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. $



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