A) \[-1.5D\]
B) \[~-6.5\text{ }D\]
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, \[{{f}_{1}}=40\,cm=0.4m,\] \[{{f}_{2}}=-25\,cm=-0.25\,m\] \[\therefore \] \[P=\frac{1}{0.4}-\frac{1}{0.25}=2.5-4=-1.5\,D\]
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