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 $
