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, \[{{f}_{1}}=25\,cm\] For concave lens, \[{{f}_{2}}=-25\,cm\] 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, \[P=\frac{1}{F}=0\,D\] Note: As a convex and a concave lens of same focal lens are placed in contact, so we get achromatism i.e., combination if free from chromatic aberration.You need to login to perform this action.
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