Question

Two thin lenses of focal lengths +10 cm and -5 cm are kept in contact. What is the focal length and power of the combination

1) -10 m and -10 D
2) -10 cm and -10 D
3)1 m and 10 D
4) -1 m and 10 D

Answer 1

Explanation:

1/fnet =1/f1+1/f2

1/fnet =1/10-1/5 = -1/10

thus, fnet = -10cm

Pnet = 1/fnet = 1/(-0.1m) = -10D