A) 20mm
B) 100mm
C) 120mm
D) 80mm
Correct Answer: C
Solution :
To obtain final image at infinity, object which is the image formed by objective should be at focal distance of eye-piece. By lens formula (for objective) \[\frac{1}{{{v}_{0}}}-\frac{1}{{{u}_{0}}}=\frac{1}{{{f}_{0}}}\]or\[\frac{1}{{{v}_{0}}}-\frac{1}{-25}=\frac{1}{20}\] \[\Rightarrow \]\[\frac{1}{{{v}_{0}}}=\frac{1}{20}-\frac{1}{25}=\frac{5-4}{100}=\frac{1}{100}mm\] \[\therefore \]\[{{v}_{0}}=100mm\] Therefore the distance between the lenses \[{{v}_{0}}=+{{f}_{e}}=100\,mm+20mm=120mm\]You need to login to perform this action.
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