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 $



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