A) concave lens
B) convex lens
C) mirror
D) none of these
Correct Answer: A
Solution :
Key Idea: Focal length of a lens depends upon the refractive index of material of lens and the radius of curvature of its surfaces. From lens formula \[\frac{1}{f}=(n-1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] When lens is dipped in liquid, its focal length is \[{{\,}_{l}}{{n}_{g}}=\frac{_{a}{{n}_{g}}}{{{\,}_{a}}{{n}_{l}}}\] Given, \[{{\,}_{l}}{{n}_{g}}=1.44,{{\,}_{a}}{{n}_{l}}=1.49\] \[\therefore \] \[{{\,}_{l}}{{n}_{g}}=\frac{1.44}{1.49}<1\] Hence, focal length \[({{f}_{1}})\]of lens becomes \[\frac{1}{{{f}_{1}}}=({{\,}_{1}}{{n}_{g}}-1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] negative. Hence lens behaves like a concave lens. Note: If focal length was positive it will behave like a convex lens, when infinite it behaves like a plane transparent plate.
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