Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. The effective power of the combination is

Question

Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. The effective power of the combination is (A) 9 D (B) 2 D (C) 3 D (D) 7 D

Tardigrade Admin · Accepted Answer

We have the effective power given by Peff=P1+P2 Also we know, P = (1/f) Hence, Peff= (1/f1)+(1/f2) = (1/0.20) + (1/0.25) = 5+ 4 = 9D

Q. Two thin lenses of focal lengths $20 c m$ and $25 c m$ are placed in contact. The effective power of the combination is