Question

what should be the focal length and nature of lens to be put in contact with a lens of f=15 cm to get power of combination equal to 10 dioptre?

Answer 1

Focal length of first lens = 15 cm = 0.15m

Power = 1/f = 1/0.15 = 6.67 D

Power of combination= 10 D

Power of second lens = 10 -6.67 = 3.33 D

Focal length o second lens = 1/P = 1/3.33 = 3/10 m = 30cm

Since power of second lens is positive, it is a convex lens and its focal length is 30cm