A) \[+21.25\text{ }cm,\] convex lens
B) \[-21.25\text{ }cm,\]concave lens
C) \[-18.75\text{ }cm,\]concave lens
D) \[+18.75\text{ }cm,\]convex lens
Correct Answer: D
Solution :
Here: Distance of object\[=-25\text{ }cm\] Distance of image \[\upsilon =+75\text{ }cm\] The relation for focal length of the lens is given by, \[\frac{1}{f}=\frac{1}{\upsilon }-\frac{1}{u}\] \[=\frac{1}{75}-\frac{1}{-25}=\frac{4}{75}\] Or \[f=\frac{75}{4}=18.75\,cm\] The focal length is positive so the lens is convex.
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