A) \[-1.5\,D\]
B) \[-6.5\,D\]
C) \[+6.5D\]
D) \[+\,6.67D\]
Correct Answer: A
Solution :
Focal length of combination can be calculated as \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] \[\Rightarrow \] \[\frac{1}{F}=\frac{1}{40}+\frac{1}{(-25)}\] \[\Rightarrow \] \[\frac{1}{F}=\frac{-200}{3}\,cm\] \[\therefore \] \[P=\frac{100}{F}\] \[=\frac{100}{-200/3}\] \[=-1.5\,D\]
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