Question

A thin converging lens of focal length 15 cm is kept in contact with a thin diverging lens of focal length 20 cm. Find the focal length of this combination.

Answer 1

Given,

f₁ = 15 cm (convex), 

f₂ = -20 cm (concave)

We have,

\(\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}\)

\(=\frac{1}{15}-\frac{1}{20}\)

\(=\frac{20-15}{300}\)

\(\frac{5}{300}=\frac{1}{60}\)

F = 60 cm.