A) - 0.4 m
B) 0.4 m
C) - 0.1 m
D) 0.1 m
Correct Answer: A
Solution :
Equivalent focal length (F) of two lenses separated by distance d is given by \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}-\frac{d}{{{f}_{1}}{{f}_{2}}}\] \[=\frac{1}{0.2}+\frac{1}{0.2}-\frac{0.5}{(0.2)(0.2)}\] \[=5+5-0.5\times 5\times 5\] \[=10-12.5\] \[=-2.5\] \[\therefore \] \[F=-\frac{1}{2.5}=-0.4m\]
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