Optics Question 591
Question: A thin convex lens of refractive index 1.5 has a focal length of 15 cm in air. When the lens is placed in liquid of refractive index 4/3 , its focal length will be
[CPMT 1974, 77; MP PMT 1992]
Options:
A) 15 cm
B) 10 cm
C) 30 cm
D) 60 cm
Show Answer
Answer:
Correct Answer: D
Solution:
$ \frac{f _{l}}{f _{a}}=\frac{{( _{a}}{\mu _{g}}-1)}{{( _{l}}{\mu _{g}}-1)} $
$ \Rightarrow \frac{f _{l}}{15}=\frac{(1.5-1)}{( \frac{1.5}{4/3}-1 )} $
$ \Rightarrow f _{l}=60cm $
