Optics Question 697
Question: If two lenses of +5 diopters are mounted at some distance apart, the equivalent power will always be negative if the distance is
[BCECE 2005]
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:
Using $ P=P _{1}+P _{2}-d\times P _{1}P _{2} $
for equivalent power to be negative $ d\times P _{1}P _{2}>P _{1}+P _{2} $
Therefore $ d\times 25>10 $
Therefore $ d>\frac{10}{25}m $
Therefore $ d>\frac{10\times 100}{25} $
Therefore $ d>40cm. $
