Optics Question 599
Question: Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact convex. The effective power of the combination is
[CBSE PMT 1990; RPMT 2001]
Options:
A) 45 dioptres
B) 9 dioptres
C) 1/9 dioptre
D) 6 dioptres
Show Answer
Answer:
Correct Answer: B
Solution:
$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}}=\frac{1}{20}+\frac{1}{25} $
$ \Rightarrow $ $ F=\frac{100}{9} $
$ cm=\frac{1}{9} $
$ metre $
$ \therefore $ $ P=\frac{1}{1/9}D=9D $
