A) Become infinite
B) Become small, but non-zero
C) Remain unchanged
D) Become zero
Correct Answer: A
Solution :
The focal length of a lens is given by \[\frac{1}{f}=\left( \frac{{{\mu }_{2}}}{{{\mu }_{1}}}-1 \right)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] \[{{\mu }_{1}}\]and \[{{\mu }_{2}}\]are the refractive index of medium and material of lens respectively. If \[{{\mu }_{1}}={{\mu }_{2}},\]then the length of lens will be infinite. In this case the lens acts as a plane sheet of glass.
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