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 $



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