power: P = ((mul)/(mug)-1)((1)/(R₁)-(1)/(R₂))
Step 2: Substituting values gives P = -(1)/(66.6)cm⁻1