Optics Question 597
Question: Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact. The combined focal length is
[CPMT 1973, 89; BVP 2003]
Options:
A) + 15 cm
B) -15 cm
C) + 30 cm
D) -30 cm
Show Answer
Answer:
Correct Answer: D
Solution:
$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}}=\frac{1}{60}+\frac{1}{(-20)}\Rightarrow F=-30 $
