A convex lens of power S D and a concave lens of power 7.5 D are placed in contact with each other. What is the:
(a) power of this combination of lenses?
(b) focal length of this combination of lenses ?
Power of convex lens = 5 D
Power of concave lens = -7.5 D
(a) Power of the combination of the lens = P1+ P2 = 5 – 7.5= -2.5 D
(b) P = 1/f
f = 1/P = 1/ -2.5 = -40 cm