A thin convex lens and a thin concave lens are kept in contact and are coaxial. Which statement about the combination is correct?
Options

Question

A thin convex lens and a thin concave lens are kept in contact and are coaxial. Which statement about the combination is correct?

G1 Physics · Accepted Answer

For two thin lenses in contact, 1 ÷ f(eq) = 1 ÷ f(convex)+1 ÷ f(concave). The convex-lens focal length is positive and the concave-lens focal length is negative. Therefore, 1 ÷ f(eq) = 1 ÷ |f(convex)| - 1 ÷ |f(concave)|. If |f(convex)| > |f(concave)|, then 1 ÷ |f(convex)| < 1 ÷ |f(concave)|, so the equivalent power is negative and the combination behaves as a concave lens. Also, the equivalent focal length of thin lenses in contact is independent of their order. Correct option: A.