Optics Question 651

Question: A convex lens has 9 cm focal length and a concave lens has -18 cm focal length. The focal length of the combination in contact will be

[RPMT 1999]

Options:

A) 9 cm

B) - 18 cm

C) - 9 cm

D) 18 cm

Show Answer

Answer:

Correct Answer: D

Solution:

$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}}\Rightarrow \frac{1}{F}=\frac{1}{(+18)}\Rightarrow F=18cm $



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