Optics Question 663
Question: If two + 5 D lenses are mounted at some distance apart, the equivalent power will always be negative if the distance is
[UPSEAT 2002]
Options:
A) Greater than 40 cm
B) Equal to 40 cm
C) Equal to 10 cm
D) Less than 10 cm
Show Answer
Answer:
Correct Answer: A
Solution:
$ P=P _{1}+P _{2}-dP _{1}P _{2}\Rightarrow P=10-25d $
$ \Rightarrow $ For $ P $ to be negative $ 25d>10 $
$ \Rightarrow $ $ d>0.4m $ or $ d>40 $ cm
