Question

A plano-convex lens made of refractive index \(1.5\) and having radius of curvature \(R = 4\,\text{cm}\) fits exactly into a plano-concave lens made of refractive index \(1.3\) and having the same radius of curvature \(R = 4\,\text{cm}\) such that their plane surfaces are parallel to each other. The focal length of the combination is:

• A. \(5\,\text{cm}\)
• B. \(50\,\text{cm}\)
• C. \(2\,\text{cm}\)
• D. \(20\,\text{cm}\)

Hint:

For lenses in contact, add their powers. A convex lens has positive power and a concave lens has negative power.

Answer 1

The Correct Option is D