Optics Question 627
Question: A concave lens of glass, refractive index 1.5, has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a
[IIT-JEE 1999]
Options:
A) Convergent lens of focal length 3.5 R
B) Convergent lens of focal length 3.0 R
C) Divergent lens of focal length 3.5 R
D) Divergent lens of focal length 3.0 R
Show Answer
Answer:
Correct Answer: A
Solution:
$ \frac{f _{l}}{f _{a}}=\frac{ _{a}{\mu _{g}}-1}{ _{l}{\mu _{g}}-1}=\frac{1.5-1}{\frac{1.5}{1.75}-1} $ = $ -\frac{1.75\times 0.50}{0.25}=-3.5 $
$ \therefore f _{l}=-3.5\ f _{a}\Rightarrow f _{l}=+3.5R $ (Q fa = R)
Hence on immersing the lens in the liquid, it behaves as a converging lens of focal length 3.5 R.
