A) concave lens of focal length 25 cm.
B) concave lens of focal length 50 cm.
C) convex lens of focal length 25 cm.
D) convex lens of focal length 50 cm.
Correct Answer: D
Solution :
A convex lens of focal length 50 cm is to be used. \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f},\frac{1}{-50}-\frac{1}{-25}=\frac{1}{f}\] \[-\frac{1}{50}+\frac{1}{25}=\frac{1}{f},\frac{-1+2}{50}=\frac{1}{f},\frac{1}{50}=\frac{1}{f},\] \[\text{f}=\text{5}0\text{cm}\]
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