The power of three lenses A, B, and C are 1, 2 and 3 dioptre respectively. What is the power of the combination of these lenses?
Two lenses A and B have focal lengths of + 20 cm and, -10 cm, respectively. (a) What is the nature of lens A and lens B ? (b) What is the power of lens A and lens B ? (c) What is the power of combination if lenses A and B are held close together ?