At what distance should a point object be placed from planoconvex lens, so that position of the final image is independent of b?
A) 40 cm
B) 60 cm
C) 30 cm
D) 20 cm
Correct Answer: D
Solution :
The position of final image will be independent of b only when the rays are incident parallel to principal axis on concave lens, which is possible only when the object is kept at focus of palnoconvex lens. Therefore, \[\frac{1}{a}=\frac{1}{{{f}_{1}}}=(\mu -1)\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] \[\Rightarrow \]\[\frac{1}{a}=\left( \frac{3}{2}-1 \right)\left[ \frac{1}{10}-\frac{1}{\infty } \right]\Rightarrow a=20\,cm\]
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