Optics Question 666

Question: If convex lens of focal length 80cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power

[AFMC 2002]

Options:

A) + 6.5 D

B) ? 6.5 D

C) + 7.5 D

D) ? 0.75 D

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Answer:

Correct Answer: D

Solution:

$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}}\Rightarrow \frac{1}{F}=\frac{1}{+80}+\frac{1}{-50} $

$ \Rightarrow $ $ F=-\frac{400}{3}\Rightarrow P=\frac{-3}{4}D $



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