Question

Two thin lenses of powers 4 D and -2D are placed
in contact. What is the focal length of the
combination?​

Answer 1

P1 = +3.5 D            P2 = -2.5 D

P = P1 + P2                  (Note: P is the combined lens power)

P = (+3.5 -2.5) D

P = 1 D

P = 1/F

1 =  1/F

F = 1m   OR  F = 100cm.           (F is the focal length of the combined lens)