A) + 80 cm
B) ? 80 cm
C) ? 24 cm
D) ?100 cm
Correct Answer: D
Solution :
\[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}+\frac{1}{{{f}_{3}}}\] \[\frac{1}{{{f}_{1}}}=(1.6-1)\left( \frac{1}{\infty }-\frac{1}{20} \right)=-\frac{0.6}{20}=-\frac{3}{100}\] ?(i) \[\frac{1}{{{f}_{2}}}=(1.5-1)\left( \frac{1}{20}-\frac{1}{-20} \right)=\frac{1}{20}\] ?(ii) \[\frac{1}{{{f}_{3}}}=(1.6-1)\left( \frac{1}{-20}-\frac{1}{\infty } \right)=-\frac{3}{100}\] ?(iii) \[\Rightarrow \] \[\frac{1}{F}=-\frac{3}{100}+\frac{1}{20}-\frac{3}{100}\Rightarrow F=-100\]\[cm\]You need to login to perform this action.
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