| A plan convex lens becomes an optical system of \[28cm\] focal length when its plane surface is silvered and illuminated from left to right as shown in Fig-A. If the same lens is instead silvered on the curved surface and illuminated from other side as in Fig. B, it acts like an optical system of focal length\[10cm\]. The refractive index of the material of lens if: [JEE Online 15-04-2018] |
 |
A) \[1.50\]
B) \[1.55\]
C) \[1.75\]
D) \[1.51\]
Correct Answer: B
Solution :
| [b] \[\frac{l}{F}=\frac{2}{f}+\frac{l}{{{f}_{m}}}\] |
| \[{{f}_{m}}\Rightarrow \infty \] |
| This gives, \[f=56\] |
| \[{{f}_{m}}=\frac{R}{2}\backslash \] |
| Hence |
| \[\frac{1}{F}=\frac{2}{f}+\frac{2}{R}\] |
| This gives, \[R=31.11\] |
| \[\frac{1}{f}=(\mu -1)(\frac{1}{R})\] |
| \[\mu =1.55\] |
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