question_answer

A thin glass (refractive index 1.5) lens has optical power of - 5 D in air. Its optical power in a liquid medium with refractive index 1.6 will be     AIEEE  Solved  Paper-2005

A) 1 D    

B)                                        -1 D                       

C)        25 D                      

D)        - 25 D

Correct Answer: A

Solution :

By lens maker formula, \[\frac{1}{{{f}_{a}}}=(\mu -1)\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)=(1.5-1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\]    ...(i) and                                               ?(ii) Thus, \[{{f}_{m}}=-8\times {{f}_{a}}=-8\,\times \,\frac{-1}{5}\]                         \[{{P}_{m}}=\frac{\mu }{{{f}_{m}}}\,=\frac{1.6}{1.6}\,=1\,D\]