A) L = 20 cm
B) L = 10 cm
C) L = 40 cm
D) L = \[\frac{20}{3}\]cm
Correct Answer: A
Solution :
Here, \[n=1.5,\]as per sign convention followed \[{{R}_{1}}=+20\,\,cm\,\] and \[{{R}_{2}}=-20\,cm\] \[\therefore \] \[\frac{1}{f}=(n-1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] \[=(1.5-1)\left[ \frac{1}{(+20)}-\frac{1}{(-20)} \right]\] \[=0.5\times \frac{2}{20}=\frac{1}{20}\] \[\Rightarrow \] \[f=20\,cm\] Incident rays travelling parallel to the axis of lens will converge at its second principal focus. Hence, \[L=+\,20\,cm\]
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