A) \[-1.5\,\,D\]
B) \[-6.5\,\,D\]
C) \[6.5\,\,D\]
D) \[6.67\,\,D\]
Correct Answer: A
Solution :
Key Idea: Power of lens is inverse of focal length in metres. The power of the combination of lenses is\[P={{P}_{1}}+{{P}_{2}}\] where \[{{P}_{1}}=\frac{1}{{{f}_{1}}}\]and\[{{P}_{2}}=\frac{1}{{{f}_{2}}}\] Given, \[{{f}_{1}}=40\,\,cm=0.4\,\,m\] \[{{f}_{2}}=-25\,\,cm=-0.25\,\,cm\] \[\therefore \] \[P=\frac{1}{0.4}-\frac{1}{0.25}=2.5-4=1.5\,\,D\]
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