A) \[\frac{M}{4}\]
B) \[3\,\,M\]
C) \[2\,\,M\]
D) \[4\,\,M\]
Correct Answer: C
Solution :
Key Idea: Minimum magnifying power is equal to ratio of focal length of objective to that of eyepiece. Magnifying power \[M\] is given by \[M=\frac{{{f}_{o}}}{{{f}_{e}}}\] where, \[{{f}_{o}}\] is focal length of objective, and \[{{f}_{e}}\] is focal length of eyepiece. Given, \[f_{e}^{'}=\frac{{{f}_{e}}}{2}\] \[\therefore \] \[M'=\frac{{{f}_{o}}}{{{f}_{e}}/2}=2\cdot \frac{{{f}_{o}}}{{{f}_{e}}}=2M\]
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