A) L=20 cm
B) L=10cm
C) L=40 cm
D) \[\operatorname{L}=\frac{20}{3}cm\]
Correct Answer: A
Solution :
when rays incident parallel to lens axis then it will converge at the focus and vice versa \[\operatorname{F}=d, {{R}_{1}}=+20cm, {{R}_{2}}=-20cm\] \[\frac{1}{\operatorname{f}}=\left( \mu -1 \right)\left[ \frac{1}{{{\operatorname{R}}_{1}}}-\frac{1}{{{\operatorname{R}}_{2}}} \right]\] \[\frac{1}{\operatorname{d}}=\left( 1.5-1 \right)\left[ \frac{1}{20}-\frac{1}{-20} \right]\] \[\operatorname{d}=20cm\]
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