Question

The power of a concave lens is 10 D and that of a convex lens is 6 D. When these two lenses are placed in contact with each other, the power of their combination will be :
(a) +16 D
(b) +4 D
(c) −16 D
(d) −4 D

Answer 1

 -4 D
 The powers of the lenses add up when brought in contact.
So, net power = +6 D + (-10 D) = -4 D.