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



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