Question

You are given lenses with powers + 10 D, + 5 D, – 5 D, – 20 D, and – 10 D. Taking a pair of lenses at a time, which two lenses will you select to have a combination of total focal length when the two lenses are kept in contact in each case?

(a) 20 cm

 (b) – 10 cm

(c) – 20 cm

 (d) 20/3 cm

Answer 1

(a) When lenses of 10 D and – 5 D are taken, total power