A) 25
B) 50
C) infinite
D) zero
Correct Answer: D
Solution :
Focal length of combination of lenses placed in contact is \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] For convex lens, \[{{f}_{1}}=25cm\] For concave lens, \[{{f}_{2}}=-25cm\] Hence, \[\frac{1}{F}=\frac{1}{25}+\frac{1}{-25}\] \[=\frac{1}{25}-\frac{1}{25}=0\] \[\therefore \] \[F=\frac{1}{0}=\infty \] Hence, power of combination, \[P=\frac{1}{F}=0\]
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