A) -1.5 dioptre
B) -6.5 dioptre
C) +6.5 dioptre
D) +6.67 dioptre
Correct Answer: A
Solution :
\[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] Let the power of the combination be P. \[\therefore \] \[P=\frac{1}{0.4\,m}+\frac{1}{-0.25\,m}\] or, \[P=-1.5\,\text{dioptre}\] Hence, the correction option is [a].
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